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TerryG TerryG is offline
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Default New member, pentode g2 operation question.

Hi all,

I am new to this forum but have known several members here for quite some time. I know not many of you will recognise me from this handle, but you will I know.

Anyway I have been enjoying SEP amplifiers this last year an 1/2. Don't know why they have the reputation they do compared to SET's, but that is another issue.

I have designed and built several pentode single ended amplifiers at this point. And the one point that still alludes me is: How from what the datasheets say can you figure out what resistor to start with on g2?

I understand the DC voltage value and that is not difficult to figure out, but I like pentodes best when there is an unbypass resistor on g2.

I have found this does several things, it moves the top bias lines down, and the bottom bias lines up at the same time raising ( or lowering depending on how you look at it) the cut off bias voltage. Usually by putting a unbypassed g2 resistor on g2 your current drops compared to bypassing, and a little more wattage is possible for the same operating point. It seems most g2's have a certain impedance, EL84s you can start with 1.5K and go to above 10K with the resistor have more effect on the signal traces. This also lowers distortion it appears, but there is always a point of diminished returns for each tube.

It sure would be nice to be able to do a few math problem when looking at datasheets and know where to start with a tube.

I will introduce a controversial subject with g2 later, but for now I just want to know what information on the datasheet will tell me what I want to know.

Terry

Last edited by TerryG : May 17th 12 at 12:22 AM
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Roger Jones Roger Jones is offline
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Default New member, pentode g2 operation question.

On May 16, 7:18*pm, TerryG wrote:
Hi all,

I am new to this forum but have known several members here for quite
some time. *I know not many of you will recognise me from this handle,
but you will I know.

Anyway *I have been enjoying SEP amplifiers this last year an 1/2.
Don't know why they have the reputation they do compared to SET's, but
that is another issue.

I have designed and built several pentode single ended amplifiers at
this point. *And the one point that still alludes me is: *How from what
the datasheets say can you figure out what resistor to start with on g2?

I understand the DC voltage value and that is not difficult to figure
out, but I like pentodes best when there is an unbypass resistor on g2.

I have found this does several things, it moves the top bias lines down,
and the bottom bias lines up at the same time raising ( or lowering
depending on how you look at it) the cut off bias voltage. *Usually by
putting a unbypassed g2 resistor on g2 your current drops compared to
bypassing, and a little more wattage is possible for the same operating
point. *It seems most g2's have a certain impedance, EL84s you can start
with 1.5K and go to above 10K with the resistor have more effect on the
signal traces. *This also lowers distortion it appears, but there is
always a point of diminished returns for each tube.

It sure would be nice to be able to do a few math problem when looking
at datasheets and know where to start with a tube.

I will introduce a controversial subject with g2 later, but for now I
just want to know what information on the datasheet will tell me what I
want to know.

Terry

--
TerryG


Terry, I've played with quite a few Single-Ended Pentode (SEP) amps
over the years...in my student days it was for economic reasons! I
still do them now in vintage radio restoration. I sometimes use a g2
resistor to ensure the g2 voltage does not exceed the plate voltage
(OPT primary resistance drops the latter a bit.) I do the same in
pentode/beam tetrode P-P amps for the same reason (if they're not UL)
- just done it with an Eico HF12 (EL84's); actually I just moved the
g2 tap further down the B+ filter and adjusted the R values to get the
right voltages back. My reasoning is to reduce screen power and
heating. However, in both, I always decouple the g2 resistor (above B+
filter does it anyway.) I would expect this to change the dynamic
operating point, tube impedance and max. power wrt a naked R...
obviously, it can't change the DC operation. But I've never studied
the effect... mea culpa! I trust the tube modellers will help us.
Cheers,
Roger
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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default New member, pentode g2 operation question.


"TerryG"

I am new to this forum but have known several members here for quite
some time. I know not many of you will recognise me from this handle,
but you will I know.

Anyway I have been enjoying SEP amplifiers this last year an 1/2.
Don't know why they have the reputation they do compared to SET's, but
that is another issue.



** Just curious - but do you use any NFB with your SEP designs ?

BTW:

the first valve amp I built used a 6V6GT and a decent size SE tranny from
A&R of Melbourne.

The amp also used a war surplus 6AC7 as a preamp and a 5Y3 rectifier.

http://www.r-type.org/exhib/aaa0053.htm



..... Phil




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Patrick Turner Patrick Turner is offline
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Posts: 3,964
Default New member, pentode g2 operation question.

On May 19, 5:29*pm, "Phil Allison" wrote:
"TerryG"



I am new to this forum but have known several members here for quite
some time. *I know not many of you will recognise me from this handle,
but you will I know.


Anyway *I have been enjoying SEP amplifiers this last year an 1/2.
Don't know why they have the reputation they do compared to SET's, but
that is another issue.


** Just curious - but do you use any NFB with your SEP designs ?

BTW:

the first valve amp I built used a 6V6GT and a decent size SE tranny from
A&R of Melbourne.

The amp also used a war surplus 6AC7 as a preamp and a 5Y3 rectifier.

http://www.r-type.org/exhib/aaa0053.htm

.... * Phil


The screen of a pentode or beam tetrode, ie, G2, acts like a
electrostatic screen to prevent the change of voltage causing much
effect on the electron flow. If G2 is held at an unchanging potential
Vdc between cathode and G2 then the tube exhibits the high Ra, and
high amplification factor µ.

If you consider that G2 is bypassed to cathode, and cathode is
bypassed to 0V, then while the tube is operated by G1 grid, there is a
signal flow from B+ to G2.
If there is an added resistance between G2 and B+, and this resistance
is not bypassed to cathode, then a signal voltage is developed across
the R which has an effect on the electron flow to reduce the gain of
the tube below the fully bypassed state.

Let us say you have a 6550 with Ia at 55mA, Ea = 450V and Eg2 = 350V,
you would find that G1 gm, transconductance, = 5.5mA/V approx. You
would also find that if you allow the signal voltage between G2 and
cathode, VG2-k, to vary, then the G2 screen has gm = about 0.83mA/V,
considerably less than the G1, but very significant in audio amps
especially where Ultralinear operation or triode operation s
considered. To really understand the what is happening inside a
pentode I suggest you try to undertsand the *equivalent models* of the
tube.
I have a number of web pages which may allow a better understanding,
see http://www.turneraudio.com.au/basic-tube-1.html
This page plus many others following might be of use.

One of the simplest models of a pentode for its signal ac operation
without being
confused by the DC operation is to **imagine** it this way :-
Ra is a resistance between anode terminal and cathode terminal, about
32,000 ohms
with the 6550 sample above.
Then you have a current generator connected between anode terminal and
cathode, This CS is an imaginery device with high grid input impedance
but with infinitely high output impedance which produces current at
the rate of G1 Vin x gm, ie, 5.5mA ac change for 1Vac Vin. The Screen
is also a CS strapped across anode terminal and cathode and also
shunting the 32k.

While ever VG2 is without any Vac relative to cathode, the Ra measured
is always going to be just 32k, because the two CS I've mentioned have
infinite R. Hence tube gain = Va / Vg = G1 gm x ( Ra in parallel with
RL ).

But if you connect the anode to the screen, then whatever Vac is
applied to the G2 changes tube current and current in Ra according to
screen gm x anode Vac.
When you work out what is happening simultaneously, you should find
the combined effect of Vac applied to G1 and G1 gives triode
operation, and the measured Ra and µ are both very much reduced, along
with distortion spectra, so triodes are favoured for hi-fi.

With a fairly large value of unbypassed R between B+ and G2, say more
than 1k5, then you may find the distortion and Ra are not much reduced
and less maximum PO is available and the operation is
dissapointing.

Pentodes tend to give much more circuit gain than triodes, especially
with high gm input pentods like the 6CA7 to which Phil refers, and
which has a very high "figure of merit" so it could be used in many RF
amps and AF apps. I have one used as a gain tube ahead of a 6CM5
cathode follower buffer which together form the second stage after a
12AT7 differential gain stage in a 6 band Wien Bridge oscillator I
made about 10 years ago. After 6CA7 which was around in WW2, many
other pentodes with high gm became available. Some, depending on the
use, could be unstable because of high gain, and then the series R to
G2 was employed to help prevent oscillations at HF, because there is
significant C between G2 and G1, and with some added R you have a low
pass shunt FB filter which tends to stop oscillations above F where
the tube is intended to operate.

I'm not much of a 6V6 fan, so when I do restore an old radio, I might
junk the 5Y3, install Si diodes, then put in an EL34 in triode with
same Ia but higher available B+, and the OPT will usually work well
with the EL34 in triode. The EL34 goes into the 5Y3 socket, and a
6SH7, 6SJ7, 6CA7, 6SL7 can be used where the 6V6 was, and a useful
amount of global NFB applied, about 12dB is plenty, and the you have a
really nice sounding SE amp where before no NFB at all with 6V6, you
had 10 times the THD/IMD as the EL34 in triode. I would much rather
listen with 4Watts from an EL34 in triode with GNFB than a poor 3Watts
from 6V6 in pure beam tetrode. One cannot usually avoid the common 20%
widing losses in old radio OPTs, but often their speakers are exremely
sensitive and 4 Watts for a 12" Rola DeLux hi-fi speaker made in Oz in
1953 will fill a kitchen with plate rattling bass tones and music to
make the glass of shiraz taste all the better.

Patrick Turner.

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flipper flipper is offline
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Default New member, pentode g2 operation question.

On Wed, 16 May 2012 23:18:38 +0000, TerryG
wrote:


Hi all,

I am new to this forum but have known several members here for quite
some time. I know not many of you will recognise me from this handle,
but you will I know.

Anyway I have been enjoying SEP amplifiers this last year an 1/2.
Don't know why they have the reputation they do compared to SET's, but
that is another issue.


Because, all else being equal (which it seldom is), a SE pentode will
have higher output impedance (worse damping factor) and more
distortion than a SET. The saving graces are it takes less input to
drive and produces more output power.

I have designed and built several pentode single ended amplifiers at
this point. And the one point that still alludes me is: How from what
the datasheets say can you figure out what resistor to start with on g2?

I understand the DC voltage value and that is not difficult to figure
out, but I like pentodes best when there is an unbypass resistor on g2.


I think you answered your own question. The common purposes of a G2
resistor are to bias, which you note is easy enough to determine from
the existing data, and to kill parasitic oscillations, which are not
'static' characteristics and, so, not something for a static graph.

An unpypassed G2, of the nature you're describing, is not 'typical'
operation.

I have found this does several things, it moves the top bias lines down,
and the bottom bias lines up at the same time raising ( or lowering
depending on how you look at it) the cut off bias voltage. Usually by
putting a unbypassed g2 resistor on g2 your current drops compared to
bypassing, and a little more wattage is possible for the same operating
point.


G2 current will be less because you are, in effect, applying negative
feedback to the screen with an unbypassed G2 resistor. Plate current
will also be less because you are, in effect, applying negative
feedback to the screen with an unbypassed G2 resistor.

I don't know how you arrive at the 'more wattage' conclusion.

As a side note, the 'opposite polarity' Rg2 derived screen signal was
used for the, so called, "self split" (Class A) PP amplifiers. It
drives the second grid so you don't need 'another tube'.

Negative feedback is also why the SET has lower impedance, lower
distortion, less power and also why it takes more signal to drive it.
Ia of a triode depends on *BOTH* G1 and plate voltages so if you stick
an impedance between plate and B+ the generated signal is negative
feedback to the plate. That is precisely what the 'fixed' screen
voltage in a pentode prevents, plate voltage feedback, hence it's
higher plate impedance, gain, and distortion. Putting an unbypassed
screen resistor on G2 means you're applying 'a little' of the feedback
a triode would see, plus lowering screen voltage.

It seems most g2's have a certain impedance,


Clearly it has impedance or else screen voltage would be 0 with any
resistance between G2 and B+.

EL84s you can start
with 1.5K and go to above 10K with the resistor have more effect on the
signal traces. This also lowers distortion it appears,


Negative feedback.

but there is
always a point of diminished returns for each tube.


Sure, because the larger Rg2 the lower screen voltage, lower Ia, and
less power.

The 'better' way of doing it is UL. You still get less power but at
least screen voltage isn't also reduced so you only 'sacrifice' that
which goes directly to reducing distortion. (Allegedly there's a
'sweet spot' ratio where 'maximum benefit' of both is achieved).

It sure would be nice to be able to do a few math problem when looking
at datasheets and know where to start with a tube.


The rest is usually contained in the 'typical' operating
characteristics or example circuits.

I will introduce a controversial subject with g2 later, but for now I
just want to know what information on the datasheet will tell me what I
want to know.


I imagine that, with enough perseverance, one could derive screen
current magnitude, the resulting Rg2 voltage, and the degree of
feedback but it's not 'straight on the graphs' because they don't
expect them to be operated that way.

Terry


Something that might be interesting to try would be a sort of 'poor
man's UL' with a tube like the 6CW5 that can use 250V B+ but is screen
limited to 200V. There a 'significant size' unbypassed Rg2 of 10k
would put screen right at the spec'd 200 V.


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Patrick Turner Patrick Turner is offline
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Default New member, pentode g2 operation question.

On May 20, 2:05*pm, flipper wrote:
On Wed, 16 May 2012 23:18:38 +0000, TerryG

wrote:

Hi all,


I am new to this forum but have known several members here for quite
some time. *I know not many of you will recognise me from this handle,
but you will I know.


Anyway *I have been enjoying SEP amplifiers this last year an 1/2.
Don't know why they have the reputation they do compared to SET's, but
that is another issue.


Because, all else being equal (which it seldom is), a SE pentode will
have higher output impedance (worse damping factor) and more
distortion than a SET. The saving graces are it takes less input to
drive and produces more output power.

I have designed and built several pentode single ended amplifiers at
this point. *And the one point that still alludes me is: *How from what
the datasheets say can you figure out what resistor to start with on g2?


I understand the DC voltage value and that is not difficult to figure
out, but I like pentodes best when there is an unbypass resistor on g2.


I think you answered your own question. The common purposes of a G2
resistor are to bias, which you note is easy enough to determine from
the existing data, and to kill parasitic oscillations, which are not
'static' characteristics and, so, not something for a static graph.

An unpypassed G2, of the nature you're describing, is not 'typical'
operation.

I have found this does several things, it moves the top bias lines down,
and the bottom bias lines up at the same time raising ( or lowering
depending on how you look at it) the cut off bias voltage. *Usually by
putting a unbypassed g2 resistor on g2 your current drops compared to
bypassing, and a little more wattage is possible for the same operating
point.


G2 current will be less because you are, in effect, applying negative
feedback to the screen with an unbypassed G2 resistor. Plate current
will also be less because you are, in effect, applying negative
feedback to the screen with an unbypassed G2 resistor.

I don't know how you arrive at the 'more wattage' conclusion.

As a side note, the 'opposite polarity' Rg2 derived screen signal was
used for the, so called, "self split" (Class A) PP amplifiers. It
drives the second grid so you don't need 'another tube'.

Negative feedback is also why the SET has lower impedance, lower
distortion, less power and also why it takes more signal to drive it.
Ia of a triode depends on *BOTH* G1 and plate voltages so if you stick
an impedance between plate and B+ the generated signal is negative
feedback to the plate. That is precisely what the 'fixed' screen
voltage in a pentode prevents, plate voltage feedback, hence it's
higher plate impedance, gain, and distortion. Putting an unbypassed
screen resistor on G2 means you're applying 'a little' of the feedback
a triode would see, plus lowering screen voltage.

*It seems most g2's have a certain impedance,


Clearly it has impedance or else screen voltage would be 0 with any
resistance between G2 and B+.

EL84s you can start
with 1.5K and go to above 10K with the resistor have more effect on the
signal traces. *This also lowers distortion it appears,


Negative feedback.


I've always wondered how much good is the "negative feedback" that is
obtained in this very simple manner.

First of all, you ought to define what sort of NFB, and where there is
a series R to G2 then to me the NFB must be negative current FB
because the more Ia you get, the more screen current flows so the Vg2
- k signal voltage increases, so the screen resistor sort of works
like an unbypassed Rk, except that the screen current which generates
the Vg2-k signal voltage is fairly non linear.

Nobody has ever provided test results to show the resulting Ra if you
apply say 1Vrms to G1, and have say 5k0 primary load on an OPT and
then vary RG2 from zero to say 10k0 or say 1 x EL34 in SEP. Everyone
so far has found the use of large value RG2 to give worse performance
than can be gathered by other conventional well known circuit
arrangements.

Having unbypassed Rk gives negative current FB and Ra rises,
distortion reduces according to the change in closed loop gain. The
open loop pentode gain will always be highest where G2 is well
bypassed to cathode, but sometimes one sees a pentode deliberately set
up with well bypassed G2 to k, and with fairly high Rk, and such a
thing has very high effecxtive Ra and the Rout from the pentode anode
is mainly determined by the value of dc carrying anode RL and
following cap coupled RL in parallel.

Take an EL84 in pentode for a preamp. One might have Ia at 20mA, RL dc
= 20k0, cap coupled to a volume pot of 50k0. So RL total = 8.33k. Open
loop gain with G2 well bypassed to k is going to be about µ x RL /
( RL + Ra ) = roughly 400 x 8k33 / ( 8k33 + 70k0 ) = 42.4,
approximately. One might find that this could produce about 50Vrms
easily at anode, but THD spectra looks quite ****house, with
2,3,4,5,6,7,8,9H all present at all levels and too high to sound very
well, although tolerable at very low levels when gain is turned up.

For a line level preamp, open loop gain would better be at about say
4.2x, ie, about 1/10 of the open loop gain of the pentode. So, for say
4.15Vrms ouput into 8.3k anode load, we would need need just 0.1Vrms
applied between g1 and cathode, so to reduce closed loop gain to 4.2
the Grid input to 0V signal has to be 4.15 / 4.2 = 0.988Vrms, and the
current FB voltage at k must be 0.988V - 0.1V = 0.888V. The cathode
current = anode current + screen current, and we don't know what the
lesser screen current is, so lets just neglect if for now. Cathode
current might be about 4.15V / 8.33k = 0.5mA, so Rk MUST be
approximately 0.888V / 0.5mA = 1.776k, so let's say 1k8.

In the real world, using Rk = 1k8 with Ik dc at say about 25mA
( including both Ia dc and IG2 dc ) means Ek = +45Vdc, and biasing for
G1 still needs to be maybe -12Vdc, so G1 must be fed a bias voltage of
around +33Vdc, and of course B+ and Eg2 must be raised by +45V to
accomodate the high Ek. Its all very well me talking about doing
simple things in circuits, but always the biasing and idle conditions
must all be changed in response to anything that is changed with Rk.
Don't ever expect total perfect spoon feeding from me because I expect
anyone reading the posts here to have a pair of ears exuding steam
because of high power thinking about the Overall Picture, and Aim For
The Mission, and all curly awkward details.

The THD/IMD will be reduced around about -17dB, not just by a factor
of -20dB ie, by 1/10, which is the gain reduction factor we have aimed
for. I am simplifying the analysis a bit, and the only REAL WAY to
findoutabout is to get away from the PC and go solder up a circuit and
measure it all.

So, if THD at 50Vrms output with no NCFB was a typical 7%, then with
NCFB it would be about 0.08% at 50Vrms and at 0.5Vrms we might expect
0.008% because THD reduces about proportionately to Va. So, if you
have closed loop gain of 4.2 instead of 42.0, and you have a CD player
producing an average level of 0.2Vrms, not uncommon, then average Va =
0.84, and average THD might be about 0.01%, and regardless of the
volume control setting the THD will always be 0.01%, and I'd say most
ppl would find the sound to be OK.

And probably a huge amount better than using a weak feeble EF86 with
Ia at 1mA or less.

I say all this because anyone asking the original poster's question
needs to begin to ask 100 MORE QUESTIONS, and get out to his workshop
where he should become at least +20dB MORE ACTIVE with soldering iron,
voltmeters, THD measuring gear, and oscilloscope!

Only then will all answers be revealed about what likely outcomes will
be, and whether any outcomes are better or worse than doing it some
other way, and whether the criteria for citing "better way" is valid
or not.

So after several busy days I'd say any moderately intelligent person
should fill an exercise book up with carefully recorded results which
he/she can apply later when building any amp.

The conventional approach to using an EL84 as a preamp, ( just for
this crazy example ) is to strap it in triode with low value Rg2 grid
stopper of say 220 ohms between anode and G2.

Then you can still have total anode load of 8k3, and get OLG = 15
approx, and THD at 50Vrms might be 3%, with mainly 2H, and much lower
relative levels of other H compared to the "wild" natural untamed
pentode gain. NCFB could still be used to reduce CLG to the wanted
4.2, and gain reduction = 1/ 3.57, and THD at 50Vrms = approx 1.2%,
and at 0.5Vrms output its going to be around 0.012%, and still mainly
all 2H, and I suggest maybe you get better sound. I would suggest that
THD will rise if the anode to screen R is increased. One result with
increased R in series with G2 is much lower Eg2 relative to Ea. Most
pentodes "like" Eg2 slightly lower than Ea, maybe 20% lower for normal
class A operation especially in SE power stages or pentode signal
driver/preamp stages. Pentode IG2 is mostly a higher % than IG2 for
beam tetrodes, and for signal pentodes, Ig2 can be 30% of Ia. I can't
remember EL84 Ig2 figures, but if Ia was 20mA, expect Ig2 at 5mAdc if
Eg2 = Ea, and if series R to G2 was 10k0, then the drop across the R
G2 = 50Vdc, and one would find the EL84 would be a bit sick in triode
if the R between a and G2 = 10k0, so please try to use 220 ohms, my
humble request, so you might get better music. One could use a choke
from screen to a regulated B+ just for the screen and cap couple the
screen to anode OR TO THE CATHODE, and this will keep Eg2 high and at
the same level as Ea because a choke has fairly low wire resistance.
The one can instantly compare the difference between triode signal op
and pentode op. Chokes for such things need to be above 50H for low
Idc, and they don't grow on trees, and are a PIA to wind. So that's
another reason to use 220 ohms between a and G2 for triode, and for
pentode, you'll need a resistance of about 3 times the value of the
anode RL dc, and if that's 25k, then R supplying G2 from the same B+
of say +350Vdc might be 82k.
The bypass cap from G2 to k would be say 10uF, and one is free to add
in some extra series R between top of bypass cap and G2. Then you can
also try using G2 bypassed to 0V instead of to k, so that whille you
have NCFB applied in series with the VG1-0V signal, there is also a
Vg2-k signal voltage present. Don't ask me to explain further what
exact effects you might find, or what's the best.

I like shunt negative FB much more than any form of current FB,
because the tube Ra is lowered as well as the THD. The down side is
that Rin is lower, but the R1 arm of shunt FB can be kept at say 47k
and no lower, and R2 arm is typically 270k, thus not adding to the ac
loading on anode, and to findoutabout more, read my website on
preamps.

I've never used much shunt FB in power amp OP stages with pentodes,
because I favour cathode FB windings on the OPT.

but there is
always a point of diminished returns for each tube.


Sure, because the larger Rg2 the lower screen voltage, lower Ia, and
less power.

The 'better' way of doing it is UL. You still get less power but at
least screen voltage isn't also reduced so you only 'sacrifice' that
which goes directly to reducing distortion. (Allegedly there's a
'sweet spot' ratio where 'maximum benefit' of both is achieved).

It sure would be nice to be able to do a few math problem when looking
at datasheets and know where to start with a tube.


The rest is usually contained in the 'typical' operating
characteristics or example circuits.

I will introduce a controversial subject with g2 later, but for now I
just want to know what information on the datasheet will tell me what I
want to know.


I imagine that, with enough perseverance, one could derive screen
current magnitude, the resulting Rg2 voltage, and the degree of
feedback but it's not 'straight on the graphs' because they don't
expect them to be operated that way.

Terry


Something that might be interesting to try would be a sort of 'poor
man's UL' with a tube like the 6CW5 that can use 250V B+ but is screen
limited to 200V. There a 'significant size' unbypassed Rg2 of 10k
would put screen right at the spec'd 200 V.


Ah, the poor man's UL! aka Free Lunch UL. It doesn't work too well
because screen input resistance is fairly low, and where you have RG2
high, then VG2 becomes high, but screen FB applied this way is a non-
linear way of applying FB. The conventional UL OPT supplies the screen
input power, ie, Vg2 x Ig2 input from a relatively low source
resistance.
Efforts have been made to make a voltage divider between anode and 0V,
and then use a direct coupled cathode follower ( or a damn mosfet ) as
a low impedance driver of the screen. This sort of UL operation is
that of the un-poor man, or an Un-free Lunch, aka a bodge, or fudge,
because it just does something in a complex way that should and can be
so much simpler, ie, taps on anode windings. But the divider+CF buffer
drive to G2 can provide an excellent low impedance drive where it is
wanted, ie, to the screen, so it can be made to work OK but its more
to go wrong, so there ain't one commercially built amp with that
included.

But where you have say a damn lousy PP OPT with UL taps at only 20%,
and a lousy pri to sec ratio and where you want the RLa-a to be
higher, for the same sec speaker load, then the use of a circlotron
for the two OP tubes can be used, with EG2 being a fixed Vdc. The OPT B
+ tap is to 0V! The two B+ supplies aof say 350Vdc each are between
anode and primary, and cathodes are connected to UL taps. Fixed bias
might be used, and I leave you all to work out the possibilities and
outcomes with such an arrangement. Its FAR BETTER than any poor man's
UL, or the lunch that poor man might afford. In fact to avoid the
horrible poor man's free lunch, you are going to have to spend a bit
more on a different PT for circlotron operation.

I never go to restaurants, and always cook at home, as I know I get
better nutrition that way, and sure, its more expensive than fast food
from McMuckkers, and for amplifiers, I apply the same idea because
what I end up with for music consumption seems to work better than if
I shopped for amps.

My 3 dracma's worth.

Patrick Turner.


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flipper flipper is offline
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Default New member, pentode g2 operation question.

On Sun, 20 May 2012 04:28:58 -0700 (PDT), Patrick Turner
wrote:

On May 20, 2:05*pm, flipper wrote:
On Wed, 16 May 2012 23:18:38 +0000, TerryG

wrote:

Hi all,


I am new to this forum but have known several members here for quite
some time. *I know not many of you will recognise me from this handle,
but you will I know.


Anyway *I have been enjoying SEP amplifiers this last year an 1/2.
Don't know why they have the reputation they do compared to SET's, but
that is another issue.


Because, all else being equal (which it seldom is), a SE pentode will
have higher output impedance (worse damping factor) and more
distortion than a SET. The saving graces are it takes less input to
drive and produces more output power.

I have designed and built several pentode single ended amplifiers at
this point. *And the one point that still alludes me is: *How from what
the datasheets say can you figure out what resistor to start with on g2?


I understand the DC voltage value and that is not difficult to figure
out, but I like pentodes best when there is an unbypass resistor on g2.


I think you answered your own question. The common purposes of a G2
resistor are to bias, which you note is easy enough to determine from
the existing data, and to kill parasitic oscillations, which are not
'static' characteristics and, so, not something for a static graph.

An unpypassed G2, of the nature you're describing, is not 'typical'
operation.

I have found this does several things, it moves the top bias lines down,
and the bottom bias lines up at the same time raising ( or lowering
depending on how you look at it) the cut off bias voltage. *Usually by
putting a unbypassed g2 resistor on g2 your current drops compared to
bypassing, and a little more wattage is possible for the same operating
point.


G2 current will be less because you are, in effect, applying negative
feedback to the screen with an unbypassed G2 resistor. Plate current
will also be less because you are, in effect, applying negative
feedback to the screen with an unbypassed G2 resistor.

I don't know how you arrive at the 'more wattage' conclusion.

As a side note, the 'opposite polarity' Rg2 derived screen signal was
used for the, so called, "self split" (Class A) PP amplifiers. It
drives the second grid so you don't need 'another tube'.

Negative feedback is also why the SET has lower impedance, lower
distortion, less power and also why it takes more signal to drive it.
Ia of a triode depends on *BOTH* G1 and plate voltages so if you stick
an impedance between plate and B+ the generated signal is negative
feedback to the plate. That is precisely what the 'fixed' screen
voltage in a pentode prevents, plate voltage feedback, hence it's
higher plate impedance, gain, and distortion. Putting an unbypassed
screen resistor on G2 means you're applying 'a little' of the feedback
a triode would see, plus lowering screen voltage.

*It seems most g2's have a certain impedance,


Clearly it has impedance or else screen voltage would be 0 with any
resistance between G2 and B+.

EL84s you can start
with 1.5K and go to above 10K with the resistor have more effect on the
signal traces. *This also lowers distortion it appears,


Negative feedback.


I've always wondered how much good is the "negative feedback" that is
obtained in this very simple manner.


Then I guess it's about time for you to "get away from the PC and go
solder up a circuit and measure it all."

On the other hand, the OP's question was where he could find G2 values
in the datasheets and no amount of bench work will answer that.

snip puffery

but there is
always a point of diminished returns for each tube.


Sure, because the larger Rg2 the lower screen voltage, lower Ia, and
less power.

The 'better' way of doing it is UL. You still get less power but at
least screen voltage isn't also reduced so you only 'sacrifice' that
which goes directly to reducing distortion. (Allegedly there's a
'sweet spot' ratio where 'maximum benefit' of both is achieved).

It sure would be nice to be able to do a few math problem when looking
at datasheets and know where to start with a tube.


The rest is usually contained in the 'typical' operating
characteristics or example circuits.

I will introduce a controversial subject with g2 later, but for now I
just want to know what information on the datasheet will tell me what I
want to know.


I imagine that, with enough perseverance, one could derive screen
current magnitude, the resulting Rg2 voltage, and the degree of
feedback but it's not 'straight on the graphs' because they don't
expect them to be operated that way.

Terry


Something that might be interesting to try would be a sort of 'poor
man's UL' with a tube like the 6CW5 that can use 250V B+ but is screen
limited to 200V. There a 'significant size' unbypassed Rg2 of 10k
would put screen right at the spec'd 200 V.


Ah, the poor man's UL! aka Free Lunch UL.


No one said a blooming thing about a 'free lunch' and a "poor man's
insert anything" always refers to an inferior 'cheap' substitute for
the otherwise preferred solution.

It doesn't work too well
because screen input resistance is fairly low, and where you have RG2
high, then VG2 becomes high, but screen FB applied this way is a non-
linear way of applying FB.


That's the second time you made an arbitrary declaration of
"non-linear," after 'always wondering' about it, and with no
explanation of what the heck that means. Non-linear how? Like Ia is
non-linear?

I don't have any data one way or the other but it was apparently
considered 'linear enough', whatever that criteria might have been,
for self split PP amps. And while that's not entirely equivalent it's
a heck of a lot more substantive than you declaring it 'sort of works
like' Rk and then never again speaking of screen feedback in the
previous rambling.

The conventional UL OPT supplies the screen
input power, ie, Vg2 x Ig2 input from a relatively low source
resistance.


If you are externally *driving* G2 then you need a low source
impedance but a large value Rg2 is not 'driving' G2, the voltage
produced is the *result* of G2 current. G2 does the 'driving'.

That's akin to bitching that Rl on a triode is 'too high an impedance'
to 'drive the plate'.

snip


Patrick Turner.

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Roger Jones Roger Jones is offline
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Default New member, pentode g2 operation question.

On May 18, 5:26*pm, Roger Jones wrote:
On May 16, 7:18*pm, TerryG wrote:









Hi all,


I am new to this forum but have known several members here for quite
some time. *I know not many of you will recognise me from this handle,
but you will I know.


Anyway *I have been enjoying SEP amplifiers this last year an 1/2.
Don't know why they have the reputation they do compared to SET's, but
that is another issue.


I have designed and built several pentode single ended amplifiers at
this point. *And the one point that still alludes me is: *How from what
the datasheets say can you figure out what resistor to start with on g2?


I understand the DC voltage value and that is not difficult to figure
out, but I like pentodes best when there is an unbypass resistor on g2.


I have found this does several things, it moves the top bias lines down,
and the bottom bias lines up at the same time raising ( or lowering
depending on how you look at it) the cut off bias voltage. *Usually by
putting a unbypassed g2 resistor on g2 your current drops compared to
bypassing, and a little more wattage is possible for the same operating
point. *It seems most g2's have a certain impedance, EL84s you can start
with 1.5K and go to above 10K with the resistor have more effect on the
signal traces. *This also lowers distortion it appears, but there is
always a point of diminished returns for each tube.


It sure would be nice to be able to do a few math problem when looking
at datasheets and know where to start with a tube.


I will introduce a controversial subject with g2 later, but for now I
just want to know what information on the datasheet will tell me what I
want to know.


Terry


--
TerryG


Terry, I've played with quite a few Single-Ended Pentode (SEP) amps
over the years...in my student days it was for economic reasons! *I
still do them now in vintage radio restoration. I sometimes use a g2
resistor to ensure the g2 voltage does not exceed the plate voltage
(OPT primary resistance drops the latter a bit.) *I do the same in
pentode/beam tetrode P-P amps for the same reason (if they're not UL)
- just done it with an Eico HF12 (EL84's); actually I just moved the
g2 tap further down the B+ filter and adjusted the R values to get the
right voltages back. My reasoning is to reduce screen power and
heating. However, in both, I always decouple the g2 resistor (above B+
filter does it anyway.) *I would expect this to change the dynamic
operating point, tube impedance and max. power wrt a naked R...
obviously, it can't change the DC operation. But I've never studied
the effect... mea culpa! *I trust the tube modellers will help us.
Cheers,
Roger


Good replies... many thanks. I conclude that a lower g2 voltage (than
plate), fully decoupled, is advantageous. FWIW, in vintage radio
restoration (keeping same 6K6, 41, etc), I invariably add NFB from OPT
secondary to the cathode of the triode AF tube and mostly replace the
detector diode with a 1N34A point contact s/s diode. Audio gain goes
down, of course, but since I just listen to local AM radio on these
sets, no problem. Actually, it makes the range on the volume control
better... it's further "off the end".
Cheers,
Roger
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Patrick Turner Patrick Turner is offline
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Default New member, pentode g2 operation question.

snip,


It doesn't work too well
because screen input resistance is fairly low, and where you have RG2
high, then VG2 becomes high, but screen FB applied this way is a non-
linear way of applying FB.


That's the second time you made an arbitrary declaration of
"non-linear," after 'always wondering' about it, and with no
explanation of what the heck that means. Non-linear how? Like Ia is
non-linear?


I often type stuff which baffles many people, sometimes for a second
time, just as you mentioned, so to be sure about what you are doing,
get away from PC and be the the Real Scientist for yourself, in your
workshop, and thus findoutabout all things confusing involving more
than 4 interactive variables.



I don't have any data one way or the other but it was apparently
considered 'linear enough', whatever that criteria might have been,
for self split PP amps. And while that's not entirely equivalent it's
a heck of a lot more substantive than you declaring it 'sort of works
like' Rk and then never again speaking of screen feedback in the
previous rambling.


Anyway, I'm a bit confused by just what you mean now. what was "it"
that was apparently considered "linear enough"? please try to extend
your descriptions and definitions so all the rest of the dummies
reading your post will have some idea what you are saying.

The conventional UL OPT supplies the screen
input power, ie, Vg2 x Ig2 input from a relatively low source
resistance.


If you are externally *driving* G2 then you need a low source
impedance but a large value Rg2 is not 'driving' G2, the voltage
produced is the *result* of G2 current. G2 does the 'driving'.


Unlike G1, G2 has low input impedance. And current flow into isn't
marvellously linear with an applied linear voltage. The larger the
added series R between a voltage source and G2 becomes, the more the
G2 acts independantly and its voltage change is due to Ig2 x series,
and that isn't a very linear application of NFB. I'm saying :- don't
use large series R between any voltage source and G2 because it leads
to worse general operation of the tube in a number of ways. The screen
tap on the OPT at say 40% of P turns would have source impedance of
pentode Ra x 0.4 squared, because there is an Ra transformation here,
so Rout from the g2 tap for 6550 with Ra at 32k = 5.12k. However, as
soon as one connects G2 to the G2 tap at 40% on OPT then whatever
voltage change is applied to the G2 works to reduce Ra, perhaps to
about 3k5, so the G2 tap output resistance becomes 3k5 x 0.4 x 0.4 =
1.k4 approx, and in fact the UL connection lowers the circuit
impedance from any point to 0V. But the screen does have to be driven,
ie, supplied power, both current and voltage to do things to Ia,
whereas G1 does not have to be driven because its input power is
negligible because its input impedance is many megohms, and Iin is
tiny. But this changes with F because of Miller and stray C.

A series R does not drive anything, because its a passive element, but
where there is a series R between G2 tap and G2 on OPT, the anode
drives the OPT, and the tap applies voltage change and hence current
change to G2 and the series R tends to reduce the effectiveness of
what the G2 tap is there for. The ultimate screen NFB occurs when the
pentode or beam tube is triode connected. One never ever sees a high
value R between G2 and a for a triode connected tube. But you should
try it anyway, just to see why nobody uses high value R between a
voltage source and G2, without bypassing G2 to cathode.

That's akin to bitching that Rl on a triode is 'too high an

impedance'
to 'drive the plate'.


I doubt you.

Patrick Turner.
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Patrick Turner Patrick Turner is offline
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Posts: 3,964
Default New member, pentode g2 operation question.

On May 21, 11:02*pm, Roger Jones wrote:
On May 18, 5:26*pm, Roger Jones wrote:





On May 16, 7:18*pm, TerryG wrote:


Hi all,


I am new to this forum but have known several members here for quite
some time. *I know not many of you will recognise me from this handle,
but you will I know.


Anyway *I have been enjoying SEP amplifiers this last year an 1/2.
Don't know why they have the reputation they do compared to SET's, but
that is another issue.


I have designed and built several pentode single ended amplifiers at
this point. *And the one point that still alludes me is: *How from what
the datasheets say can you figure out what resistor to start with on g2?


I understand the DC voltage value and that is not difficult to figure
out, but I like pentodes best when there is an unbypass resistor on g2.


I have found this does several things, it moves the top bias lines down,
and the bottom bias lines up at the same time raising ( or lowering
depending on how you look at it) the cut off bias voltage. *Usually by
putting a unbypassed g2 resistor on g2 your current drops compared to
bypassing, and a little more wattage is possible for the same operating
point. *It seems most g2's have a certain impedance, EL84s you can start
with 1.5K and go to above 10K with the resistor have more effect on the
signal traces. *This also lowers distortion it appears, but there is
always a point of diminished returns for each tube.


It sure would be nice to be able to do a few math problem when looking
at datasheets and know where to start with a tube.


I will introduce a controversial subject with g2 later, but for now I
just want to know what information on the datasheet will tell me what I
want to know.


Terry


--
TerryG


Terry, I've played with quite a few Single-Ended Pentode (SEP) amps
over the years...in my student days it was for economic reasons! *I
still do them now in vintage radio restoration. I sometimes use a g2
resistor to ensure the g2 voltage does not exceed the plate voltage
(OPT primary resistance drops the latter a bit.) *I do the same in
pentode/beam tetrode P-P amps for the same reason (if they're not UL)
- just done it with an Eico HF12 (EL84's); actually I just moved the
g2 tap further down the B+ filter and adjusted the R values to get the
right voltages back. My reasoning is to reduce screen power and
heating. However, in both, I always decouple the g2 resistor (above B+
filter does it anyway.) *I would expect this to change the dynamic
operating point, tube impedance and max. power wrt a naked R...
obviously, it can't change the DC operation. But I've never studied
the effect... mea culpa! *I trust the tube modellers will help us.
Cheers,
Roger


Good replies... many thanks. *I conclude that a lower g2 voltage (than
plate), fully decoupled, is advantageous. FWIW, in vintage radio
restoration (keeping same 6K6, 41, etc), I invariably add NFB from OPT
secondary to the cathode of the triode AF tube and mostly replace the
detector diode with a 1N34A point contact s/s diode. *Audio gain goes
down, of course, but since I just listen to local AM radio on these
sets, no problem. *Actually, it makes the range on the volume control
better... it's further "off the end".


Yikes, WTF are you discussing. Please post a schematic somewhere so a
picture says a thousands words you can't seem to type.

As one reduces Eg2, the G2 biasing Vdc, then the set of Ra curves for
Ia vs Ea for given Eg1 values changes. Most ppl won't use AB2 and the
limit of operation and maximum PO is determined by the Ra curve for
Eg1 = 0.0V. This curve rises steeply at first from 0V, 0A, often on a
slope = 200 ohms, then with Ea at about 50V to 100V the line rolls
over to the right and assumes a slope of perhaps 38k, for an EL84. The
KNEE of the curve and the height of the flat part of the curve above
Ea = 100V appear lower on the graph as Eg2 is reduced.

For class AB and highest possible PO, Eg2 is kept as high as possible
to get maximal Ia swing into minimal RLa which is 1/4 x RLa-a for AB
operation. But for class A operation the RLa = 1/2 RLa-a and Ia change
for this higher RLa value is LESS than for class AB, so you **don't
need** to have Eg2 as close to Ea as possible. This is especially true
for SE pentodes.

Tube data from old books etc often show 3 sets of Ra curves for
differing values of fixed Eg2, and for an EL34 in SE mode, Ea might be
420V, but Eg2 at only 300V. This means Ig2 is much lower than if Eg2 =
Ea = +420V. One will find that the class A performance is no worse for
the lower Eg2, ie, Ra and THD is not significantly higher, see my
pages on SE35 to find out more.
http://www.turneraudio.com.au/se35cfb-monobloc.htm

I use CFB windings wherever I can for maximum linearity, and to get Ra
effectively lower than triode before adding a little GNFB.

The lower Eg2 means that Eg1 can be lower, so that for cathode biasing
there is less power wasted in Rk because Ek is low.

For high PP AB PO, Eg2 is high as possible, and this causes Ia dc to
increase, so Eg1 must be increased to counter the effect of high Eg2,
but of course this is OK because one wants a high Vg1 swing without
getting any grid current.

In guitar amps, you often see 6L6GC with Ea = +450V, and Eg2 a fixed
+440V, and grid stopper series R to G2 are often 470 ohms, 1 Watt.
Some Rg2 are special fusable resistors and they look like they have a
high Watt rating but actually don't. When they fuse, there are no
flames and smoke. Normal G2 power input at idle might be 4mA x 440V =
1.76 Watts, and so Pd in 470 ohms is 0.0075 Watt only, so the 470 ohms
could have a rating of 0.25 Watts, just a small metal film R. If the R
heat went to 1W, R would fail, and to cause this the Ig2 would be
46mA,
and I have seen such G2 resistors go open circuit often where an OP
tube has gone into thermal runaway, red hot anode with Ia dc = 300mA
perhaps, and hence Ig2 increases hugely.
Once Rg2 fuses open, Eg2 collapses to 0V, and Ia is cut off. I've used
low power rated Rg2 as an extra layer of protection against bias
failure. It isn't a good protection measure because after Rg2 fuses
open someone has to solder in a new Rg2, and the best form of
protection is via active monitoring of idle bias current and a circuit
which automatically turns the amp off when Ia dc exceeds 1.5 x idle
value for longer than 4 seconds. My website has details.

Patrick Turner.


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TerryG TerryG is offline
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Location: California USA
Posts: 6
Default

Quote:
Originally Posted by Patrick Turner View Post
I say all this because anyone asking the original poster's question
needs to begin to ask 100 MORE QUESTIONS, and get out to his workshop
where he should become at least +20dB MORE ACTIVE with soldering iron,
voltmeters, THD measuring gear, and oscilloscope!

Only then will all answers be revealed about what likely outcomes will
be, and whether any outcomes are better or worse than doing it some
other way, and whether the criteria for citing "better way" is valid
or not.

So after several busy days I'd say any moderately intelligent person
should fill an exercise book up with carefully recorded results which
he/she can apply later when building any amp.


My 3 dracma's worth.

Patrick Turner.
Well I guess it is time to fix my audio signal generator and provide actual circuit measurements here and as much more specific questions.

I would say at this point I have made at least a few hundred pentode circuits and performed tests on them. One thing I did do was what as suggested here, I put a 10K 12watt pot as a reostat on g2, with a 5K OPT load, with an EL84, and ran the pot up and down with another 1K 10 watt pot on the cathode.

What I found was:

1. As I increased the unbypassed resistance value on g2 this lessened the idle current of the circuit, and so had to compensate with Rk to a lower value to keep the idle current.

Obviously g2's unbypassed resistor effected the current, but the question was: Is this because of lower g2 voltage. So I added back voltage on g2 to bring it back up to the original value (without changing the resistor value), this did bring the current back up some, but not to its original value. The unbypassed g2 resistor was effecting the circuit in a static sense, to some degree.

2. As g2 was increased, Rk was reduced to maintain the same current, and voltage on g2 was increased but still within the tubes operational maximum wattage, output wattage on the dummy load resistor would increase. Because the sensitivity of the circuit and tube was effected by the unbypassed resistor more input voltage swing was required, but now the tube would obtain higher output wattages before clipping.

I will set up a circuit and do all these tests again, and measure distortion. My distortion meter only does THD, I do not have a spectrum analyzer to see what order/s are being increased or decreased by the unbypassed g2 resistor.

Also, tubes differ in the effects obtained by an unbypassed g2 resistor. Therefore I will perform measurements on a few different tubes.



Rather than start a new thread on this topic I will post here to this one as it is be relevant to this discussion here.

My findings so far are that the g2 resistor are both dynamic and static in nature, how and why is what I intend to figure out. I guess since this is "unintended" pentode operation, there are no formulas to calculate an unbypassed g2 resistors effect, from a datasheet.

Any suggestions on how to fairly set up my test circuit are welcome.

Terry
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TerryG TerryG is offline
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Location: California USA
Posts: 6
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This chart explains a few things. Not of g2 specifically, but of distortion behaviors of Pentodes. An unbypasses resistor on g2 could essentially be helping by lower reflected impedance to the plate, so a little is good, but too much isn't.

This could be why small value resistors (relatively) make things sound better and larger values do not.

Terry
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flipper flipper is offline
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Posts: 2,366
Default New member, pentode g2 operation question.

On Wed, 23 May 2012 21:29:06 +0000, TerryG
wrote:


Patrick Turner;956843 Wrote:

I say all this because anyone asking the original poster's question
needs to begin to ask 100 MORE QUESTIONS, and get out to his workshop
where he should become at least +20dB MORE ACTIVE with soldering iron,
voltmeters, THD measuring gear, and oscilloscope!

Only then will all answers be revealed about what likely outcomes will
be, and whether any outcomes are better or worse than doing it some
other way, and whether the criteria for citing "better way" is valid
or not.

So after several busy days I'd say any moderately intelligent person
should fill an exercise book up with carefully recorded results which
he/she can apply later when building any amp.


My 3 dracma's worth.

Patrick Turner.


Well I guess it is time to fix my audio signal generator and provide
actual circuit measurements here and as much more specific questions.

I would say at this point I have made at least a few hundred pentode
circuits and performed tests on them. One thing I did do was what as
suggested here, I put a 10K 12watt pot as a reostat on g2, with a 5K OPT
load, with an EL84, and ran the pot up and down with another 1K 10 watt
pot on the cathode.

What I found was:

1. As I increased the unbypassed resistance value on g2 this lessened
the idle current of the circuit, and so had to compensate with Rk to a
lower value to keep the idle current.

Obviously g2's unbypassed resistor effected the current, but the
question was: Is this because of lower g2 voltage.


The plate curves answer that question, yes. For any value of grid,
lower screen volts is lower Ia.

Btw, in that context the word is "affected."

So I added back
voltage on g2 to bring it back up to the original value (without
changing the resistor value), this did bring the current back up some,
but not to its original value. The unbypassed g2 resistor was effecting
the circuit in a static sense, to some degree.


That doesn't make sense to me. There is nothing in the plate curves
that specify 'at what resistance' screen volts goes through. Screen
volts are volts.

2. As g2 was increased, Rk was reduced to maintain the same current, and
voltage on g2 was increased but still within the tubes operational
maximum wattage, output wattage on the dummy load resistor would
increase. Because the sensitivity of the circuit and tube was effected
by the unbypassed resistor more input voltage swing was required, but
now the tube would obtain higher output wattages before clipping.


'Higher wattage' also doesn't make sense to me. Lets do a quickie
theoretical analysis. Power is I(rms)^2 times R, which can also be
represented for a sine as (Ipp/2.828)^2 times R. Current can go no
lower than 0, which pins down one end of the peak. Imax is then the
current available at 0 grid (since it will grid clamp above 0) but as
Ia increases so does screen current, which lowers screen voltage, and
at any grid value lower screen voltage means lower Ia. Max current,
with an unbypassed Rg2, must be lower than if screen remained static,
so Ipp is less, and, so, power must be lower.

That presumes everything is adjusted for maximum power like, for
example, idle bias at exactly half Ip, and I suspect what's happening
is the 'max power' load line changes when you change the bias
conditions so you're getting 'less than ideal' results.

I will set up a circuit and do all these tests again, and measure
distortion. My distortion meter only does THD, I do not have a spectrum
analyzer to see what order/s are being increased or decreased by the
unbypassed g2 resistor.

Also, tubes differ in the effects obtained by an unbypassed g2 resistor.


That would be because the screen mu factor is different.

Therefore I will perform measurements on a few different tubes.

[image:
http://sphotos.xx.fbcdn.net/hphotos-... 710908_n.jpg]

Rather than start a new thread on this topic I will post here to this
one as it is be relevant to this discussion here.

My findings so far are that the g2 resistor are both dynamic and static
in nature, how and why is what I intend to figure out. I guess since
this is "unintended" pentode operation, there are no formulas to
calculate an unbypassed g2 resistors effect, from a datasheet.

Any suggestions on how to fairly set up my test circuit are welcome.

Terry

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Patrick Turner Patrick Turner is offline
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Default New member, pentode g2 operation question.

On May 24, 11:19*am, TerryG wrote:
[image:http://sphotos.xx.fbcdn.net/hphotos-...5421323913...]

This chart explains a few things. *Not of g2 specifically, but of
distortion behaviors of Pentodes. *An unbypasses resistor on g2 could
essentially be helping by lower reflected impedance to the plate, so a
little is good, but too much isn't.

This could be why small value resistors (relatively) make things sound
better and larger values do not.

Terry


You have referred the group to a very interesting graph for 2H and 3H
production in a pentode or beam tetrode. But the graph does not
explain much about having various values of series R between G2 and B
+.

Notice how the 2H is high for low RLa values, falls to ZERO at RLa =
2k2, then rises for RLa 2k2.

How can this be?

Well, the exact mechanism is a little difficult to explain
mathematically or any other way, but when you plot the load line
across the tube curves, you will see Ea change in + and - directions
being equal for RLa = 2k2. What seems to happen is that as RLa is
increased to to 2k2, 2H reduces, and then above 2k2 the PHASE of the
2H reverses 180d, Just one of those lesser known things. In fact for
low Rla values, a triode's THD driving an SE pentode cancels that
produced in the pentode, but only up to RLa = 2k2. Above 2K2, the
triode's 2H adds to the 2H of the pentode. All rather messy, and
needing lots of NFB to fix.

There is more to all this THD business, and I suggest you read RDH4
where a comparison between 2A3 and 6F6 is made, and the 6F6 produces a
much larger number of H than just 2H and 3H.

But harder to see from the anode Ea + Ia Ra curves is the 3H which
occurs at all RLa, increasing with RLa. The 3H in fact usually looks
like flattening of + and - peaks in the sine wave, and is a form of
wave compression. But some tubes produce increased peaking of + and -
sine wave peaks, which simply means the 3H PHASE is 180d opposite to
the 3H flattening wave.

From the curves we see on the picture, we can say that if the RLa-a in
a PP circuit had a fairly LOW value, then 2H current in each tube is
high, and 3H current low, and that if the tubes work in class A then
the 2H can cancel out itself, and 3H won't, but it will remain low.
The result means that a pair of 6L6GC with RLa-a = 4k0 can produce
22Watts of class A PO with only 2% THD, almost all 3H, and that's the
best we might expect from a couple of beam tubes. Its about the same
as can be expected from triode connected tubes in class A.
But in SE mode, the beam tetrode / pentode THD is very much higher,
regardless of load. With triode connection, the NULL in 2H production
does not happen, and the phase the 2H remains constant, and although
2H is high with low RLa for triode, 2H becomes minimal and quite low
when RL becomes maximal, approaching a constant current source. The 3H
produced by the triode output tube, including triode connected
pentodes or beam tubes is relatively much lower.
And where an SE triode drives an SE output triode, the THD always
cancels, although there is a second order effect by way of
intermodulation so that some extra 3H is made in addition to the sum
of the 3H of each of the two triodes.

Fiddling with a series R to G2 probably won't improve anything, ie,
allow lower THD or increased power, or give a better damping factor. I
recall nothing in RDH4 or any other book I've read.

If there was to be any benefit in a large value Rg2, it would have
been well explained in text books used in old days.

Patrick Turner.


--
TerryG


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TerryG TerryG is offline
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Patrick,

Does not the unbypassed resistor on g2 effect plate impedance? If this were the case, then by altering g2 your altering the impedance relationship between the plate and load messing with where these distortion curves land on a plot like the one above for any tube.

If I use a small reostat on the secondary of the output transformer, I should be able to see how the distortion changes with load, B+, bias, and g2 values. Then find the ideal impedance for lowest distortion of any given pentode circuit.

At present the method I found to calculate the output transformer primary impedance seems lacking. Multiply the B+ by itself and divide by the max plate dissipation. So in the 6Y6 above the ideal plate impedance would be 1,656Ω

I can't say why anything would or would not make sense I just perform the experiments, we will know more once I get a test circuit set up and we can all examine the results. You know what they say about theory and practice.

Terry

P.S. BTW, I use a variable regulated supply on g2 so I can change its value on the fly. That is why I can increase the g2 resistor and then increase the voltage back on g2 as well by adjusting the supply voltage.

Last edited by TerryG : May 24th 12 at 07:15 PM


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John L Stewart John L Stewart is offline
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In a SEP amplifier an unbypassed R supplying G2 results in degeneration of the signal. Too much R & the power handling ability will be restricted.
If you think of it, the entire plate family is being modulated at the signal rate. Just like a freq conversion mixer, so lots of Dist products are possible.

In a PP amp an unbypassed G2 R supplies provides some balancing of the signal, similar to an unbypassed cathode R as used in a diff amp, but not as much. Works OK for Class A, but fails badly otherwise.

One of the primary reasons for using low value resistors on G1 & G2 is to prevent parasitic oscillation of the amplfier. In this case the R should be physically small & attached as close as possible to the socket. The R lowers the RF Q of the parasitic circuit. Needs to be physically small in order to avoid skin effect if possible. I've seen parasitics to 150 MHz in audio amps I've built.

Regulated G2 is nice & gets the most out of a power amp. But then you will need a stiff plate supply to take full advantage. Otherwise you will simply modulate the B+ at twice the audio rate.

As Patrick has pointed out, get the soldering iron going & make lots of observations. Apologies to others if this has already been covered. Too busy here with manual labour & the bicycle!

Cheers, John
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On Wed, 23 May 2012 02:13:59 -0700 (PDT), Patrick Turner
wrote:

snip,


It doesn't work too well
because screen input resistance is fairly low, and where you have RG2
high, then VG2 becomes high, but screen FB applied this way is a non-
linear way of applying FB.


That's the second time you made an arbitrary declaration of
"non-linear," after 'always wondering' about it, and with no
explanation of what the heck that means. Non-linear how? Like Ia is
non-linear?


I often type stuff which baffles many people, sometimes for a second
time, just as you mentioned,


That's because you often babble gibberish.

so to be sure about what you are doing,
get away from PC and be the the Real Scientist for yourself, in your
workshop, and thus findoutabout all things confusing involving more
than 4 interactive variables.


You're the one who "always wondered" so get to it.

I don't have any data one way or the other but it was apparently
considered 'linear enough', whatever that criteria might have been,
for self split PP amps. And while that's not entirely equivalent it's
a heck of a lot more substantive than you declaring it 'sort of works
like' Rk and then never again speaking of screen feedback in the
previous rambling.


Anyway, I'm a bit confused by just what you mean now. what was "it"
that was apparently considered "linear enough"? please try to extend
your descriptions and definitions so all the rest of the dummies
reading your post will have some idea what you are saying.


The "it" is what's been the topic of this thread: the voltage signal
generated by an unbypassed screen resistor.

Look up self split push pull amp.

According to RH4, using a screen resistor was done to get 'less
distortion' than using a divider off the plate signal.

The conventional UL OPT supplies the screen
input power, ie, Vg2 x Ig2 input from a relatively low source
resistance.


If you are externally *driving* G2 then you need a low source
impedance but a large value Rg2 is not 'driving' G2, the voltage
produced is the *result* of G2 current. G2 does the 'driving'.


Unlike G1, G2 has low input impedance. And current flow into isn't
marvellously linear with an applied linear voltage.


You miss the whole point in that one is not 'applying' an external
voltage *to* the screen. Screen current results in a voltage.

The larger the
added series R between a voltage source and G2 becomes, the more the
G2 acts independantly and its voltage change is due to Ig2 x series,


That is 'the point' of it.

and that isn't a very linear application of NFB.


That claim is obviously pulled whole cloth from thin air because the
first blooming thing you said about this subject was you "always
wondered" how good it was.

I'm saying :- don't
use large series R between any voltage source and G2 because it leads
to worse general operation of the tube in a number of ways.


I know what you've been 'saying' and it doesn't make sense to "always
wonder" and then claim to know what you have "always wondered" about.

The screen
tap on the OPT at say 40% of P turns would have source impedance of
pentode Ra x 0.4 squared, because there is an Ra transformation here,
so Rout from the g2 tap for 6550 with Ra at 32k = 5.12k. However, as
soon as one connects G2 to the G2 tap at 40% on OPT then whatever
voltage change is applied to the G2 works to reduce Ra, perhaps to
about 3k5, so the G2 tap output resistance becomes 3k5 x 0.4 x 0.4 =
1.k4 approx, and in fact the UL connection lowers the circuit
impedance from any point to 0V. But the screen does have to be driven,
ie, supplied power, both current and voltage to do things to Ia,


You just said a series screen resistor won't do anything to Ia because
it doesn't 'drive' G2. So much for the claim of 'non-linearity'.

Stop babbling.

whereas G1 does not have to be driven because its input power is
negligible because its input impedance is many megohms, and Iin is
tiny. But this changes with F because of Miller and stray C.


Nobody made a comparison to G1, except your nonsense.

A series R does not drive anything, because its a passive element,


You're the only one stuck on this 'driving' nonsense. Everyone else
has known from the get go it doesn't 'drive' G2 and, as I said, G2
current is what causes the voltage change.

but
where there is a series R between G2 tap and G2 on OPT, the anode
drives the OPT, and the tap applies voltage change and hence current
change to G2 and the series R tends to reduce the effectiveness of
what the G2 tap is there for. The ultimate screen NFB occurs when the
pentode or beam tube is triode connected. One never ever sees a high
value R between G2 and a for a triode connected tube. But you should
try it anyway, just to see why nobody uses high value R between a
voltage source and G2, without bypassing G2 to cathode.


Your the one who "always wondered" so get to it and find out.

That's akin to bitching that Rl on a triode is 'too high an

impedance'
to 'drive the plate'.


I doubt you.


A change in Ia causes a voltage across the load resistor and that
voltage on the plate is the cause of 'inherent triode feedback'.

The 'large' load resistor is not, however, 'driving the plate'. The
voltage, and resulting feedback, is the result of Ia through the load.

Whether that 'works' for an unbypassed screen resistor would likely
depend on how well it tracks plate current and, as I said, I have no
data one way or the other but at least I don't pretend to know what
you "always wondered."

Patrick Turner.

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On Thu, 24 May 2012 01:19:25 +0000, TerryG
wrote:


[image:
http://sphotos.xx.fbcdn.net/hphotos-... 009207_n.jpg]


This chart explains a few things. Not of g2 specifically, but of
distortion behaviors of Pentodes. An unbypasses resistor on g2 could
essentially be helping by lower reflected impedance to the plate, so a
little is good, but too much isn't.


An unbypassed screen resistor might lower plate impedance, and lord
knows what else, but it isn't going to do a blessed thing to OPT
impedance.


This could be why small value resistors (relatively) make things sound
better and larger values do not.

Terry

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On May 25, 4:13*am, TerryG wrote:
Patrick,

Does not the unbypassed resistor on g2 effect plate impedance?


Indeed it does. I have previously invited everyone to spend time
finding out just how.


When tube designers dreamed up a tetrode in around 1926 and later a
pentode ( Philips in 1929 I recall ), the idea was to STOP the
negative FB inside triodes. Triodes have low Ra relative to RL values
*because* of the the electrostatic effects of anode AND grid 1 action
on the electron space charge between grid and cathode. Professor Child
described all this stuff in 1930s. I suggest all who are unfamiliar
with just exactly what goes on in triodes at the very basic level
should proceed forthwith to a library archive building where Books Of
the Ancients are still being kept and have not been chucked out by
newer books full of crap we don't need or want to know.

The screen interrupts the electrostatic field *change* associated with
voltage changes at the anode from having much effect to the grid1 +
cathode area of the tube. In other words, the NFB loop is largely made
inneffective, and thus Ra goes very high, instead of hundreds of ohms,
it goes to many thousands of ohms, even megohms, and the tube becomes
a current source with finite Ra much higher than RL, a quite different
animal to a triode where Ra is lower then RL, which allows us to say a
triode is a voltage source.

The ambition of tube designers was to allow much higher frequencies to
be amplified without grid to anode capacitance ruining their efforts,
and NFB keeping the Ra low. Radio F amps operate best with parallel
tuned circuits when the tube is a current source, and anode C to 0V or
anything else is small as possible, so the pentode allowed much
improved efficiency in RF amps of all kinds, including millions of old
radios where there is usually one 6U7 IF amplifier, or a similar tube.
In just about all such RF apps, the G2 has no series R and is well
bypassed to the cathode with adequate C, so 0.1uF at 455kHz gives
bypass reactance of 3.5 ohms. RF cathodes are usually grounded, with
g1 negative bias developed via an AVC circuit. At audio F, say 1kHz,
0.1uF bypass cap would be inadequate to bypass screen to cathode,
because 0.1uF has XC = 1k6, and variable screen G2 current would allow
G2 voltage to change, and affect the precious gain most designers like
to be as high as possible. Many tubes have a series screen feed R of
say 1k0 to perhaps 1M0, depending on the type of tube and function,
and one wants the tube to operate similarly from below F1 to and above
F2 of the intended BW. So usually bypass cap values are high enough
for the intended band. Where a high series R for G2 is used, and not
bypassed, then a high VG2 is generated, and associated Vg2 and altered
value of Ra and changed THD character. I suggest readers go to their
workshops to explore, measure, and understand the phenomena.

In audio amps with resistance loads, the use of tubes, mosfets, or
bipolar transistors all with very high Ra, Rd, Rc relative to loads
always leads to very high THD if no NFB is allowed to be present, and
usually devices have been favoured if they merely have a high Gm,
regardless of how high Ra is, and not much worrying about THD because
the NFB is always configured to take advantage of the high reductionj
of open loop gain to closed loop gain thus reducing THD from say 10%
to less than 0.01%, regardless of whatever spectral harmonics are
present, or what their phase might be, relative to the fundememental
F.

So as far as I know, nobody has done a very well detailed article for
all to read on the effects - good or bad -of using various umbypassed
series feed resistors to G2 in pentodes, and I suspect theory predicts
a "useless outcome" for a give amp, ie, the map will have less power,
worse DF, and higher THD for a given amount of applied NFB and various
load values. To get away from doubt, I suggest someone cabable *do the
research* because I sure don't have that time to waste. I'm uncertain
about the issue, and would be first to admit I don't know all that
could be known, but my mission plan is to design and build excellent
sounding and excellent measuring amplifiers - of any kind - and I
doubt I would ever need or want to feed a screen via a large value of
series R without feeding it from a low impedance source, or from a
source whichj becomes low impedance because of the feed point, as is
the case with UL taps.

If this
were the case, then by altering g2 your altering the impedance
relationship between the plate and load messing with where these
distortion curves land on a plot like the one above for any tube.


One cannot predict what a set of curves might look like before
building and testing the circuit to get to the REAL TRUTH, when one
fiddles around with one of many variables.

If I use a small reostat on the secondary of the output transformer, I
should be able to see how the distortion changes with load, B+, bias,
and g2 values. Then find the ideal impedance for lowest distortion of
any given pentode circuit.


Good luck. What you might find is that to keep Eg2 as high as it
should be, the pot value stapped across an OPT primary must be
reasonably low to prevent the Vdc drop to G2. Then you'll have the Va
having to supply power to the pot. Say the pot needs to be 5k0, and
you have one SE power pentode able to produce 9Watts into 4k, typical
for a lone EL34, ie, Va = 189Vrms then the 5k0 pot to get a screen
voltage would mean you'd have to make an extra 7W, all of which is
wasted.

Using a 50k means 0.7Watts is "wasted" but the Vdc drop is too much.
The VG2 generated by Ig2 ac will be horrid, and probably not do any
good, well, it only what I suspect until I'm proven wrong. Then one
must remember that if there ever was any benefit in your idea, you'd
think someone@somewhere over the last 70 years would have patented it,
made use of it, and generated income from it. All the good possible
circuit tricks invented for use around around pentodes and beam
tetrodes have already been invented, and anyone able to configure
something novel and new is unlikely to succeed. Everyone wants to be
unique, to stroke their own ego, but its generally impossible. Some
manage to be unique, and invent much, but its usually with emerging
new technology, as was done by Steve Jobs, and others. But just among
us tube farnarklers, its really difficult to be original, just as it
is difficult to create a new genre in art or music.


At present the method I found to calculate the output transformer
primary impedance seems lacking. *Multiply the B+ by itself and divide
by the max plate dissipation. *So in the 6Y6 above the ideal plate
impedance would be 1,656Ω


?????????????????????????????

I can't say why anything would or would not make sense I just perform
the experiments, we will know more once I get a test circuit set up and
we can all examine the results. *You know what they say about theory and
practice.


Well nothing wrong in trying to be the investigative scientist. But
scientist you have to be to convince anyone of what you've found, so
you need the discipline of the dedicated, the self doubt you must
disprove, to get at the truth and nothing but, and that means some
very hard distance to cover, long nights, and carefully worded web
page with many circuit details ans re-producable graphs of THD etc.

I fill up a 128 page exercise book each fortnight with notes and
drawings of what I do in my workshop to repair old amps and speakers
and while building new stuff. During my learning period of several
years after 1993, I filled a stack of exercise books 4 feet high. I
distilled the contents and have gradually built a website from it all,
so all that paper could then be re-cycled.
I have a stack of critical saved paper pages about a foot high
awaiting distilation and publishing online, but nothing about
unbypassed Rg2.

Terry

P.S. BTW, I use a variable regulated supply on g2 so I can change its
value on the fly. *That is why I can increase the g2 resistor and then
increase the voltage back on g2 as well by adjusting the supply voltage.


Fair enough, but why?

Patrick Turner.

--
TerryG


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Patrick Turner Patrick Turner is offline
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On May 25, 3:44*pm, flipper wrote:
On Wed, 23 May 2012 02:13:59 -0700 (PDT), Patrick Turner





wrote:
snip,


It doesn't work too well
because screen input resistance is fairly low, and where you have RG2
high, then VG2 becomes high, but screen FB applied this way is a non-
linear way of applying FB.


That's the second time you made an arbitrary declaration of
"non-linear," after 'always wondering' about it, and with no
explanation of what the heck that means. Non-linear how? Like Ia is
non-linear?


I often type stuff which baffles many people, sometimes for a second
time, just as you mentioned,


That's because you often babble gibberish.

so to be sure about what you are doing,
get away from PC and be the the Real Scientist for yourself, in your
workshop, and thus findoutabout all things confusing involving more
than 4 interactive variables.


You're the one who "always wondered" so get to it.

I don't have any data one way or the other but it was apparently
considered 'linear enough', whatever that criteria might have been,
for self split PP amps. And while that's not entirely equivalent it's
a heck of a lot more substantive than you declaring it 'sort of works
like' Rk and then never again speaking of screen feedback in the
previous rambling.


Anyway, I'm a bit confused by just what you mean now. what was "it"
that was apparently considered "linear enough"? please try to extend
your descriptions and definitions so all the rest of the dummies
reading your post will have some idea what you are saying.


The "it" is what's been the topic of this thread: the voltage signal
generated by an unbypassed screen resistor.

Look up self split push pull amp.

According to RH4, using a screen resistor was done to get 'less
distortion' than using a divider off the plate signal.

The conventional UL OPT supplies the screen
input power, ie, Vg2 x Ig2 input from a relatively low source
resistance.


If you are externally *driving* G2 then you need a low source
impedance but a large value Rg2 is not 'driving' G2, the voltage
produced is the *result* of G2 current. G2 does the 'driving'.


Unlike G1, G2 has low input impedance. And current flow into isn't
marvellously linear with an applied linear voltage.


You miss the whole point in that one is not 'applying' an external
voltage *to* the screen. Screen current results in a voltage.

The larger the
added series R between a voltage source and G2 becomes, the more the
G2 acts independantly and its voltage change is due to Ig2 x series,


That is 'the point' of it.

and that isn't a very linear application of NFB.


That claim is obviously pulled whole cloth from thin air because the
first blooming thing you said about this subject was you "always
wondered" how good it was.

I'm saying :- don't
use large series R between any voltage source and G2 because it leads
to worse general operation of the tube in a number of ways.


I know what you've been 'saying' and it doesn't make sense to "always
wonder" and then claim to know what you have "always wondered" about.

The screen
tap on the OPT at say 40% of P turns would have source impedance of
pentode Ra x 0.4 squared, because there is an Ra transformation here,
so Rout from the g2 tap for 6550 with Ra at 32k = 5.12k. However, as
soon as one connects G2 to the G2 tap at 40% on OPT then whatever
voltage change is applied to the G2 works to reduce Ra, perhaps to
about 3k5, so the G2 tap output resistance becomes 3k5 x 0.4 x 0.4 =
1.k4 approx, and in fact the UL connection lowers the circuit
impedance from any point to 0V. But the screen does have to be driven,
ie, supplied power, both current and voltage to do things to Ia,


You just said a series screen resistor won't do anything to Ia because
it doesn't 'drive' G2. So much for the claim of 'non-linearity'.

Stop babbling.

whereas G1 does not have to be driven because its input power is
negligible because its input impedance is many megohms, and Iin is
tiny. But this changes with F because of Miller and stray C.


Nobody made a comparison to G1, except your nonsense.

A series R does not drive anything, because its a passive element,


You're the only one stuck on this 'driving' nonsense. Everyone else
has known from the get go it doesn't 'drive' G2 and, as I said, G2
current is what causes the voltage change.

but
where there is a series R between G2 tap and G2 on OPT, the anode
drives the OPT, and the tap applies voltage change and hence current
change to G2 and the series R tends to reduce the effectiveness of
what the G2 tap is there for. The ultimate screen NFB occurs when the
pentode or beam tube is triode connected. One never ever sees a high
value R between G2 and a for a triode connected tube. But you should
try it anyway, just to see why nobody uses high value R between a
voltage source and G2, without bypassing G2 to cathode.


Your the one who "always wondered" so get to it and find out.

* That's akin to bitching that Rl on a triode is 'too high an
impedance'
to 'drive the plate'.


I doubt you.


A change in Ia causes a voltage across the load resistor and that
voltage on the plate is the cause of 'inherent triode feedback'.

The 'large' load resistor is not, however, 'driving the plate'. The
voltage, and resulting feedback, is the result of Ia through the load.

Whether that 'works' for an unbypassed screen resistor would likely
depend on how well it tracks plate current and, as I said, I have no
data one way or the other but at least I don't pretend to know what
you "always wondered."


As usual, Flipper cannot offer any advice on how to build a better
amp.

Patrick Turner.


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On May 25, 11:26*am, John L Stewart
wrote:
In a SEP amplifier an unbypassed R supplying G2 results in degeneration
of the signal. Too much R & the power handling ability will be
restricted.
If you think of it, the entire plate family is being modulated at the
signal rate. Just like a freq conversion mixer, so lots of Dist products
are possible.

In a PP amp an unbypassed G2 R supplies provides some balancing of the
signal, similar to an unbypassed cathode R as used in a diff amp, but
not as much. Works OK for Class A, but fails badly otherwise.

One of the primary reasons for using low value resistors on G1 & G2 is
to prevent parasitic oscillation of the amplfier. In this case the R
should be physically small & attached as close as possible to the
socket. The R lowers the RF Q of the parasitic circuit. Needs to be
physically small in order to avoid skin effect if possible. I've seen
parasitics to 150 MHz in audio amps I've built.

Regulated G2 is nice & gets the most out of a power amp. But then you
will need a stiff plate supply to take full advantage. Otherwise you
will simply modulate the B+ at twice the audio rate.

As Patrick has pointed out, get the soldering iron going & make lots of
observations. Apologies to others if this has already been covered. Too
busy here with manual labour & the bicycle!


During the last 12 months I rode 11,000 km. Speed average at 23kph.
Nobody over 55yo has overtaken me in last 3 years, except a few blokes
who ride in Canberra's Pedal Power group. There's plenty of blokes
fitter than me. But I continue with activities to keep me sane, and
out of hospital and out of jail, if not quite off the streets. But I
see a life as a stack of cards, and as one grows older, the stack
becomes taller, and ever so much more likely to tumble down. Life is
but a temporay therapy, with a final side effect. Doctors just delay
it.

Ya can't beat working out in a shed with adequate gear to teach
yourself, while doubting yourself, but then proving your doubts wrong.
When ya return from shed back to life after learning, one should be
better equipped to make better amps, otherwise, why go to the shed in
the first place? Why not paint a room, repair a floor, repair a toilet
cistern, or go for a bike ride instead? I like having reasons to
exist.

G2 is a control grid like G1, but with much lower gm. G2 input
resistance is low, and not linear.

I have models at my website for pentodes, in terms of current
generators for G2 and G1, with Ra as a shunt R, but what I have just
realised is that my models for the G2 current generator don't include
the input resistance to G2 in the models, however, in the models
explained, the outcome for a given pentode or beam tetrode can be
predicted better than otherwise methods all much more vague, more
approximate and unclear. Even the best of brains in 1955 didn't quite
create the best of equivalent models for tubes as can be devised, to
allow the examination and prediction of what happens when you have a
high value R in series wuth G2.

In such modelling, voltage gain and Effective Ra can be calculated
fairly well without worrying about THD because one always works with
small signal levels when THD is always going to be low because the
tubes is considered while working in class A, and the macro
transition between class A and AB, or AB2 etc is not considered. In
practice, PP OP tubes are usually always working in pure class A even
though they are biased at low Ia with high Ea to allow maybe 10 times
the possible pure class A PO in class AB, or AB2.

I have never ever seen any commercially successful tube amp design
which has high value R between B+ and G2, or between G2 and UL taps on
OPTs. The old Mullard notes about using EL84 and EL34 in UP PP amps
for class AB1 do include mention of using maybe 1k0 series R to G2
from UL taps and I recall they say THD is lower, but it ain't much
lower.

One would think everyone would have used a couple of cheap resistors
to feed G2 and leave it fed without any C bypassing if it led to
better amp performance, because resistors are cheaper to implement
than a tranny tap, and leaving out 1 bypass cap means cheaper and the
CEO can more likely afford the Cadillac.
But we see the vast majority of PP amps made using the UL method, and
with many having no R between G2 and OPT taps.

The only way to avoid the *possible* horrid effects of high R between
G2 and either some fixed Wg2 supply OR some Vac source is to feed G2
by a direct coupled cathode follower, which means its anode must be
say 300V above the B+ for OP tubes, and then the its grid can be fed
from a high value pot strapped across an OP primary to vary the UL
tapping, without using the tap. One could also use a mosfet. By the
time one finishes the experiments one might conclude plain old UL
works fine, and that the doubt one had about learning anything useful
before using a CF like this was well founded, and no use to persue the
alternative and more costly/complex way to skin the poor little cat.

Patrick Turner.





Cheers, John

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  #22   Report Post  
TerryG TerryG is offline
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Location: California USA
Posts: 6
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Quote:
Originally Posted by flipper View Post
On Thu, 24 May 2012 01:19:25 +0000, TerryG
wrote:


[image:
http://sphotos.xx.fbcdn.net/hphotos-... 009207_n.jpg]


This chart explains a few things. Not of g2 specifically, but of
distortion behaviors of Pentodes. An unbypasses resistor on g2 could
essentially be helping by lower reflected impedance to the plate, so a
little is good, but too much isn't.


An unbypassed screen resistor might lower plate impedance, and lord
knows what else, but it isn't going to do a blessed thing to OPT
impedance.


This could be why small value resistors (relatively) make things sound
better and larger values do not.

Terry

Why restate the obvious, can not anyone understand what I was inferring?

My point was that there is a relationship between plate impedance, OPT impedance, and distortion type and amount. Therefore by effecting a change in plate impedance with an unbypassed G2 resistor, we are essentially controlling where this relationship lands on the chart I provided for any tube.

Therefore aside from the assumption that G2 produces feedback (linear or not), it also can lower or increase distortions depending on the relationship the tubes plate impedance has with the OPT load impedance according to the chart for the 6Y6. I have a feeling every tube has a similar plot like the one in the 6Y6 chart, I would really like to make my own charts of current production pentode power tubes.

You know this thread is typical of what happens often on the internet in technical forums. There are egos and issolated facts presented in each post. A lot of good information and I want to thank everyone that has contributed so far, I have gotten some information from almost every post. But there isn't much here that seems to be presented in the spirit of genuine interest in helping explain what is happening with a pentode with an unbypassed resistor on G2. Personally I feel several things are happening all at the same time, stopper, affecting of plate impedance, perhaps feedback, and a change in the total Gm and mu of the circuit

We will see how this progresses further.

Terry
  #23   Report Post  
Posted to rec.audio.tubes
flipper flipper is offline
external usenet poster
 
Posts: 2,366
Default New member, pentode g2 operation question.

On Sat, 26 May 2012 04:15:25 -0700 (PDT), Patrick Turner
wrote:

On May 25, 3:44*pm, flipper wrote:
On Wed, 23 May 2012 02:13:59 -0700 (PDT), Patrick Turner





wrote:
snip,


It doesn't work too well
because screen input resistance is fairly low, and where you have RG2
high, then VG2 becomes high, but screen FB applied this way is a non-
linear way of applying FB.


That's the second time you made an arbitrary declaration of
"non-linear," after 'always wondering' about it, and with no
explanation of what the heck that means. Non-linear how? Like Ia is
non-linear?


I often type stuff which baffles many people, sometimes for a second
time, just as you mentioned,


That's because you often babble gibberish.

so to be sure about what you are doing,
get away from PC and be the the Real Scientist for yourself, in your
workshop, and thus findoutabout all things confusing involving more
than 4 interactive variables.


You're the one who "always wondered" so get to it.

I don't have any data one way or the other but it was apparently
considered 'linear enough', whatever that criteria might have been,
for self split PP amps. And while that's not entirely equivalent it's
a heck of a lot more substantive than you declaring it 'sort of works
like' Rk and then never again speaking of screen feedback in the
previous rambling.


Anyway, I'm a bit confused by just what you mean now. what was "it"
that was apparently considered "linear enough"? please try to extend
your descriptions and definitions so all the rest of the dummies
reading your post will have some idea what you are saying.


The "it" is what's been the topic of this thread: the voltage signal
generated by an unbypassed screen resistor.

Look up self split push pull amp.

According to RH4, using a screen resistor was done to get 'less
distortion' than using a divider off the plate signal.

The conventional UL OPT supplies the screen
input power, ie, Vg2 x Ig2 input from a relatively low source
resistance.


If you are externally *driving* G2 then you need a low source
impedance but a large value Rg2 is not 'driving' G2, the voltage
produced is the *result* of G2 current. G2 does the 'driving'.


Unlike G1, G2 has low input impedance. And current flow into isn't
marvellously linear with an applied linear voltage.


You miss the whole point in that one is not 'applying' an external
voltage *to* the screen. Screen current results in a voltage.

The larger the
added series R between a voltage source and G2 becomes, the more the
G2 acts independantly and its voltage change is due to Ig2 x series,


That is 'the point' of it.

and that isn't a very linear application of NFB.


That claim is obviously pulled whole cloth from thin air because the
first blooming thing you said about this subject was you "always
wondered" how good it was.

I'm saying :- don't
use large series R between any voltage source and G2 because it leads
to worse general operation of the tube in a number of ways.


I know what you've been 'saying' and it doesn't make sense to "always
wonder" and then claim to know what you have "always wondered" about.

The screen
tap on the OPT at say 40% of P turns would have source impedance of
pentode Ra x 0.4 squared, because there is an Ra transformation here,
so Rout from the g2 tap for 6550 with Ra at 32k = 5.12k. However, as
soon as one connects G2 to the G2 tap at 40% on OPT then whatever
voltage change is applied to the G2 works to reduce Ra, perhaps to
about 3k5, so the G2 tap output resistance becomes 3k5 x 0.4 x 0.4 =
1.k4 approx, and in fact the UL connection lowers the circuit
impedance from any point to 0V. But the screen does have to be driven,
ie, supplied power, both current and voltage to do things to Ia,


You just said a series screen resistor won't do anything to Ia because
it doesn't 'drive' G2. So much for the claim of 'non-linearity'.

Stop babbling.

whereas G1 does not have to be driven because its input power is
negligible because its input impedance is many megohms, and Iin is
tiny. But this changes with F because of Miller and stray C.


Nobody made a comparison to G1, except your nonsense.

A series R does not drive anything, because its a passive element,


You're the only one stuck on this 'driving' nonsense. Everyone else
has known from the get go it doesn't 'drive' G2 and, as I said, G2
current is what causes the voltage change.

but
where there is a series R between G2 tap and G2 on OPT, the anode
drives the OPT, and the tap applies voltage change and hence current
change to G2 and the series R tends to reduce the effectiveness of
what the G2 tap is there for. The ultimate screen NFB occurs when the
pentode or beam tube is triode connected. One never ever sees a high
value R between G2 and a for a triode connected tube. But you should
try it anyway, just to see why nobody uses high value R between a
voltage source and G2, without bypassing G2 to cathode.


Your the one who "always wondered" so get to it and find out.

* That's akin to bitching that Rl on a triode is 'too high an
impedance'
to 'drive the plate'.


I doubt you.


A change in Ia causes a voltage across the load resistor and that
voltage on the plate is the cause of 'inherent triode feedback'.

The 'large' load resistor is not, however, 'driving the plate'. The
voltage, and resulting feedback, is the result of Ia through the load.

Whether that 'works' for an unbypassed screen resistor would likely
depend on how well it tracks plate current and, as I said, I have no
data one way or the other but at least I don't pretend to know what
you "always wondered."


As usual, Flipper cannot offer any advice on how to build a better
amp.


As usual, Patrick fabricates a strawman to beat up.

Besides the OP never asking how to "build a better amp" I told him how
anyway; UL. To which you then bitched about the use of "better" even
though you argued the exact same damn thing.

Btw, when are you going to stop 'always wondering' and do some
measurements like the OP did?

Patrick Turner.

  #24   Report Post  
Posted to rec.audio.tubes
Patrick Turner Patrick Turner is offline
external usenet poster
 
Posts: 3,964
Default New member, pentode g2 operation question.


As usual, Flipper cannot offer any advice on how to build a better
amp.


As usual, Patrick fabricates a strawman to beat up.


As usual, Flipper cannot offer any advice on how to build a better
amp.

I've concluded that discussions with Flipper lead nowhere, and even a
single line reply like mine is pointless.

Besides the OP never asking how to "build a better amp" I told him how
anyway; UL. To which you then bitched about the use of "better" even
though you argued the exact same damn thing.


The OP seems puzzled by the little he is seeing so far with pentodes.
There is a lot more to see and understand, and of course the OP is
only ONE of many many ppl who read the group each day hoping to better
understand things. Last time I looked, I saw it was a free world, and
I can say what I like to say without kissing Flippers's ****ty arse.
Flipper is the agrieved bitch here, not me.

Btw, when are you going to stop 'always wondering' and do some
measurements like the OP did?


I've already done enough IMHO. I will NEVER act as a slave to solve
some other persons query if I see he could do it as well as I could if
he / she applied themselves to the task.

Folks should recall when I doubted Flipper's understanding of a Brooks
10C biasing circuit, and Flipper could never prove to me he had any
real idea how the Brook circuit worked, and then Flipper spent a huge
long time and thousands of words to zero effect to say that the Brook
Patent said it right, and that I had no grounds for doubt, and that
because a patent said something, it must be right. Flipper convinced
everyone he was a big fool, because he never could explain the Brook
circuit right, and ended up saying he was right because he said so.

But arguments based on "I'm right because I say so" is UTTER COMPLETE
BULL****, and Flipper hates me for saying this. Flipper is a
psychopathic hater of many things, a insular man with tunnel vision
and zero sense of humour, and without enough courage to use his own
name here on the Internet.

Flipper commanded me to build a Brooks circuit and measure it, but it
wasn't me who needed to know how it worked, it was the OP and Flipper,
and these ppl would not lift a finger to answer their own questions by
using their own resources, so, they don't get my respect, or anyobe
else's..

So, the working of the Brook 10C remains unclear to all who may have
read the posts many months ago. Nobody built a sample amp with a
Brooks bias adjuster. Just one 6SN7 involved, along with any generic
triode output stage needed. I don't care, and do not have to, and I do
have my own preferred methods for tube biasing spelt out at my website
in many amps I have supplied to ppl. I do not need to use the method
Brook patented so many years ago. The Brook biasing circuit has
shortcomings which make its employment a mere gimick. It is my opinion
that Brook patented an idea about which they could then make claims
which might convince more PPL to buy more of them. Its a sample of the
american way of buiness, be seen to invent something, use lawyers to
word up a patent, stop other people using the idea, and actually keep
the real truth and working of the patent concealed by means of
preserving its confusing nature. Well, such marketting ploys just
don't work with Anti Bull**** Warriors like me. Nice try Mr Brooks,
but ya haven't got me thinking you had much to crow about, and Mr
Brooks, youse can still confuse ppl, all these years later, and Mr
Brooks, the ****s who are confused are so stubborn and lazy they won't
lift a finger to understand whatcha really meant all those years ago.

So, if I could hand out a prize for being a sneaky curmudgeon, I'd
award it to Mr Brooks, he really knew the art of bull****ting.

Flipper has not tendered a method of biasing which works any better.

I've concluded that trying to discuss anything with Flipper is
unrewarding, inconclusive, oppressive, unpleasant, uninformative, and
if Flippper asked me the time of day, I'd not give it to him, and
direct him to find his own ****ing clock.

Meanwhile, there is more than has come to my mind about using G2 as a
controlling grid of some sort.

One can always opt for driving an OP to produce anode power tube by
using G2 as the signal input instead of G1. If G1 is biased at say 0V
dc, then Eg2 can lower than normal, maybe +150V instead of the usual
+300V, and by operating the G2 with +/- 150V pk, one achieves a
current change across a range of say +0.2Amps to 0.0amps. One will
find this form of triode working will have an Ra about the same as the
original tube used as a pentode.
Gain is very low, because the rules have to be followed, ie, µ = gm x
Ra, and if g2 gm = say 0.8ma/V, and Ra = 32k, ( for a 6550 with Ia =
55mA, Ea = 400V, ) then µ = 0.0008 x 32,000 = 25.6, and if RL = 4k,
gain A = µ x RL / ( RL + Ra ) = 25.6 x 4 / ( 32 + 4 ) = 2.8, very low,
and still not very linear. So how could NFB be used? One could use a
shunt NFB with two R between an input voltage and a speaker output
voltage and the junction of the 2 would have 1/2 the THD at the
speaker but no fundemental voltage, so you get distortion reduction
without gain reduction due to FB application. I've now said enough
about and leave others to investigate further.

But the exact properties of the screen should be well understood no
matter how one uses the screen to do anything, including having it
dangling at the end of an unbypassed supply resistance.
The properties can be observed at very low signal levels of a few Vac
without having to worry much about distortion, and to measure the
current flow into screen, a sensitive current meter is needed, or by
more simply reading a Vac across say 100 ohms in series wuth G2. 1Vac
across 100 ohms = 10mA ac, easy to read. If there was say some other
larger unbypassed series R of say 2k2 in series with g2, and there was
say 10Vac at G2, then Rin to G2 = 10V / 10mA = 1k0. If one applies a
small signal to g2, say 10Vac, from some external source, with at
least 100 ohms used as a current sensor, does Rg2 in stay constant? I
can think of many things to measure, but its important to measure
without loading the anode circuit, and allowing the screen voltage
changing circuit to work without beig changed. Hence test gear must
have input impedance much higher than circuit impedances.

Just my 3.1428 dracmas.

Patrick Turner.

  #25   Report Post  
John L Stewart John L Stewart is offline
Senior Member
 
Location: Toronto
Posts: 301
Smile

[quote=Patrick Turner;957412]On May 25, 11:26*am, John L Stewart
wrote:
In a SEP amplifier an unbypassed R supplying G2 results in degeneration
of the signal. Too much R & the power handling ability will be
restricted.
If you think of it, the entire plate family is being modulated at the
signal rate. Just like a freq conversion mixer, so lots of Dist products
are possible.

In a PP amp an unbypassed G2 R supplies provides some balancing of the
signal, similar to an unbypassed cathode R as used in a diff amp, but
not as much. Works OK for Class A, but fails badly otherwise.

One of the primary reasons for using low value resistors on G1 & G2 is
to prevent parasitic oscillation of the amplfier. In this case the R
should be physically small & attached as close as possible to the
socket. The R lowers the RF Q of the parasitic circuit. Needs to be
physically small in order to avoid skin effect if possible. I've seen
parasitics to 150 MHz in audio amps I've built.

Regulated G2 is nice & gets the most out of a power amp. But then you
will need a stiff plate supply to take full advantage. Otherwise you
will simply modulate the B+ at twice the audio rate.

As Patrick has pointed out, get the soldering iron going & make lots of
observations. Apologies to others if this has already been covered. Too
busy here with manual labour & the bicycle!


During the last 12 months I rode 11,000 km. Speed average at 23kph.
Nobody over 55yo has overtaken me in last 3 years, except a few blokes
who ride in Canberra's Pedal Power group. There's plenty of blokes
fitter than me. But I continue with activities to keep me sane, and
out of hospital and out of jail, if not quite off the streets. But I
see a life as a stack of cards, and as one grows older, the stack
becomes taller, and ever so much more likely to tumble down. Life is
but a temporay therapy, with a final side effect. Doctors just delay
it.

Ya can't beat working out in a shed with adequate gear to teach
yourself, while doubting yourself, but then proving your doubts wrong.
When ya return from shed back to life after learning, one should be
better equipped to make better amps, otherwise, why go to the shed in
the first place? Why not paint a room, repair a floor, repair a toilet
cistern, or go for a bike ride instead? I like having reasons to
exist.

G2 is a control grid like G1, but with much lower gm. G2 input
resistance is low, and not linear.

I have models at my website for pentodes, in terms of current
generators for G2 and G1, with Ra as a shunt R, but what I have just
realised is that my models for the G2 current generator don't include
the input resistance to G2 in the models, however, in the models
explained, the outcome for a given pentode or beam tetrode can be
predicted better than otherwise methods all much more vague, more
approximate and unclear. Even the best of brains in 1955 didn't quite
create the best of equivalent models for tubes as can be devised, to
allow the examination and prediction of what happens when you have a
high value R in series wuth G2.

In such modelling, voltage gain and Effective Ra can be calculated
fairly well without worrying about THD because one always works with
small signal levels when THD is always going to be low because the
tubes is considered while working in class A, and the macro
transition between class A and AB, or AB2 etc is not considered. In
practice, PP OP tubes are usually always working in pure class A even
though they are biased at low Ia with high Ea to allow maybe 10 times
the possible pure class A PO in class AB, or AB2.

I have never ever seen any commercially successful tube amp design
which has high value R between B+ and G2, or between G2 and UL taps on
OPTs. The old Mullard notes about using EL84 and EL34 in UP PP amps
for class AB1 do include mention of using maybe 1k0 series R to G2
from UL taps and I recall they say THD is lower, but it ain't much
lower.

One would think everyone would have used a couple of cheap resistors
to feed G2 and leave it fed without any C bypassing if it led to
better amp performance, because resistors are cheaper to implement
than a tranny tap, and leaving out 1 bypass cap means cheaper and the
CEO can more likely afford the Cadillac.
But we see the vast majority of PP amps made using the UL method, and
with many having no R between G2 and OPT taps.

The only way to avoid the *possible* horrid effects of high R between
G2 and either some fixed Wg2 supply OR some Vac source is to feed G2
by a direct coupled cathode follower, which means its anode must be
say 300V above the B+ for OP tubes, and then the its grid can be fed
from a high value pot strapped across an OP primary to vary the UL
tapping, without using the tap. One could also use a mosfet. By the
time one finishes the experiments one might conclude plain old UL
works fine, and that the doubt one had about learning anything useful
before using a CF like this was well founded, and no use to persue the
alternative and more costly/complex way to skin the poor little cat.

Patrick Turner.



[color=blue][i]

Cheers, John
------------------------------------------------------------
I managed just 5200 km on the bike for the year ending Apr 30/12, not bad for an old fart in his 80th year. A lot of that during the Winter which is fairly real here. That & many km on the XCSkis.

Didn't do much with toobz over the past year but the manual labor here on more than 2 hectares here keeps me buzy. Lots of fire wood to cut & split.

Dug out the access to pump the septic tank a couple of weeks ago. Its under the brick patio. So got the bricks are back in now so its on to the next thing. One of the tractors needs some fixing. Living in the country for more than 40 years brings lots of physical labour.

Still beats sitting inside with the computor!

Cheers to all, John
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  #26   Report Post  
Posted to rec.audio.tubes
flipper flipper is offline
external usenet poster
 
Posts: 2,366
Default New member, pentode g2 operation question.

On Sun, 27 May 2012 04:10:04 +0000, TerryG
wrote:


flipper;957307 Wrote:
On Thu, 24 May 2012 01:19:25 +0000, TerryG
wrote:
-

[image:
http://tinyurl.com/bnnoyv5


This chart explains a few things. Not of g2 specifically, but of
distortion behaviors of Pentodes. An unbypasses resistor on g2 could
essentially be helping by lower reflected impedance to the plate, so a
little is good, but too much isn't.-


An unbypassed screen resistor might lower plate impedance, and lord
knows what else, but it isn't going to do a blessed thing to OPT
impedance.

-
This could be why small value resistors (relatively) make things sound
better and larger values do not.

Terry-



Why restate the obvious, can not anyone understand what I was inferring?


I get *now* what you were 'inferring', or what you're 'now' inferring,
but can only go by what you said and, in that post, you were talking
about affecting "reflected impedance to the plate," which it can't.

My point was that there is a relationship between plate impedance, OPT
impedance, and distortion type and amount. Therefore by effecting a
change in plate impedance with an unbypassed G2 resistor, we are
essentially controlling where this relationship lands on the chart I
provided for any tube.


Possibly but I think this perhaps explains what may be contradictory
results rather than being necessarily 'practical' because speaker load
is anything but constant.

What I'm suggesting is there may be more than one thing interacting in
the results. For example, let us imagine that, despite having nothing
to support the opinion, Patrick is correct and Rg2 feedback is
'non-linear' (g1 is also 'non-linear' or else we'd never have
distortion to begin with), which he then apparently presumes would
result in 'more' distortion rather than less. It's possible that at
some serendipitous point the reduction in distortion from a 'magic'
load line intersection might swamp that increase, but maybe only at
that one point under specific conditions. Now, if that were the case
you'd likely be better off to simply change the load, for the supposed
'lower distortion' intersect, without also adding the Rg2 distortion.
Except, of course, if it's only at 'one point', with the rest worse
off, it might not be of any 'real world' benefit since, as mentioned,
a speaker is anything but a constant load.

Just between you and me let's get one thing clear, since nothing is
ever clear with Patrick, I never said his opinion was 'wrong'. What I
said was he has little or nothing to back it up and that it's absurd
to open with "I always wondered" and then, in the next breath, start
making declarations of alleged 'fact' about what he "always wondered."

Therefore aside from the assumption that G2 produces feedback (linear or
not), it also can lower or increase distortions depending on the
relationship the tubes plate impedance has with the OPT load impedance
according to the chart for the 6Y6. I have a feeling every tube has a
similar plot like the one in the 6Y6 chart, I would really like to make
my own charts of current production pentode power tubes.


Given the parameters of the 'experiment' in which you readjust so
screen volts and idle current are returned to 'normal', lower plate
impedance would be the result of negative feedback so it isn't an
"aside" thing.

Every pentode will have a 'similar' set of curves because it's
characteristic of the 'pentode shape', so to speak, and, in
particular, the plate knee.

If you look at the vertical on the plate curves you see the plate
lines 'compress' the further down from 0 grid you go. That is 2nd
harmonic distortion. Now, if you look to the left at the knee you see
the upper end becomes 'compressed', relative to the vertical case. At
some 'magic point' knee compression will match the bottom end
compression and, so, we have '0' 2nd harmonic distortion, since the
peak excursions match, but that means increased higher order
distortion because the plate lines now go from 'compressed' to
(partially) expanded and back to 'compressed'. It 'balloons in the
middle', so to speak.

Go even further down the knee with your load line, past the 'magic
point', and we get 2nd harmonic distortion again but the phase is now
reversed because the 'top' is compressed more than the 'bottom',
rather than the other way around in the vertical case, and the higher
order harmonics keep increasing because we've retained the 'balloon
middle'.


You know this thread is typical of what happens often on the internet in
technical forums. There are egos and issolated facts presented in each
post. A lot of good information and I want to thank everyone that has
contributed so far, I have gotten some information from almost every
post. But there isn't much here that seems to be presented in the
spirit of genuine interest in helping explain what is happening with a
pentode with an unbypassed resistor on G2. Personally I feel several
things are happening all at the same time, stopper, affecting of plate
impedance, perhaps feedback, and a change in the total Gm and mu of the
circuit

We will see how this progresses further.

Terry


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  #27   Report Post  
Posted to rec.audio.tubes
flipper flipper is offline
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Posts: 2,366
Default New member, pentode g2 operation question.

On Sun, 27 May 2012 00:34:36 -0700 (PDT), Patrick Turner
wrote:


As usual, Flipper cannot offer any advice on how to build a better
amp.


As usual, Patrick fabricates a strawman to beat up.


As usual, Flipper cannot offer any advice on how to build a better
amp.


Better than what, strawman?

I've concluded that discussions with Flipper lead nowhere, and even a
single line reply like mine is pointless.


They're pointless because they're B.S.


Besides the OP never asking how to "build a better amp" I told him how
anyway; UL. To which you then bitched about the use of "better" even
though you argued the exact same damn thing.


The OP seems puzzled by the little he is seeing so far with pentodes.


He's got a notebook full of measurements on the circuit in question
and, since anything divided by 0 is infinity, that's infinitely more
than you've provided.

There is a lot more to see and understand, and of course the OP is
only ONE of many many ppl who read the group each day hoping to better
understand things. Last time I looked, I saw it was a free world,


Maybe you should consult a few Iranians on that 'free world' nonsense
and there's, no doubt, more than a few Chinese who could offer some
enlightenment.

and
I can say what I like to say without kissing Flippers's ****ty arse.
Flipper is the agrieved bitch here, not me.

Btw, when are you going to stop 'always wondering' and do some
measurements like the OP did?


I've already done enough IMHO. I will NEVER act as a slave to solve
some other persons query if I see he could do it as well as I could if
he / she applied themselves to the task.


That's funny as hell since it's exactly what you 'demand' everyone
else do.


Folks should recall when I doubted Flipper's understanding of a Brooks
10C biasing circuit, and Flipper could never prove to me he had any
real idea how the Brook circuit worked, and then Flipper spent a huge
long time and thousands of words to zero effect to say that the Brook
Patent said it right, and that I had no grounds for doubt, and that
because a patent said something, it must be right. Flipper convinced
everyone he was a big fool, because he never could explain the Brook
circuit right, and ended up saying he was right because he said so.


The inventor explained how it worked, the patent explained how it
worked, and, after doing numerous simulations of both solid state and
tube implementations, I explained to you how it worked. That you are
unable to grasp it from the multiple sources and detailed information
provided is your problem, not mine.

But arguments based on "I'm right because I say so" is UTTER COMPLETE
BULL****,


Which is why nothing more than 'Patrick said' is "UTTER COMPLETE
BULL****."

and Flipper hates me for saying this. Flipper is a
psychopathic hater of many things,


And that comes from a person who has decided to type a book volume
explaining why he 'hates' me.

a insular man with tunnel vision
and zero sense of humour, and without enough courage to use his own
name here on the Internet.


Not at all. I often find you quite funny.

Flipper commanded me to build a Brooks circuit and measure it, but it
wasn't me who needed to know how it worked, it was the OP and Flipper,
and these ppl would not lift a finger to answer their own questions by
using their own resources, so, they don't get my respect, or anyobe
else's..


I already know how it works and all I said to you was that if you
couldn't grasp it from the provided materials then build one to find
out. And that is because *you* keep claiming that is 'the only way to
know' so if that is what it takes for *you* to figure it out then so
be it.

Have you figured out how a triode plate detector works yet?

So, the working of the Brook 10C remains unclear to all who may have
read the posts many months ago.


Oh really? Post your poll results on how just how 'dumb' you claim
"all who may have read the posts" are.

Nobody built a sample amp with a
Brooks bias adjuster. Just one 6SN7 involved, along with any generic
triode output stage needed. I don't care, and do not have to, and I do
have my own preferred methods for tube biasing spelt out at my website
in many amps I have supplied to ppl. I do not need to use the method
Brook patented so many years ago. The Brook biasing circuit has
shortcomings which make its employment a mere gimick.


Since you've demonstrated you do not understand how it works any
comments about "shortcomings" are unfounded.

It is my opinion
that Brook patented an idea about which they could then make claims
which might convince more PPL to buy more of them. Its a sample of the
american way of buiness, be seen to invent something, use lawyers to
word up a patent, stop other people using the idea, and actually keep
the real truth and working of the patent concealed by means of
preserving its confusing nature.


I have no problem understanding how it works from what is apparently
to you 'confusing' so, despite your hated of Brook, 'Americans',
business, patents, and lawyers, it would appear the problem is with
you.

Well, such marketting ploys just
don't work with Anti Bull**** Warriors like me. Nice try Mr Brooks,
but ya haven't got me thinking you had much to crow about, and Mr
Brooks, youse can still confuse ppl, all these years later, and Mr
Brooks, the ****s who are confused are so stubborn and lazy they won't
lift a finger to understand whatcha really meant all those years ago.


Let's see, the person who just claimed it was 'confusing' to him has
that to say about himself?

So, if I could hand out a prize for being a sneaky curmudgeon, I'd
award it to Mr Brooks, he really knew the art of bull****ting.

Flipper has not tendered a method of biasing which works any better.


Better than what? The circuit you don't understand?

Actually I have but since you never listen to anything it's not worth
going into again.

I've concluded that trying to discuss anything with Flipper is
unrewarding, inconclusive, oppressive, unpleasant, uninformative, and
if Flippper asked me the time of day, I'd not give it to him, and
direct him to find his own ****ing clock.


What a pal.

Meanwhile, there is more than has come to my mind about using G2 as a
controlling grid of some sort.

One can always opt for driving an OP to produce anode power tube by
using G2 as the signal input instead of G1. If G1 is biased at say 0V
dc, then Eg2 can lower than normal, maybe +150V instead of the usual
+300V, and by operating the G2 with +/- 150V pk, one achieves a
current change across a range of say +0.2Amps to 0.0amps. One will
find this form of triode working will have an Ra about the same as the
original tube used as a pentode.
Gain is very low, because the rules have to be followed, ie, µ = gm x
Ra, and if g2 gm = say 0.8ma/V, and Ra = 32k, ( for a 6550 with Ia =
55mA, Ea = 400V, ) then µ = 0.0008 x 32,000 = 25.6, and if RL = 4k,
gain A = µ x RL / ( RL + Ra ) = 25.6 x 4 / ( 32 + 4 ) = 2.8, very low,
and still not very linear. So how could NFB be used? One could use a
shunt NFB with two R between an input voltage and a speaker output
voltage and the junction of the 2 would have 1/2 the THD at the
speaker but no fundemental voltage, so you get distortion reduction
without gain reduction due to FB application. I've now said enough
about and leave others to investigate further.

But the exact properties of the screen should be well understood no
matter how one uses the screen to do anything, including having it
dangling at the end of an unbypassed supply resistance.
The properties can be observed at very low signal levels of a few Vac
without having to worry much about distortion, and to measure the
current flow into screen, a sensitive current meter is needed, or by
more simply reading a Vac across say 100 ohms in series wuth G2. 1Vac
across 100 ohms = 10mA ac, easy to read. If there was say some other
larger unbypassed series R of say 2k2 in series with g2, and there was
say 10Vac at G2, then Rin to G2 = 10V / 10mA = 1k0. If one applies a
small signal to g2, say 10Vac, from some external source, with at
least 100 ohms used as a current sensor, does Rg2 in stay constant? I
can think of many things to measure, but its important to measure
without loading the anode circuit, and allowing the screen voltage
changing circuit to work without beig changed. Hence test gear must
have input impedance much higher than circuit impedances.

Just my 3.1428 dracmas.

Patrick Turner.

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Patrick Turner Patrick Turner is offline
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On May 27, 11:40*pm, John L Stewart
wrote:
Patrick Turner;957412 Wrote:

snip,

------------------------------------------------------------
I managed just 5200 km on the bike for the year ending Apr 30/12, not
bad for an old fart in his 80th year.


I'll probably will have been cremated well before I turn 80, so
congratulations, 100km a week is not too bad at all for anyone, let
alone anyone 80.

A lot of that during the Winter
which is fairly real here. That & many km on the XCSkis.


Didn't do much with toobz over the past year but the manual labor here
on more than 2 hectares here keeps me buzy. Lots of fire wood to cut &
split.


Dug out the access to pump the septic tank a couple of weeks ago. Its
under the brick patio. So got the bricks are back in now so its on to
the next thing. One of the tractors needs some fixing. Living in the
country for more than 40 years brings lots of physical labour.


I just live in a house on 1/4 acre. I finally caved in about using
hand operated gear and bought a Husqvarna petrol powered hedge cutter
and a chain saw, prices here were on special, and garden can now be
controlled easier, in far less time, and no more sore ankle, knees,
back and hands after spending a day doing real work. And I need all
the time I have to get things done to maintain everything, and be
cook, bottle washer, and sweeper-upperer plus CEO of a tiny little
activity called turneraudio.com. I'm still making new pages. But I
ain't workin as much as I used to, and no good if I do because it
jepodizes me getting the old age pension beginning August 1. Sooo, I'm
just planning to stay reasonably fit, remain a source of useful info,
and do as little as possible for ppl wanting commercial stuff. The
trouble with being a craftsman and fixer-upperer of all manner of tube
audio gear is the lousy pay. The average wage for a tradesman here is
$80 an hour. A typical fridge repair guy charges $80 call out fee, and
$20 for each 15 minutes. I might take 10 hours to fix a lotta things
on some old POS amp, and do you think anyone is happy to pay $800? No
way, and they expect maybe $200. So, **** 'em all now, I can afford to
say no, or charge so high they run a mile, or just tell 'em I'm busy
for 10 years. Its amazing when I have argued with customers over
issues of social wage justice, and insisted they tell me what they
think I am worth per hour. Its like lecturing a brick wall, they just
are so afraid to give an answer, for they fear they'll be up for more
money than they'd like to pay, but meanwhile, they get paid far more
than average weekly earnings here, some $66,000 per anum, which is
about $830 a week after the Govt or company pays their tax for them.
They'd say they pay tax, but its taken out of pay before they get it,
so they never have to actually pay it.
If a wife wants a new kitchen, sure, here's $15 grand honey, whack it
on the credit card, but if they want their Shanling amp fixed, well,
they expect it done for $100, but it might take me all day to find out
why it was repaired twice before, and why there had been a small fire
on the PCB board. So I end up saying take it away to one of the many
other ppl who can fix it cheaply. Of course there is nobody. So it
ends up cluttering my shed, another terrible example of how not to
build a tube amp by a dopey Chinese company. I had a Shanling CD
player here to fix a few years, my tech said it was the worst bit of
junk he'd ever worked on.

Still beats sitting inside with the computor!


Remember when Apple made a PC which had transparent sides and you
could look in to see everything working? Well, if ya had one of those
PCs now, you could sit inside and look out at the world whiring around
being silly most days. Much better than sitting inside an early IBM
from 1955. You'd be **** scared to move an inch. Might get ozoned out
by all those mechanical relays, go deaf as well, die of heat
exhaustion.

But I had a friend who used to like working in the old telephone
exchanges. He'd be on night duty, and when things were quiet, he'd
have a joint, and when some ******* phoned up someone else at 4 am, he
found it fascinating to hear the relays going off around the building,
like the parts of the machine were talking to each other, and, of
course, they were.

The town sewer system was eventually run past my parents house in
Sydney in about 1977. We had septic tanks, and my father and me as
child helper had to clean out the fukken thing on a couple of
occasions. before 1965. At that time, everyone ate food cooked in
animal fats, and much fat was tipped down the sink, and it's gradually
build up faster than it'd degrade, boy, wotta mess. Plus all the other
unmentionables. Kids today would never have to help their old man up
to his calves in ****. I used to swim in the creek which would get
nicely full after rain, but it also was pretty grubby. But there were
eels and frogs in that creek, and mosquitoes with 2 foot wing spans,
so it really was healthy if all those critters could survive. Its not
much changed despite a tripling of human population. Humans tip all
sorts of crap down stormwater drains, washings from paint, concrete
truck deliveries, paint thinners, engine oil, lots of detergents and
cleaning chemicals, swimming pool dregs, whatever, yet frogs and
mozzies and eels remain.

My anti-environment footprint has become -20dB lighter than it was, no
more wasting fats during cooking, no more fats, butter, margerine etc
period. Hardly any chemicals go down the drain where it then goes to a
creek, and on to a local lake here in Canberra. The sewer treatment
works here for a city of 300k produces clear water at the output, but
salt and chemicals like hormones in drugs and many queer things end up
going downstream in the local big river, Murrumbidgee, on a long
journey to the sea. But upstream from us is a town of 30k which under
different jurisdiction and their bloody sewer plant is always faulty
with overflows and so our lake gets bad algae often, and as population
grows its getting worse and there are months when its dangerous to
swim, and rowers are even banned. Our Lake Burley Griffin is an
artificial lake formed with a damn across the tributory Molongolo
river than flows into 'bidgee.
Whenever someone plonks such an artifical pool on an inland river in
the part of the country where average rain is 20 inches a year, and
then they add 10 times the population envisaged 100 years ago when
they thought a lake would be nice, expect problems with water quality.
Lots of talk among experts about what to do about the water, and I
never heard so much hot air.
https://www.google.com.au/search?q=m...w=1440&bih=697

If greenhouse warming continues after humans have put all the buried
carbon back up into the atmosphere, then sea levels will rise about
300 feet - take a awhile, but the fossil record shows sea levels have
been up and down hundreds of feet like a yo-yo over millions of years.
Rise is only 1.5mm each year now, hardly anything to worry about, but
over 10,000 years its +49 feet. Anyway, large areas of Oz will
dissappear under sea water because the Murray river has a vast low
level flood plain. By the time sea rises go up 30 feet, maybe we will
have cures for all the trouble we are causing the environment. We
surely would know by then whether we caused the ice to melt and sea to
rise, I imagine the Missisippi flood plain will also flood out wide.
Is mankind destined to learn +50dB more about everything, but to do
-12dB less about fixing problems? If kids today don't know where their
**** goes after they flush the toilet, and don't know where the food
comes from, or who made their clothes and shoes, then I expect
problems to all be ignored, so, don't worry, be happy. And I won't be
here.


Cheers to all, John


+-------------------------------------------------------------------+
|Filename: IMG_0034 Wood Splitting NOV 11A.jpg * * * * * * * * * * *|
|Download:http://www.audiobanter.com/attachment.php?attachmentid=286|


Got your hands full eh.
I have a gum tree that has to come down soon, probably cost me plenty
to get down. Its too big to just fell it at 3 am like I felled an old
willow that got too big and started to die. Its a bit too tall to
reach up with a pole saw. Maybe there's 2+ tonnes of hardwood in it. I
could saw it to 1.5 foot lengths, but have to split it for the fire
place. Just do an hour a day at this **** work, body will take that if
I spread out the effort. I don't plan to use a hammer splitter or axe,
or sledge hammer and steel wedges, been there done that with my father
50 years ago, and am unable to now, so I guess I have to use a
hydraulic splitter, so maybe I have to hire one. Depends what quotes I
get for getting the tree down. The blokes doing this can be real
cowboys, and charge like wounded bulls.

Patrick Turner.

John L Stewart-

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GRe GRe is offline
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Posts: 67
Default New member, pentode g2 operation question.


"Patrick Turner" wrote:

[...]

...and be cook, bottle washer, and sweeper-upperer plus CEO of a tiny
little activity called turneraudio.com


CEO? Cathode Eager Oddball?

:-) Gio.


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Patrick Turner Patrick Turner is offline
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Posts: 3,964
Default New member, pentode g2 operation question.

On May 30, 9:15*pm, "GRe" wrote:
"Patrick Turner" wrote:

[...]

...and be cook, bottle washer, and sweeper-upperer plus CEO of a tiny
little activity called turneraudio.com


CEO? Cathode Eager Oddball?


Well, certainly that's a good title, and possibly actually true. But
I'm getting less eager, and more fundementalist as I age. Typical OB.
If I had any religion, I probably don't go as far as the Amish, who
dissallow electricity, and must ride round in buggy pulled by horse,
although I believe state laws insist the buggies have lights fitted,
red ones for stop lights, green for left, orange for right, and a big
brown light for when the horse has **** itself, so I think maybe they
have compromised to allow batteries, and one might hope they'd maybe
adopt to green technologies, they wear overalls, like living in
agrarian rural communities, almost hippy.

But I don't like using silicon to spoil music, especially old music
which was better than most that followed, and I do like bicycles, and
I chuckled with agreement when the Indonesians banned Lady Ga-Ga from
performing in Jakata, where 100 million members of the religious right
down there threatened to each throw a large brick at Parliment if that
G-Ga Devil was allowed to insult human dignity. Transfering most of
the bricks in Indo to a large pile on top of parliment with
politicians buried underneath would bring the nation to its knees.
Lady GG is so prone to offend so many in order to afford an obscenely
rich greedy lifestyle. There are aspects of modernity which are quite
appalling, repellant, fuggly, etc, and although I couldn't be a Muslim
any more than I could be Amish its good to see both being severe about
crap in the world.

If anyone did see me riding around aimlessly on a Sunday on a bike,
they could be forgiven for thinking I was lost, and that I was having
trouble locating the Cherch of Cathode Followers.

But last time I attended the cherch, there were all these old gizas
there, and two old ladies, both wearing blue jumpers, and I figured
neither knew what a bj was.

Patrick Turner.


:-) Gio.




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flipper flipper is offline
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Posts: 2,366
Default New member, pentode g2 operation question.

On Wed, 30 May 2012 17:07:50 -0700 (PDT), Patrick Turner
wrote:

On May 30, 9:15*pm, "GRe" wrote:
"Patrick Turner" wrote:

[...]

...and be cook, bottle washer, and sweeper-upperer plus CEO of a tiny
little activity called turneraudio.com


CEO? Cathode Eager Oddball?


Well, certainly that's a good title, and possibly actually true. But
I'm getting less eager, and more fundementalist as I age. Typical OB.
If I had any religion, I probably don't go as far as the Amish, who
dissallow electricity, and must ride round in buggy pulled by horse,
although I believe state laws insist the buggies have lights fitted,
red ones for stop lights, green for left, orange for right, and a big
brown light for when the horse has **** itself, so I think maybe they
have compromised to allow batteries, and one might hope they'd maybe
adopt to green technologies, they wear overalls, like living in
agrarian rural communities, almost hippy.

But I don't like using silicon to spoil music, especially old music
which was better than most that followed, and I do like bicycles, and
I chuckled with agreement when the Indonesians banned Lady Ga-Ga from
performing in Jakata, where 100 million members of the religious right
down there threatened to each throw a large brick at Parliment if that
G-Ga Devil was allowed to insult human dignity. Transfering most of
the bricks in Indo to a large pile on top of parliment with
politicians buried underneath would bring the nation to its knees.
Lady GG is so prone to offend so many in order to afford an obscenely
rich greedy lifestyle. There are aspects of modernity which are quite
appalling, repellant, fuggly, etc, and although I couldn't be a Muslim
any more than I could be Amish its good to see both being severe about
crap in the world.


Just what the world needs, another self-righteous fascist.

And how typical of you to equate the pacifist Amish with terrorist
thugs.

If anyone did see me riding around aimlessly on a Sunday on a bike,
they could be forgiven for thinking I was lost, and that I was having
trouble locating the Cherch of Cathode Followers.

But last time I attended the cherch, there were all these old gizas
there, and two old ladies, both wearing blue jumpers, and I figured
neither knew what a bj was.

Patrick Turner.


:-) Gio.

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Patrick Turner Patrick Turner is offline
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Flipper farted these turds of crap at the world......


Just what the world needs, another self-righteous fascist.

And how typical of you to equate the pacifist Amish with terrorist
thugs.


Flipper is a nutcase who has serious difficulties with comprehension
of plain english language emails. Just how he concludes that I "equate
the pacifist Amish with terrorist thugs" is hard for most to
understand, IMHO. I think its the Ga-Ga link. Most probably, the Amish
elders and the moderate peace loving god loving people of Indonesia
share the same view that Lady Ga-Ga is performing trollop, a plain
slut, an un-readeemable sample of brazen ****ery, someone to be
despised and banned and disrespected whenever she / it appears in some
place in their territory. So, with a bit of a shove and shout at
Government, she can be persuaded to **** off, and take her devilish
corrupting, unworthy, dopey, evil crap elsewhere to people who have
lesser standards and morals, in places like the USA.

We had a small band of shielas put on a "slut walk" through city
streets. Lady Ga-Ga is their heroine. Well, these whingers about the
usual war of the sexes got mainly a yawn from the public. Luckily, Oz
is a fairly tolerant country and if 1/2 a dozen shielas wanna do
something wacky like parade half naked etc, and its not life
threatening, we just let 'em, they get it out of their system, and
probably in 5 years nobody will recall who Lady Ga-Ga was, or remember
her extremely forgetable talent. In Afghanistan, Gaga would be stoned
as she walked in the street. The country has lots of stones
everywhere. Its a whole country fulla fukkin stones. So its only
natural a cultural habit of stoning has arisen. The same culture just
does not evolve where most open spaces have lawns and good looking
fields of nice grass.
Indonesia is a predominantly Muslim nation, about 200 million, and
sure, like in the USA and Norway recently, they get a few terrorists.
A few blew up a crowded Bali nightspot. They don't like the sluttish
habits of westerers misbehaving. I went to Bali for a month in 1981,
and I know what they mean. I am just wondering what the crime rates
are for the USA and Indo, say, shootings per 100,000 people, drug
abuse % etc. I'm always amused by self righteous americans saying how
marvellously good the USA is, Same here, ppl say Oz is just only
wonderful. But I know that everywhere one goes there is both good and
bad, and in many places in the world, addopting the USA style of life
is considered a huge step backwards.

I always think anyone including flipper should be challenged when
calling anyone else a fascist. Spouting abuse is flipper's speciality.

Oh, and BTW flipper, I AM a pacifist too, and in the 1960s when our
Government had a national ballot system of selection of 20 yr olds for
national Service, they could not count on me to help their amoral
efforts to "Go all the way with LBJ" and send conscripted soldiers to
Vietnam. I could, with a sizable % of other Australians, see that the
Viet conflict was plain wrong. Half the men in Oz who'd been to WW2
only 20 years before held the views I had. Luckily for me, my number
in the ballot didn't come up, so I didn't have to become a real draft
dodger like those who did who spent years hiding from the police and
ultra right wing behaviours of western governments at that time.
Remember when the CIA and USA government in particular constantly
interferred with whole countries to advance domination of them and
exploit them for business interests, and to "stop the spread of
socialist communism". Well, fear of socialism was unfounded because it
imploded by itself, because if you deny private ownership and freedom,
soon you have a dysfunctional society which produces nothing but
nothing, and they basically go broke, especially if we point thousands
of nuclear armed missiles at them so they have to make a few to point
at us. So had the USA just ignored Vietnam, and let Vietnam decide its
own fate and politics, the later inevitable swing to capitalism would
have occurred faster.

Robert McNamara appeared in a famous movie to explain how wrong the
USA had been about Vietnam. Lord Valve said to me once the West won
because although 3 millions "gooks" died, the US "only" lost 50,000.
Well, what an utterly insane comment to make, but a lot in the USA and
here too think the same evil way, and they didn't like copping the
defeat which Vietnam was. There are still rightwing ppl who, like
fascists, think anyone who don't agree with them is a traitor and
should be executed if they won't serve in War X or War Y or War Z.

But the trend is that people have woken up a bit about joining up to
go to Afghanistan, and of course to win a war there means shooting
most of the men, women and children, maybe 20 million, and ordinary
peace loving americans won't do it, and if they were willing, it'd
cost too much. So instead, the US is investing very heavily in UAV or
"drones" and I heard these drones are to be equipped with software to
make the decision about what to bomb. So, just launch these things,
and have 'em hovering over your possible enemy, and they just take out
ppl here and there for the next 20 years, lots of women and children
included. Now who is the terrorist? Of course in Ghan and Pakistan,
there ARE terroists and old fashioned plain old arsoles, so why pick a
fight with them? The US should just defend itself. Then there is the
question of what happens after the US and Oz is forced out of Ghan?
Well, I reckon the Ghan army might break apart into waring tribes and
the economy will collapse, and evermore reliance on producing heroine
will be tried. Oh yeah, Ghan will have trouble with so many arsoles.
But, it's been like that for thousands of years. Attacking other
countries is not our defense. The expense of ME wars seems rather
unjustified, and has put the West into parlous financial conditions.
Seems like the brown skins with turbans are winning, but they get
nothing from it - the Afghans have never really taken kindly to
successive invasions over centuries, and I'm sure they'd rather not
have had to fight us.

Meanwhile I heard Karzai is makin all sorts of deals with the Chinese
so that joint ventures can proceed to exploit Ghan's vast mineral
wealth and to give useful employment to hordes of eager young Afghan
men who'd love to work rather than devise IEDs to defeat yanks, or
grovel in the dust growing opium. But Karzai and his corrupt cronies
ain't so strong, and it could all collapse so easy like a house of
cards, and nobody can ever go into Ghan to exploit it. Eventually,
Ghan might fall to a foreign power when the rest of the world's
resources dwindle and national governments fade. Armies of only
thousands armed with millions of drone UAVs may become affordable by
multinational companies to "stabilise unsympathetic populations".
There is reason to imagine the world could be a lot worse in 50 years,
or 100 years, or 1,000 years. What some would now call worse may be
seen later as better, glorious in fact. Reality becomes warped.
"Fascist" is such a limited word these days as the varieties of evil
doers diversifies. Humanity has this dreadfully inventive war gene.
But hey, I won't be here, I'm getting old. I'm real lucky to have
lived without being shot at by police of my own country, or by men of
another country who were easily conned into defending it after I was
sent over there to shoot at them.

There are a lot of people to admire in the USA, and I am much better
off to know them. But gee, I can only challenge the others with
emails, a bit like throwing eggs and tomatoes. I hope the UAV flying
overhead does not award a "high priority target point score" to me and
my evil typing habits. It used to be much more messy, remember Laos
and Cambodia and so many B52s?

Patrick Turner.



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TerryG TerryG is offline
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Location: California USA
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The webpage explains what I am talking about, but I disagree with one point it makes for feedback. I do not feel feedback is happening with an unbypassed g2 resistor. If anyone would like to explain how feedback is happening with an unbypassed g2 resistor I would be interested in hearing the theory.

http://www.freewebs.com/valvewizard2/EF86.html


All pentodes drop in distortion and then increase in distortion, as tube after tube datasheets that has this chart on them shows the same thing. But no one is arguing this.



Distortion is relative to load impedance, and a few other things. Since speaker loads vary in impedance with frequency then we can assume that some distortions will be present at some frequencies and other distortions at other frequencies. The more complex the load the worse things will sound. This should go without saying I would think.

My only other question at this point is does anyone know a way of calculating the primary impedance of the output transformer other than multiplying the plate voltage by to itself and then dividing by the plate dissipation wattage?


Terry

P.S. Sum up: Two questions, one - theory on g2 feedback, and two - output transformer primary impedance formula.

I do hate wading through all the mud slinging to find something of substance, therefore if you have mixed in an answer already with other useless banter I probably did not read it, so please post it again.

Last edited by TerryG : June 6th 12 at 05:01 PM
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flipper flipper is offline
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Default New member, pentode g2 operation question.

On Wed, 6 Jun 2012 15:58:03 +0000, TerryG
wrote:


The webpage explains what I am talking about,


That is what I described to you and notice that max (0 grid) Ia is
less with the screen 100k. It can't possibly put out the same,
certainly not more, power.

but I disagree with one
point it makes for feedback. I do not feel feedback is happening with
an unbypassed g2 resistor. If anyone would like to explain how feedback
is happening with an unbypassed g2 resistor I would be interested in
hearing the theory.


First, it's visually clear from the curves due to the loss of gain
and, so, reduced sensitivity. That is classic negative feedback.

Look at the curves down at the bottom. As screen is lowered Ia is
reduced, as I previously described. In fact, you could use the screen,
rather than the grid, as a signal input (and it's being used as an
'input' with negative feedback) Now imagine we have a fixed screen and
we apply a grid voltage that would produce Ia=3mA. There is also a
proportional screen current. Now insert the screen resistor. Screen
current through the resistor will cause screen voltage to drop, which
will act to *reduce* Ia 'counter' to what grid voltage is trying to
do. It is negative feedback.

With an AC signal applied to the control grid screen current is not
'constant'; it varies in phase with Ia. I.E. more Ia means more screen
current and vice versa. So, with a series screen resistor you have a
screen voltage changing anti-phase to the grid signal. It is negative
feedback.

http://www.freewebs.com/valvewizard2/EF86.html


I would have more confidence in the 'summary' description if he had
distortion measurements because the graph is potentially misleading,
or 'not clear', because the one on the right only shows the 'more
linear' region of the plate curves rather than to near cutoff as the
one on the left shows. I.E. it is not clear that 0V-3V on the right is
'more linear' than 0V-1.5V on the left (or whatever the 'comparable'
range would be). It might be 'better' but I can't tell from the
graphs.

However, the 'linearity' is based on a vertical load line and, for a
power tube, the 'softer', more compressed, knee is a curse rather than
a blessing. Not only do we get 2nd harmonic compression much earlier
but power output is even further reduced because it simply cannot pull
current at low plate voltage, so voltage swing is reduced (in addition
to max Ia being less).


All pentodes drop in distortion and then increase in distortion, as tube
after tube datasheets that has this chart on them shows the same thing.
But no one is arguing this.


That is an entirely different phenomena as is made clear by screen
being a constant voltage.

Now, clearly, if we alter the plate curves with screen NFB then the
parameters of the original load line no longer apply so, as I
previously mentioned, you might 'move them' to a 'singular point'
where 2nd order harmonics are less. But, if that were the goal, you'd
be better off to move the load line rather than the plate curves, for
the reasons mentioned above.

On the other hand, if you're 'stuck' with a particular load impedance
then it might seem fruitful to move the plate curves but, then,
speaker load is not constant and you have the deleterious effects of
lower power and compressed knee.


[image:
http://sphotos.xx.fbcdn.net/hphotos-...9207_n.jpg%5D]

Distortion is relative to load impedance, and a few other things. Since
speaker loads vary in impedance with frequency then we can assume that
some distortions will be present at some frequencies and other
distortions at other frequencies. The more complex the load the worse
things will sound. This should go without saying I would think.

My only other question at this point is does anyone know a way of
calculating the primary impedance of the output transformer other than
multiplying the plate voltage by to itself and then dividing by the
plate dissipation wattage?


Yes, a load line.
http://diyaudioprojects.com/mirror/m...h102/pent.html


Terry

P.S. Sum up: Two questions, one - theory on g2 feedback, and two -
output transformer primary impedance formula.

I do hate wading through all the mud slinging to find something of
substance, therefore if you have mixed in an answer already with other
useless banter I probably did not read it, so please post it again.


+-------------------------------------------------------------------+
+-------------------------------------------------------------------+

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Patrick Turner Patrick Turner is offline
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Default New member, pentode g2 operation question.

On Jun 7, 2:11*pm, flipper wrote:
On Wed, 6 Jun 2012 15:58:03 +0000, TerryG

wrote:


snip, for brevity,

My only other question at this point is does anyone know a way of
calculating the primary impedance of the output transformer other than
multiplying the plate voltage by to itself and then dividing by the
plate dissipation wattage?


Steve sure has a lotta useful insight.

I can say, to begin with, Pda = 24 watts for 1 x EL34, and if you try
Ea at +400Vdc, then Ia = 24 / 400 = 60mAdc.
This is for pentode, triode, UL or Acoustical. But its not 100%
correct because really, Ig2 might be 5mA, so to keep
the tube cool, we might use Ia = 55mA and Ig2 = 5mA, where Eg2 is
close to Ea.

For calculating loads for maximum PO for class A1 SE pentodes, RLa =
0.9 x Ea / Ia. For approximate load calculations, neglect Ig2 in power
tubes.

For pentode operation where the knee of the Ra curve for Eg = 0V is
well above 2 x Ia at idle, for max PO for class A1, SE, RLa = 0.9 x
Ea / Ia = 0.9 x 400 / 0.06 = 6,000 ohms.
If you chose Ea t 300V, then Ia = 24 / 300 = 80mAdc, so RLa = 0.9 x
300 / 0.08 = 3,375 ohms.
You can develop an an equation where you have Pda and Ia or Pda and Ea
only as the variables and I leave you to think about that.
Pentodes have dreadful spectra when in SE mode, and piles of THD which
can only be cured with NFB applied externally.
In PP mode, the RLa experienced by one tube becomes a curved line
because the current change and voltage change in one tube affects the
same in the other, so I find the composite PP load line to be
confusing, and a trifle simplistic, so I never bother with it, and
just concentrate on what is happening in ONE tube of the pair in PP,
and then settle for approximate estimations. The real outcome is
always what I measure. Most PP amps are class AB so when one tube is
cut off, the other sees 1/4 of nominal RLa-a, and most PP power is
effectively produced by SE action of each tube. Because any pair of
average PP tubes are not perfectly matched, there will aways be 2H and
4H in the PP THD. Mostly you get 3H and 5H.

Triodes are different, and the PO is limited in AB1 by the Ra line for
Eg1 = 0V having no knee, and having the similar shape to all other Ra
lines for negative values of Eg1.
Ra at a given Ia varies slightly for most triode conditions, and for
EL34 can vary between say 1k5 for Ia = 50mA down to 1k2 for Ua =
100mA.

RLa load is calculated for maximum PO, triode class A1 = ( Ea / Ia )
- ( 2 x Ra ), so if you have Ea = 400V and Ia = 60mA, and Ra = 1,350
hms, then RLa = ( 400 / 0.06 ) - ( 2 x 1,350 ) = 6,666 - 2,700 = 3,966
ohms, ie, about 4k0, and if the winding losses were 10%, load is 4k0
-10% = 3,600 ohms. Of course speakers present a wide range of loads
above and below the calculated RLa value. Tubes and any other devices
are expected to perform with a dynamically changing load for every
microsecond of their existance when being used.
Devices with adequate open loop gain and adequate capability to
produce a useful amount of power at less than 10% THD routinely used
in amplifiers with adequate NFB used to straighten out what is a usual
mess to begin with, especially when penodes are used. While a trioded
SE EL34 can produce 7.2 watts at about 5% THD, RLa = 4k0, at all other
RLa values away from 4k0 the max PO will be less because either grid
current or cut off cause assymetical clipping before 7.2Watts is
produced. If we insist on having an ability for say 3 Watts at less
than 5%, then load values can range between say 2k0 and 8k0, but you
should be able to work all this out yourself with load line analysis.
SE Pentode operation with RLa above or below RLa for max PO also shows
the limited range of RLa for a useful PO.

See my pages, http://www.turneraudio.com.au/loadma...amtetrodes.htm
Follow your nose to many other pages, and most useful is to scurry
away to your workshop and get busy with measuring gear and
oscilloscope.

For working out the class A PO for any RLa value which is less than
that for max PO, PO = 0.5 x Ia dc squared x RLa.
Eg, for 2k0, Ia = 60mAdc, PO max at clip = 3.6Watts.
For working out the class A PO for any tube value which is higher than
that for max PO, PO = ( peak negative Ea swing x 0.707 ) squared /
RLa.
Eg, for 8k0, triode, Ea peak swing might be 300V, so PO max = ( 300x
0.0707 ) squared / 8k0 = 5.6Watts. There are formulas to find
theoretical peak Ea swing using Ea, Ra, Ia and RLa etc, but just
drawing a line across the Ra curves is less confusing.

Patrick Turner.

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