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6LU8 SET or SEUL Amp



 
 
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  #1  
Old March 16th 11, 03:56 PM
John L Stewart John L Stewart is offline
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First recorded activity by AudioBanter: Jan 2011
Location: Toronto
Posts: 267
Default 6LU8 SET or SEUL Amp

I did this one about 12 years ago. It compares very well with what you can do with a 2A3, but at a much lower cost. And 6LU8s are still cheap, all built while production knew how to make a reliable tube.

The distortion tests were done with an HP302A Wave Analyzer for both triode & UL modes at One Khz. Another short set at 100 Hz, One Watt compares Hammond 125E Universal with a Hammond 1628SE. The advantage of more iron is quite obvious.

ULTRALINEAR WITH FEEDBACK WATTS 1 2 3 4 5 6

HARMONIC DISTORTION % 2ND 0.28 0.44 0.45 0.53 0.74 1.18
3RD 0.16 - - 0.35 0.36 0.96
DAMPING FACTOR = 17


ULTRALINEAR WITHOUT FEEDBACK WATTS 1 2 3 4 5 6

HARMONIC DISTORTION % 2ND 2.71 3.93 5.09 6.65 7.71 7.62
3RD 0.70 0.80 1.10 1.20 1.43 4.05
DAMPING FACTOR = 2.3


TRIODE WITH FEEDBACK WATTS 1 2 3 4 5

HARMONIC DISTORTION % 2ND 0.71 1.04 1.30 1.66 1.98
3RD 0.22 0.25 0.49 0.50 0.79

TRIODE WITHOUT FEEDBACK 2ND 3.95 5.83 7.11 8.93 8.83
3RD - 0.23 0.21 0.20 2.07

Following measured at 100 Hz

OUTPUT TRANSFORMER 125E 1628SE

A) ULTRALINEAR WITHOUT FEEDBACK

HARMONIC DISTORTION % 2ND 15.3 2.9
3RD 3.1 0.4

B) TRIODE WITHOUT FEEDBACK

HARMONIC DISTORTION % 2ND 12.2 2.8
3RD 1.5 ---

Hope this comes out your end OK. Cheers, John
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Last edited by John L Stewart : March 16th 11 at 04:21 PM. Reason: Problems with tables
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  #2  
Old March 17th 11, 12:19 PM posted to rec.audio.tubes
Patrick Turner
external usenet poster
 
Posts: 3,914
Default Determination of Maximum Positive Feedback in Bootstrapped Driver

On Mar 17, 12:36*am, John L Stewart <John.L.Stewart.
> wrote:
> First of all, calculate the driver gain in grounded cathode mode from
> published specs-
>
> For the 6SN7 family at 250 volts supply
> * * * * Mu = 20
> * * * * Rp = 7.7 K while grid volts is 8
>
> Let A1 be the gain with these conditions,
> Then A1 = ( mu*Rl ) / ( rp + Rl )
> * * * * = ( 20* 27 ) / ( 7.7 + 27 )
> * * * * = 15.56



But 27k is the RL only when the 27k is terminated by a B+ which has no
ac, or put another way, when there is no signal voltage in the OPT and
no Vac bootstrapping tap.

Beginners may have failed to understand you.

BTW, Ra at 7k7 is a bit optimistic unless Ia is fairly high; Ra varies
considerably with Ia.
>
> Then find the maximum gain possible for any non-feedback triode stage-
>
> Then A2 = mu
> * * * * = 20
>
> Gain change is A2/A1
> * * * * = 20 /15.56
> = 1.29
>
> In DB becomes 20 log A2/A1
> * * * * = 2.18 db
>
> The feed back in the case of the bootstrapped driver is to the triodes
> anode, so no gain above mu is possible. There is no gain thru the triode
> being plate driven.


OK, this point becomes more obvious the more you think about it, but
there's more to consider.

One could connect the 27k load to a tap on the OPT where twice the
6SN7 anode signal exists, ie, cross coupled to the opposite side
output tube anode, and then you have a case where the 6SN7 would rise
above , but not much, because the 6SN7 Ra is much less than 27k, and
to make a large amount of gain occur one would need to have a feed
point for bootstrapping several times the anode signal at the opposite
output tue anode.
But usually when one feeds more Vac to a bootstrapped RL one has real
PFB, not just something which leads the tube towards being loaded by a
dummy CCS, ie a load with no current change.
When I tried doing this the results were dissapointing, increased Rout
and THD


> NTL, some experimenters have actually built working amplifiers this way
> using many triodes such as the 6S4 & 12B4. Starting digging, you will
> find them.
>
> Hope this all mkes sense. Cheers to all, John


Indeed what you are saying does make sense, but the the effect of
bootstrapping where it is a form of mild PFB on THD is yet to be
formularized.

In RDH4, PFB is mentioned and a schematic is given where a PFB loop
between two cascaded small signal voltage triodes is boosted 12dB and
the same amount of global NFB is used. The effect of PB when the
signal is below say 10Vrms is to increase open loop sensitivty without
much increase in THD or reduction of bandwidth so that global NFB may
still be applied while retaining stability, at least with resistance
loads.
RDH4 goes on to say what a marvel PFB can be. Its as if PFB offers
"free voltage gain" without using expensive hardware to get it, just
resistors are needed.

But I have never ever seen any commercial amp with PFB used
deliberately to increase OL Gain except of course in samples of the
parafeed phase inverter where V1 anode output is divided down to drive
V2, to make another phase of drive signal "for free". Anything for
free in electronics usually has a price, and THD increase with any
form of PFB is one of the costs. Even in Quad-II there is 6dB of PFB
so that the two EF86 may generate just enough gain between them to
drive the KT66 AND allow about 10dB of global FB.
Some other type of input-plus-driver stage with the same tube gain and
no PFB would have input sensitivity needing 2.8Vrms input for clipping
instead of only 1.4Vrms as Quad-II requires, if the same amount of
global FB is maintained.

In earier times amp makers wished there amps to be sensitive to 0.2V
for clipping; Leak amps with GNFB needed only 0.1Vrms for clipping.
Mullard 520 had EF86 in pentode followed by a 12AX7 LTP.
But no PFB, and no bootstrapping.
Williamson had triode output stage and 6SN7 low triode drive stages
and the concertina phase inverter with lots of local NFB as a buffer
stage between SE input and balanced amp driver. Sensitivity was 2Vrms
for clipping.
Again, no bootstraping or PFB.

As I have mentioned, boostrapping RL of driver anode loads to OPT
taps is mainly used to achieve a high voltage swing without the driver
going near cut off or grid current.

Bootstrapping from an anode winding on an OPT does require a well
smoothed B+ rail for the OPT CT. Any hum at the CT also exists ant any
tapping points on the anode primary winding and for bootstrapping to
work one needs to have any PFB feed of noise to be fed back while
remaining common mode and it would be all too easy for some imbalance
of noise and the noise becomes differentially applied to OP tube grids
resulting in its amplification as any other wanted differential
signal. Many old amps like Quad-II had quite high hum at the OPT CT,
17Vrms in the Quad case because there was no beans to pay for a plate
choke and additional filter cap, and there must have been a bean
counter at Quad's head office. They had a guy there who passionately
believed in CMMR and preached its virtues so suppressing any murmers
about yet another bit more engineering to make a toy amp heavy.
So bootstrapping anything from a conveniently phased OPT tap somewhere
can be a problem unless is noise is suppressed by a good PSU.

In Quad-II, it would be possible to have say series 82k plus 100k
anode supply RL to each EF86 anode. The total of 182k is virtually the
same as what is the standard original 180k. Then you could have 22uF
electro caps from junction of the two R to the CFB winding which has a
CT that is virtually grounded. This would make the load seen by the
EF86 to become higher than the 180k, maybe double because pentodes
have high Ra not reduced by the internal NFB in a triode, and hence
the EF86 gain would nearly double, and GNFB reduced by 1/2 to maintain
the same amount of GNFB. But what's the use? I think ppl would find
the 0.7Vrms input sensitivity welcome but THD and Rout would not be
less.

The CFB winding in the Acoustical Connection does not have a common
mode hum signal to upset the apple cart. But I doubt much would be
really achieved in overall THD reduction or anyother betterments.

What works better in Quad-II is to make an LTP with EF80 with 3 times
the anode current in EF86 and therefore raise OLG by nearly 2X and
construct the input pair as a true LTP with CCS, and with input sig to
one side and NFB to the other, and no need to bootstrap or worry about
PFB and its THD increasing propensities.

6EJ7 is also a nice driver pentode. Radford used a pair in the LTP for
the ST100 PP amp with 2x KT88.

50 ways to leave a lover, slip out the back Jack, find a new plan
Stan,

and 50 ways to make an amplifier.

Patrick Turner.



>
> --
> John L Stewart


  #3  
Old March 18th 11, 05:27 AM posted to rec.audio.tubes
Patrick Turner
external usenet poster
 
Posts: 3,914
Default Determination of Maximum Positive Feedback in Bootstrapped Driver

On Mar 18, 3:05*am, flipper > wrote:
> On Thu, 17 Mar 2011 05:19:15 -0700 (PDT), Patrick Turner
>
> > wrote:
>
> <snip>
>
>
>
> >In RDH4, PFB is mentioned and a schematic is given where a PFB loop
> >between two cascaded small signal voltage triodes is boosted 12dB and
> >the same amount of global NFB is used. The effect of PB when the
> >signal is below say 10Vrms is to increase open loop sensitivty without
> >much increase in THD or reduction of bandwidth

>
> PFB is PFB and has the attendant increase in distortion and reduced
> bandwidth regardless of the 'voltage level'.


But RDH4 didn't make your simple generalisation or come to the same
conclusion about PFB.

It is possible to reduce distortion with bootstrapping which is mild
PFB.

Or PFB may be added to increase gain say by +12dB in a low voltage
level input stage/s without increasing THD by +12dB.
Then a higher amount of NFB can be applied and the overall THD is much
less than without PFB.

And, last but not least, Positive Current FB reduces Rout.

Sure, PCFB increases THD and reduces bandwith and increases
instability, but you don't need much of it to reduce the Rout of some
amp from say 2 ohms to say 0.2 ohms, or down to 0.0 ohms, or in fact -
0.5 ohms, or a negative figure which simply means VO increases if a
lower load is connected.
PCB is DANGEROUS because if RL becomes very low, then the amp tries
desperately and vainly to keep VO constant and it can oscillate itself
to death easily at some inconvenient frequency, and with some
resultant high expense.

The reason PCFB and PVFB isn't used is because many dumbcluck amp
designers and fuctard production managers will not be able to get a
mass made amp to conform to the behaviour of a well researched and
tested prototype. R&D and design time costs money, and what CEO can
afford to pay anything while he dreams about hs latest Cadilac?

> > so that global NFB may
> >still be applied while retaining stability, at least with resistance
> >loads.
> >RDH4 goes on to say what a marvel PFB can be. Its as if PFB offers
> >"free voltage gain" without using expensive hardware to get it, just
> >resistors are needed.

>
> I don't know where the heck you got that idea because RDH4 doesn't say
> a thing about 'free' or PFB being a 'marvel'.


RDH4 was kept brief. It was a 1,600 page book, and brevity needed to
be mantained, so much of it comes across dry as desert, and without
any distracting notions about "free", or "marvels" .
>
> What it does say for the referenced two stage amplifier, and what you
> left out, is if stage 1's distortion is *much lower* than stage 2 then
> the increase in stage 1 distortion from applying local PFB is more
> than offset by GNFB reducing overall distortion because that is
> dominated by stage 2, which is in the global loop but not the local.
> You are, in effect, 'redistributing' the distortion from stage 2 to 1.


Just in case you are confused about which section of RDH4's many pages
about FB to which I may have been referring to, I suggest you turn to
page 352 and 353 of RDH4, 4th Ed, 1955.

On page 352 there is a 3 stage feedback amp with two 6SN7 and a pair
of 6V6 output tubes in beam tetrode mode.

The amount of internal loop PVFB is +26dB.

The distortion results at the bottom of the page say...

"The intermodulation is 40% with no feedback, 8% with negative
feedback alone and 1.9% with combined positive and negative feedback
under the following conditions--output 8 watts, 4:1 ratio with
frequency 60 and 7000 c/s."

I rest my case with the jury....


> Every 6dB of PFB doubles the distortion but as Cyrano Jones noted when
> negotiating the price of Tribbles, "twice nothing is still nothing."
> Now, the first stage isn't quite 'nothing', of course, but if it's
> very low compared to the second stage then it's, to reference another
> famous quote, "close enough for government work." Or amplifiers. E.g.
> if you double a very small number and then halve the big one the net
> result is lower overall.
>
> But it ain't 'free' as stage 1's bandwidth is also reduced,
> complicating stability.
>
> You'd be better off with a high gain wide bandwidth section, like say
> 2 stages (assuming you don't introduce new problems) instead of the
> one with PFB, but that costs more and just "how low can you go" before
> it isn't worth dancing that hand jive?


I don't like any PFB anywhere myself.

But it is inherent with bootstrapping which is usally a very small
amount of PFB, certainly not 26dB.
Ditto with paraphase input stages as in Quad-II amps.

In amps which John Stewart was referring to which did have
bootstrapped driver RLs, the PVFB is small but it does a lot.

In Quad-II, the paraphase inverter does indeed double drive amp THD.
But overall is mainly due to the OP stage where near clipping its THD
might be 10 times the driver stage, so driver THD may be halved or
doubled without greatly affecting the overall measurements.

By the paraphase connection does the mavellous thing of increasing
amplifier sensivity to a sensible figure and thus prevent the Quad-22
preamp having to produce such a high drive voltage which might be
higher han the power amps.

Everything needs some careful weighing of many factoids before any
simple conclusion may be made.

I would guess the schematic shown in RDH4, page 353 might be a dog of
a thing to get unconditional stability as easily as the text suggests.
There's no report there of square testing at low levels with various
cap loads...Much is damn well missing!

So, what's missing must be found out about and learnt about by real
world engineers like myself, not too much dementia yet, because I
don't assume and I still ask questions.

Patrick Turner.
  #4  
Old March 18th 11, 11:49 AM posted to rec.audio.tubes
Patrick Turner
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Posts: 3,914
Default Determination of Maximum Positive Feedback in Bootstrapped Driver

On Mar 18, 9:55*pm, flipper > wrote:
> On Thu, 17 Mar 2011 22:27:28 -0700 (PDT), Patrick Turner
>
>
>
>
>
> > wrote:
> >On Mar 18, 3:05*am, flipper > wrote:
> >> On Thu, 17 Mar 2011 05:19:15 -0700 (PDT), Patrick Turner

>
> >> > wrote:

>
> >> <snip>

>
> >> >In RDH4, PFB is mentioned and a schematic is given where a PFB loop
> >> >between two cascaded small signal voltage triodes is boosted 12dB and
> >> >the same amount of global NFB is used. The effect of PB when the
> >> >signal is below say 10Vrms is to increase open loop sensitivty without
> >> >much increase in THD or reduction of bandwidth

>
> >> PFB is PFB and has the attendant increase in distortion and reduced
> >> bandwidth regardless of the 'voltage level'.

>
> >But RDH4 didn't make your simple generalisation or come to the same
> >conclusion about PFB.

>
> Yes they did and it isn't 'my conclusion'. It's a fact you'll find in
> every electronics textbook on the subject.
>
> And I quote RDH4 page 355: "Each 6dB increase in gain due to positive
> feedback will double the distortion in this stage ; for example 24dB
> increase in gain will increase the distortion in this stage by 16
> times."


Page 355 is concerned about different operating condition to those
mentioned on Page 353 which I quoted.

I remain correct about what I said, and I selected the example omn
page 353 because its content is relevant to John Stewart's OP about a
power amp including gain stages and power tubes.

I cannot find time to answer your further concerns about what I said,
maybe your right, maybe your'e wrong.

I know people who have reverse phase connected UL taps on OPTs to
increase OP tube gain to more than double OP tube gain. The THD was
reduced and Rout reduced and bandwidth remained OK and stability fine
because they knew how to do it all, unlike most who'd end up with an
oscillator.

The man who told me about it was Neville Thiele, of the famous and
very well educated Thiele and Small gang who worked out all those
incomprehensible equations for speaker enclosure design. Theile was 75
yo when I phoned him up one evening in about 1995 for a casual chat
about something else which puzzled me.
He'd also written some fine articles in Electronics Australia, one
being about the PP amp in a deluxe model of Kreisler TV sets which
were made in Oz. The tube amp had CFB OPT and a pair of 6BM8, but
with no PFB except in the paraphase inverter. Unless careful Nyquist
parameters are considered, PVFB is a nightmare.
But Neville knew all about Nyquist. For what he did at home for fun,
he could get away with using PFB which I would not bother with - too
much trouble, and lowering THD with PVFB or PCFB may not improve the
sound much.

In McIntosh MC275 amps, the 12BH7 has bootstrapped load resistors and
there is an amount of PVFB which is substantial, but not huge, but
which allows the production of high drive voltages at lower THD than
could otherwise be easily achieved.

I again rest my case with the jury....

Delete the rest of whatever you said.....

Patrick Turner.
  #5  
Old March 18th 11, 01:56 PM
John L Stewart John L Stewart is offline
Senior Member
 
First recorded activity by AudioBanter: Jan 2011
Location: Toronto
Posts: 267
Default

[quote=Patrick Turner;928239]On Mar 18, 9:55*pm, flipper > wrote:[color=blue][i]
> On Thu, 17 Mar 2011 22:27:28 -0700 (PDT), Patrick Turner
>
>
>
>
>
> > wrote:
> >On Mar 18, 3:05*am, flipper > wrote:
> >> On Thu, 17 Mar 2011 05:19:15 -0700 (PDT), Patrick Turner

>
> >> > wrote:

>
> >> <snip>

>[color=green][i][color=darkred][i]


Hey Patrick, I won't comment on your most recent except to day that Flipper is correct in his post that it is important to attack distortion in the hilevel stages first. If need be, find gain somewhere else, a low level stage perhaps if there is a shortfall.

The short equations I gave predict the worst case of +ve FB since we assume the triode gain to be simply equal to mu while bootstrapped. In reality, while bootstrapped the triode gain is somewhere between the grounded cathode value A1 & mu. So the +ve FB is always less than predicted.

In the McIntosh the local NFB in the OP stage is 15-20 db, depending on loading, Etc. So lots of room for a couple of db +ve FB.

The rp of the sample circuit given turns out to be about 10K since the cathode current in each triode is 4.3 mA. That translates to a worst case +ve FB of 2.74.

The equations can be further reduced as follows-

A1 = ( mu*Rl ) / (rp + Rl ) and

A2 = mu

Gain change is A2 / A1

So Gain change becomes ( mu ) / ( mu*Rl ) / (rp + Rl )

That reduces to Gain Change

A2 /A1 = (rp + Rl ) / Rl

Very easy to apply to various conditions.

I made some real measurements on working systems 8 or 10 years ago but can't find the results. As I recall, all less than 2 db I think. When I get some time, or if there is rain or snow I will make some measurements. I've got a Circlotron, the 6AS7/6080 Amp & two different twin coupled amps I built for publication back in the day.

Yesterday was 8K on the cross country skis, then 45K on the bicycle. Not bad for an old fart.

Cheers, John
  #6  
Old March 18th 11, 11:10 PM posted to rec.audio.tubes
Patrick Turner
external usenet poster
 
Posts: 3,914
Default Determination of Maximum Positive Feedback in Bootstrapped Driver

Delete unecessary argy bargy.

> >I remain correct about what I said,

>
> No and I've proved it by quoting RDH4 itself.


OK, I agree to disagree.

I leave the outside world to decide who's right.

RDH4 is open to interpretation.

Delete more stuff that leaves nobody wiser.

I just ain't got time to arhue about what seems true for me.

Patrick Turner.
  #7  
Old March 19th 11, 12:46 AM posted to rec.audio.tubes
Patrick Turner
external usenet poster
 
Posts: 3,914
Default 6LU8 SET or SEUL Amp

On Mar 19, 12:56*am, John L Stewart wrote


> > Hey Patrick, I won't comment on your most recent except to day that
> > Flipper is correct in his post that it is important to attack distortion
> > in the hilevel stages first. If need be, find gain somewhere else, a low
> > level stage perhaps if there is a shortfall.


I was merely quoting RDH4 where they measured an amp with NFB and
**with** PFB and **without**
..
RDH4 had reason to believe the overall effect of adding PFB was a
reduction of IMD. The PFB raises open loop gain which allows more
global FB to be applied and the NFB reduced the IMD much more than any
increase of N&D because of PFB.

All PFB increases N&D locally at least, within a circuit, and I have
no argument with the text books. But that does not mean anyone can
conclude PFB will always increase overall N&D in the presence of NFB.

Electronic behaviour can rarely be described by any simple
generalisation and in 10 times out of 20 every generalisation is
subject to conditions of operation, behaviour A happens if condition
B, C, and D are present and if condition E. F are not present, etc.

It was the overall effect of combined PFB and NFB and not the local
which the real issue, at it was in your OP.

I also believe its best to reduce the OP stage N&D locally as much as
possible without causing the driver voltage to be come so high that it
begins to contribute more N&D to the total N&D outcome than the OP
stage.

A McIntosh OP stage in plain beam tet mode with no 50% CFB windings
and biased for largely class B op with low value RLa-a would have much
more N&D than with the 50% CFB. Even with the 50% CFB, the outcome
without other additional loop FB is rather poor compared to a class A
UL amp or triode amp.
To get 50W from 6L6 or 75W from 6550 means means high N&D.

But the McI driver has to give over half the Vac between anode and
cathode to each grid, so driver N&D is high. The global NFB enclosing
all stages reduces the total N&D of all stages about equally to an
acceptable figure which in McI's case is better than many other amps
of its day. The McI Rout is especially low because of the large total
amount of all loops of NFB and despite the inclusion of PFB in the
bootstrapping.

EAR509 follow a similar idea of having lots of voltage gain which
allows lots of FB, and aided by bootstrapping, carefully done to avoid
instability to keep Nyquist happy with resistance loads.

But EAR failed the Nyquist if a pure C load was used. Paravicini must
have thought nodody was ever going to use a 0.47uF speaker load. I
rebuilt a pair of EAR509 and was able to make them unconditinally
stable while retaining the existing high OLG and all NFB loops, and
retaining a 50kHz BW. Sometimes I think master designers
likeParavicini have **** for brains.

>
> > The short equations I gave predict the worst case of +ve FB since we
> > assume the triode gain to be simply equal to mu while bootstrapped. In
> > reality, while bootstrapped the triode gain is somewhere between the
> > grounded cathode value A1 & mu. So the +ve FB is always less than
> > predicted.

>
> > In the McIntosh the local NFB in the OP stage is 15-20 db, depending on
> > loading, Etc. So lots of room for a couple of db +ve FB.

>
> > The rp of the sample circuit given turns out to be about 10K since the
> > cathode current in each triode is 4.3 mA. That translates to a worst
> > case +ve FB of 2.74.

>
> > The equations can be further reduced as follows-

>
> > A1 = ( mu*Rl ) / (rp + Rl ) and

>
> > A2 = mu

>
> > Gain change is A2 / A1

>
> > So Gain change becomes ( mu ) / ( mu*Rl ) / (rp + Rl )

>
> > That reduces to Gain Change

>
> > A2 /A1 = (rp + Rl ) / Rl

>
> > Very easy to apply to various conditions.

>
> > I made some real measurements on working systems 8 or 10 years ago but
> > can't find the results. As I recall, all less than 2 db I think. When I
> > get some time, or if there is rain or snow I will make some
> > measurements. I've got a Circlotron, the 6AS7/6080 Amp & two different
> > twin coupled amps I built for publication back in the day.

>
> > Yesterday was 8K on the cross country skis, then 45K on the bicycle. Not
> > bad for an old fart.


Don't overdo it. It can easily dull the brain. I wish Flipper would go
for a long bike ride sometime but he's excused if he fears USA
motorists. Maybe along swim would suit him, he has the right name for
it.

I have to work. I'm not a retired old fart. But I can fart well when I
have to though.

I'm presently doing a second attemt to make a pair of 100W Ming Da
amps behave.

The Chinese were way out of their brainular cababilities when they
designed these POS amps. They are great smoke producers, and an
owner's nightmare with regard to biasing..

Anyway, I have retained the tube line up, 6SN7 SET input stage to make
about 5Vrms, 6SN7 LTP phase inverter to make two 30Vrms phases, then
300B balanced amp each with Ia 7mA and each making 120Vrms to power
grids of a pair of 845 with Ea = 1,200Vdc and Ia 43mAdc each.
Initially N&D was 5 times worse than how I have it now, stability was
poor, and the list of Chinese errors from their technical
incompetence would fill several screen fulls of description.

There is no need for bootstrapping because I have revised the +420V
rail and created a -190Vdc rail for the 300B balanced driver amp. This
300B driver stage is laughable use of 300B, but the Chinese think
westerner buyers like lots of spectacular tubes. In fact the 300B with
Ia only 7mA works fine. But Ia was less when they were expected to
work from only a 400V rail originally.

OK, the amps go back to the buyer who bought them second hand from
someone who just wanted to get rid of them. He's very happy for a
month before the attacks of N&D start again, and he returns the amps
to me. This time I find an anode resistor of a drive tube had gone
open and I have to suspect it has the Chinese Carbon Film Resistor
Disease, CCFRD where the resistor sure looks nice, and like something
ARC Corp might use, (except the paint job is worse), and the resistor
has gone open without any sign of heat stress.

Well, There are about 20 of these potentially horrible Chinese made
resistors in each monobloc and during the first re-engineering attempt
I assumed they'd be OK, but they are NOT OK. I have seen several
examples where Chinese 1W carbon film resistors just go open. Usually
its because there is more than 200Vdc across a carbon film type.
Something strange happens in the resistor. The MingDa resistors look
like 2.5W rated and I thought they would be fine. WRONG!, not OK. II
am replacing all 20 R throughout each monobloc amp with metal film
types with heavy watt ratings. Where they have 4 x 470uF caps in
series for +1,200Vdc rails, there is 300Vdc across 400V rated cap
bypassed with 220k and this looks very prone to failure.

I can't bootstrap the 50k anode loads of each 300B tube from the OPT
because there are no available taps on OPT which are suitable.
I could have taken anode R loads to the +1,200V rail after making yet
another series C RC filter, but the split rails of over 700Vdc is
enough to get the swing needed for drivng 845, and I avoid HV bothers.

But all hell breaks loose if this amp is driven well into clipping;
the 300B and 845 become badly affected by temperature changes in grids
and the biasing goes very wobbly and strange, and the best cure is to
switch off the amps, wait a minute for things to cool before switching
back on. All of which doesn't happen/isn't needed in normal OP tubes
like 6550.

To force the amp to behave when over driven I will try to make a
limiting circuit where a high resistance plus low resistance diivider
is cap coupled of V1 output and if V1 voltage rises above a threshold
due to clipping, two transistor, npn and pnp will be turns on and
there collectors will shunt the input signal preventing any V1 output
increase.

Such a scheme is fine up to clipping because the transistors have
extremely high collector resistance when fully turned off.
And if someone turns up the volume with a shorted speaker it prevents
the OP tubes being turned on fully into a shortcircuit.

I rarely cannot proceed from the start of a day to its finish without
addressing the incompetence of others I am forced to deal with in my
day to day work life.

But the Turner-Ming amps do sound very well indeed when they work.
There is no doubt the shuguang Chinese made 300B and type B 845 are
superb.

But I cross my fingers about the Chinese OPTs when I think about them.
I don't expect them to last a long time. The leads coming out of the
OPT box have insulation far too thin, nowhere as good as what you'd
find on any Hammond OPT and the the Chinese saw fit to use green wire
with yellow stripes for the CT lead to B+. This color is normally used
for chassis earth wiring. I hate to think of the Feng Shui karma
issues surrounding chinese OPT designs.

Patrick Turner.

  #8  
Old March 19th 11, 12:31 PM posted to rec.audio.tubes
Patrick Turner
external usenet poster
 
Posts: 3,914
Default Determination of Maximum Positive Feedback in Bootstrapped Driver

>
> >Hope this all mkes sense. Cheers to all, John

>
> Maybe I'm not thinking straight but I don't see that answering the
> question because it depends on what appears to be an arbitrary claim
> that gain cannot exceed mu. I mean, it begs the question by declaring
> a limit, which is the essence of the question.


Its very easy to think bently instead of straightly.
>
> Are you presuming that the PFB is designed to produce a unity voltage
> feedback (relative to the anode signal swing) so that Rl 'appears'
> infinite?


The RL which is infinite is a constant current source or constant
current sink.

But the bootstrapping can be arranged to make RL effectively increase
in ohms value positively until it indeed becomes infinite and no
current change occurs in the tube. But from there the load can become
negative.
On a load line graph with load sloping down as you move from right to
left, the bootstrapping will tip the load line to flat until it is a
CCS which is a horizontal line. Then the load can be made to be a
negative load value which slopes up as you more from right to left
across the Ra curves.
Try plotting the lines and current changes while considering different
vallues of bootstrap taps on the OPT.

As the amount of bootstrapping volage increases, the gain rises above
, but it takes some doing, and my experiments showed trying to boost
gain above was completely pointless because of such rapdidly
increasing N&D and instabilities. THes s.
Usually, applying PFB through an anode is the most useless way to
apply positive FB to boost gain in order to increase open loop gain of
an amp without much increasing open loop N&D, because the latter
cannot be achieved.

But McIntosh found bootstrapping to be a real boon for achieving low
N&D and low Rout, and methinks whatever McIntosh did agrees with RDH4
conclusion that PFB can be used for an overall outcome which gives
less IMD with PFB than without PFB.

Bootstrapping is a *small amount* of PFB, and if the OP stage is
already fairly linear as is the case with a class A triode or UL or
CFB amp then the amount of THD fed back to increase itself is not
huge.
One has to analyse just what happens to an existing measurable
distortion voltage at the OPT tap as it is fed back. Forget about
other signal voltages - just think about the distortion voltages and
what MUST happen with them.
>
> If that's the case then it's trivial to show from the gain equation
> that gain approaches, and does not exceed, mu but it's not intuitively
> obvious from the schematic that's the amount of feedback being
> applied.
>
> If the amount of voltage feedback exceeds plate swing then gain can,
> indeed, be above mu.


Indeed.

PFB is used all over the joint in complex filter circuits. I have used
it myself to boost the Q of a narrow band bandpass filter. Some times
the N&D just does not matter.

The wien bridge oscillator relies on much PFB and slightly less NFB so
that it will continue oscillating, and in a recent revision of my 1kHz
oscillator I was able to get 0.0044% with just one opamp with just the
right lamp.


>
> To illustrate I spiced a simple circuit with a 250V source having
> 100Vrms on it (simulating an arbitrary transformer feedback signal)
> applied to a 6SN7 plate through a 27k and cathode to ground through an
> unbypassed 470 ohm. Opposite phase 1Vrms (for visual 'instant math'
> convenience) was then applied to the grid.
>
> Plate swing was 52Vrms for a gain of 52.- Hide quoted text -
>
> - Show quoted text -


Google prevents me seeing any further...

But try to build real cicrcuits with PFB....

Patrick Turner.

  #9  
Old March 19th 11, 12:58 PM posted to rec.audio.tubes
Patrick Turner
external usenet poster
 
Posts: 3,914
Default Determination of Maximum Positive Feedback in Bootstrapped Driver

On Mar 19, 8:39*pm, flipper > wrote:
> On Fri, 18 Mar 2011 16:10:52 -0700 (PDT), Patrick Turner
>
>
>
>
>
> > wrote:
> >Delete unecessary argy bargy.

>
> >> >I remain correct about what I said,

>
> >> No and I've proved it by quoting RDH4 itself.

>
> >OK, I agree to disagree.

>
> >I leave the outside world to decide who's right.

>
> >RDH4 is open to interpretation.

>
> >Delete more stuff that leaves nobody wiser.

>
> >I just ain't got time to arhue about what seems true for me.

>
> Sorry Pat but electronics is not a Van Gogh to be freely 'interpreted'
> by the observer.


I didn't mention Van Gogh. Only you did.


>
> The confusion is coming from using PFB to enable 'something else' that
> is beneficial, such as enabling GNFB in the example you cited. PFB
> enables it but GNFB is what 'does the good thing'.


Youv'e got it. That's exactly what I was trying to tell you.

And RDH4 gave the figures to back up their claim about the schematic
they show.

>
> Another example is John's boostrap, but it's 'something else' is
> something else.


John's circuit has PFB with NFB. There's just enough PFB to
effectively increase the RL value seen by the 6SN7 so its THD should
reduce maybe 10dB. I say that because with a plain 27k you'd have 1%
THD at 10Vrms, but with CCS you'd have 1% THD at 50Vrms, depending on
the sample.

But the bootstrapping feeds back THD created in the OP tubes plus 6SN7
back through the R divider formed by Ra and 27k so that a fraction of
D appears at the OP grid to be re-amplified and add to D already
there.
some second order products are formed on this merrygoround. But the
THD measured with bootstrapping is the results after the merrygoround;
the artifacts don't keep on increasing infinitely.

>
> If you look at my reply to John you'll see I simulated a 6SN7 plate
> PVFB circuit where the gain was greater than mu so I went back and
> measured the distortion.
>
> Gain with no PVFB was 11.97 with 2'nd harmonic at -54.6dB
>
> Gain with PVFB was 52.2 with 2'nd harmonic at -42.9dB
>
> PFB increases distortion.


Sure, and you have applied far too much PFB.

But you need to measure an amp built like John's or a Mcintosh, and
vary the amount of PFB from between no bootstrapping to where gain is
say double . Do this at a low level of output and plot the THD for
same output voltage while adjusting input voltage to keep Vo constant.
Maybe the experiment might change your mind.
Minds were changed in the McIntosh lab well before 1950......
>
> However, we can use the PFB to enable 'something else'; namely to make
> Rl 'appear' as if it's infinite (bootstrap). If I do that, by
> selecting the feedback ratio to match the expected plate swing, which
> we can estimate by mu since an infinite load should make gain equal to
> mu, then
>
> Gain with selected PFB is indeed 20 and 2'nd harmonic becomes a
> glorious -131dB. (an unrealistic number indicating the limits of the
> spice simulation)


Huh, you jest, surely?


>
> It's the special case (the 'something else') of Rl appearing to be
> (near) *infinite* that lowers the distortion, not the PFB (which was
> illustrated by the first example).


The increase in RL via bootstrapping value lowers 6SN7 THD but creates
a PFB path to make OP stage THD greater. Its about that simple.


>
> By "not the PFB" I mean one could, in theory at least, put a 'real'
> infinite resistor, with an infinite B+ to supply it, there and get
> even better distortion numbers, because PFB increases distortion over
> what the equivalent circuit has (and note than an infinite load is not
> the same circuit as one with a 27k load), but that's not terribly
> practical while PFB is.


I've done all this and have avoided bootstrapping wherever I can
because I wanted the full 5 steps forward benefit of driver linearity
without the 2 steps backward which PFB brings.
But bootstrapping can give you 3 steps forward.

One can easily arrange each side of a balanced amp to be a pair of
triodes in a follower config to make the DC carrying RL far larger
than 27k. Or one may use a solid state CCS. The cap coupled output
load is the biasing resistor, and its value becomes the main load for
the gain triode.

I found the use of a CT choke plus series RL at each end gave best
results.

http://www.turneraudio.com.au/300w-1...tput-jan06.htm

But people hate chokes, so they bootstrap instead !!!

>
> We could also achieve the same result with a constant current source
> on the plate (which also simulates a near 'infinite R') so it's
> clearly not PFB that 'does the good thing'. It's the artificially
> created (near) 'infinite load' that lowers distortion.


Indeed.


>
> PFB enables use to do (in special cases) *'something else' (apply
> GNFB, simulate an 'infinite R', etc) that would be more difficult to
> achieve by other means.
>
> In real life it won't be that good because my sim uses a 'pure' sine
> for PFB and not a distorted OPT feedback.


Using any useful amount of PFB in mass market amps has been avoided
like the plague by all manufacturers.

Its just too difficult to get right.

Its SO EASY to just arrange a suitable amount of OLG and apply only
NFB.

Patrick Turner.
>
>
>
> >Patrick Turner.- Hide quoted text -

>
> - Show quoted text -- Hide quoted text -
>
> - Show quoted text -


  #10  
Old March 19th 11, 08:42 PM
John L Stewart John L Stewart is offline
Senior Member
 
First recorded activity by AudioBanter: Jan 2011
Location: Toronto
Posts: 267
Default

Quote:
Originally Posted by flipper View Post
On Wed, 16 Mar 2011 13:36:45 +0000, John L Stewart
> wrote:

>
>First of all, calculate the driver gain in grounded cathode mode from
>published specs-
>
>For the 6SN7 family at 250 volts supply
> Mu = 20
> Rp = 7.7 K while grid volts is 8
>
>Let A1 be the gain with these conditions,
>Then A1 = ( mu*Rl ) / ( rp + Rl )
> = ( 20* 27 ) / ( 7.7 + 27 )
> = 15.56
>
>Then find the maximum gain possible for any non-feedback triode stage-
>
>Then A2 = mu
> = 20
>
>Gain change is A2/A1
> = 20 /15.56
>= 1.29
>
>In DB becomes 20 log A2/A1
> = 2.18 db
>
>The feed back in the case of the bootstrapped driver is to the triodes
>anode, so no gain above mu is possible. There is no gain thru the triode
>being plate driven.
>
>NTL, some experimenters have actually built working amplifiers this way
>using many triodes such as the 6S4 & 12B4. Starting digging, you will
>find them.
>
>Hope this all mkes sense. Cheers to all, John


Maybe I'm not thinking straight but I don't see that answering the
question because it depends on what appears to be an arbitrary claim
that gain cannot exceed mu. I mean, it begs the question by declaring
a limit, which is the essence of the question.

Are you presuming that the PFB is designed to produce a unity voltage
feedback (relative to the anode signal swing) so that Rl 'appears'
infinite?

If that's the case then it's trivial to show from the gain equation
that gain approaches, and does not exceed, mu but it's not intuitively
obvious from the schematic that's the amount of feedback being
applied.

If the amount of voltage feedback exceeds plate swing then gain can,
indeed, be above mu.

To illustrate I spiced a simple circuit with a 250V source having
100Vrms on it (simulating an arbitrary transformer feedback signal)
applied to a 6SN7 plate through a 27k and cathode to ground through an
unbypassed 470 ohm. Opposite phase 1Vrms (for visual 'instant math'
convenience) was then applied to the grid.

Plate swing was 52Vrms for a gain of 52.
------------------------------------------------------------------

Hi Flipper- Sounds intriging & I am curious as usual. Could you post a JPG of your simulation so I can try it here?

Far as I know the only way to get gain more tham mu is thru a transformer as the ancients did it before pentodes (or tetrodes) or some kind of PFB applied to the grid or cathode.

And the kind you don't want by way of parasitics.

I did some quick tests on the first of the Twin Coupled Amps I had built a few years back. The final in this thing runs 6LU8s in PP, the triode sections cathode coupled to the OP grids. At clipping it is good for 37 watts. IMD is down more than 50 db at one db below max OP.

It was nice that the caps didn't blow since it has not been run in a few years. Tests are done using a HV Diff Probe into a PICO Scope ADC216. Does scope, SA & RMS volts with 16 bit resolution.

With the 6SN7 27K plate resistors returned to the 400 volt regulated supply rather than thru the PFB circuit the gain was 11.4.

Then reconnected to the PFB circuit the SN7 gain was 16.

If you do the math, turns out to be 3 db, somewhat higher than I recall. I may have done that kind of test on one of the other bootstrapped amps I mentioned in a previous post. Oddly I found I had used a tail resistor of 33K to the -150 volt supply on this 6SN7. Usually I've put in more like 18K. That would account for some of the differance since the SN7 is working at a higher plate resistance here.

If you can't get that JPG on here then email to me direct at

Cheers, John
 




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