[Phil Allison[_3_]]

** Hi all,

customer brought me his Fender Champ clone, 5E1 version.

http://www.webphix.com/schematic%20h..._5e1_schem.pdf

There were a couple of changes including a "triode /pentode" switch and the
use of a 6X4 rectifier in lieu of a 5Y3. The amp was fairly well built on
an aluminium chassis by a local Chinese gent using a mix of Chinese,
Japanese and Russian made parts. The 6V6 was in fact 6n6c with blackened
glass, like these:

http://www.decware.com/newsite/images/6p6s.jpg

Closer inspection showed something odd about the 6X4 wiring, the plates both
went direct to ground and there was a standby switch that linked a 56 ohm 5W
to the cathode. The other end of the 56 ohm went to the negative of a bridge
rectifier made from four 1N4007s.

At idle, the amp drew about 43mA from the 300V HT and the voltage on the
cathode of the 6X4 was -7 volts.

So the only function of the 6X4 was to create a warm up delay and confuse
the owner.

Cute eh ?


.... Phil

[GRe]

"Phil Allison" wrote in message
...

** Hi all,

customer brought me his Fender Champ clone, 5E1 version.

http://www.webphix.com/schematic%20h..._5e1_schem.pdf

There were a couple of changes including a "triode /pentode" switch and
the use of a 6X4 rectifier in lieu of a 5Y3. The amp was fairly well
built on an aluminium chassis by a local Chinese gent using a mix of
Chinese, Japanese and Russian made parts. The 6V6 was in fact 6n6c with
blackened glass, like these:

http://www.decware.com/newsite/images/6p6s.jpg

Closer inspection showed something odd about the 6X4 wiring, the plates
both went direct to ground and there was a standby switch that linked a 56
ohm 5W to the cathode. The other end of the 56 ohm went to the negative of
a bridge rectifier made from four 1N4007s.

At idle, the amp drew about 43mA from the 300V HT and the voltage on the
cathode of the 6X4 was -7 volts.

So the only function of the 6X4 was to create a warm up delay and confuse
the owner.

Cute eh ?


... Phil




Something like this?

Gio.


|(-----------+
|( |
|( | 4x 1N4007
----)|( / \ /
)|( / \
~ )|( +--- -----------+---- HT
)|( | \ / |
----)|( | \ / |
|( | | ---
|( | | ---
|(-----)-----+ |
| |
| GND
+-+
| |56
| |5W
+-+
|
|
\ Standby
\ Switch
|
|
O C
_ _ 6X4
A1| |A2
| |
| |
GND GND

[Phil Allison[_3_]]

"GRe"
"Phil Allison"

So the only function of the 6X4 was to create a warm up delay and confuse
the owner.

Cute eh ?

Something like this?



|(-----------+
|( |
|( | 4x 1N4007
----)|( / \ /
)|( / \
~ )|( +--- -----------+---- HT
)|( | \ / |
----)|( | \ / |
|( | | ---
|( | | ---
|(-----)-----+ |
| |
| GND
+-+
| |56
| |5W
+-+
|
|
\ Standby
\ Switch
|
|
O C
_ _ 6X4
A1| |A2
| |
| |
GND GND



** Eeeee- yep.

You seen it done before somewhere ??



.... Phil

[GRe]

"Phil Allison" wrote in message
...

"GRe"
"Phil Allison"

So the only function of the 6X4 was to create a warm up delay and
confuse the owner.

Cute eh ?

Something like this?



|(-----------+
|( |
|( | 4x 1N4007
----)|( / \ /
)|( / \
~ )|( +--- -----------+---- HT
)|( | \ / |
----)|( | \ / |
|( | | ---
|( | | ---
|(-----)-----+ |
| |
| GND
+-+
| |56
| |5W
+-+
|
|
\ Standby
\ Switch
|
|
O C
_ _ 6X4
A1| |A2
| |
| |
GND GND



** Eeeee- yep.

You seen it done before somewhere ??



... Phil


No, I did'nt see it before. I distilled it from your description but was'nt
sure if the posted schematic matched to the modification in the Fender clone
respectively to what you described. That's why I asked "something like
this?".

BTW, maybe the only intended function of the circuit was to create a warm up
delay but in that case a waste product of the circuit is that it adds tube
rectifier impedance.

Gio.

[John L Stewart]

No, I did'nt see it before. I distilled it from your description but was'nt
sure if the posted schematic matched to the modification in the Fender clone
respectively to what you described. That's why I asked "something like
this?".

BTW, maybe the only intended function of the circuit was to create a warm up
delay but in that case a waste product of the circuit is that it adds tube
rectifier impedance.

Gio.[/quote]

But compared to the original 5Y3 rectifier cct it is an improvement. Forward drop of each 6X4 plate is less than in the 5Y3, see p1171 RDH4. And they are in parallel here for the load. Heater is only 3.6 W vs 10 W for the 5Y3.

Low cost to get & looks like an idea!

Cheers to all, John

[Phil Allison[_3_]]

"GRe"

BTW, maybe the only intended function of the circuit was to create a warm
up delay but in that case a waste product of the circuit is that it adds
tube rectifier impedance.

** The paralleled 6X4 plates act like a 150 ohm resistor - along with the 56
ohm 5W plus the resistance in the transformer secondary of about 180 ohms.
All helps to reduce the ripple voltage on the HT.

But there is no sag under load as the current draw is constant.



.... Phil

[Patrick Turner]

On Jun 1, 5:57*pm, "Phil Allison" wrote:
** Hi all,

customer brought me his Fender Champ clone, 5E1 version.

http://www.webphix.com/schematic%20h...heaven.com/fen...

There were a couple of changes including a "triode /pentode" switch and the
use of a 6X4 rectifier in lieu of a 5Y3. *The amp was fairly well built on
an aluminium chassis by a local Chinese gent using a mix of Chinese,
Japanese and Russian made parts. The 6V6 was in fact 6n6c with blackened
glass, like these:

http://www.decware.com/newsite/images/6p6s.jpg

Closer inspection showed something odd about the 6X4 wiring, the plates both
went direct to ground and there was a standby switch that linked a 56 ohm 5W
to the cathode. The other end of the 56 ohm went to the negative of a bridge
rectifier made from four 1N4007s.

At idle, the amp drew about 43mA from the 300V HT and the voltage on the
cathode of the 6X4 was -7 volts.

So the only function of the 6X4 was to create a warm up delay and confuse
the owner.

Cute eh ?

... *Phil

That's the gist of it Phil. I've tried GZ34 used as a slow turn on
series diode in the B+ path but it turns on faster than output tubes
so the B+ still soars before dropping 10 seconds later with Idc draw
in OP tubes. Use of 6X4 as they have it is probably benign, as long as
Vpeak does not exceed ratings ec, and arcing does not occur. With 56
ohms and 43mA, there should be 2.4Vdc across the 56 ohms, but there is
also Vac, got to be careful what is being measured, but 7Vdc across
the 6X4 Va-k seems about right, and indicates the Ra is higher than 56
ohms.
The Ra of 6X4 plus the 56 ohms maybe keeps B+ at about the same level
as if 5Y3 had been used as a rectifier. 5Y3 has direct heated cathode
and comes on within seconds, so the original circuit with CT winding
for full wave rect would have had el-caps rated for a high peak
Vdc.

Patrick Turner.

[John L Stewart]

Originally Posted by Phil Allison[_3_]

"GRe"

BTW, maybe the only intended function of the circuit was to create a warm
up delay but in that case a waste product of the circuit is that it adds
tube rectifier impedance.

** The paralleled 6X4 plates act like a 150 ohm resistor - along with the 56
ohm 5W plus the resistance in the transformer secondary of about 180 ohms.
All helps to reduce the ripple voltage on the HT.

But there is no sag under load as the current draw is constant.



.... Phil

And the rectifier system is symmetrical (FWB), so makes better use of the HV winding when compared to a full wave CT, something called Utilty Factor. As well, symmetry contributes no Power Frequency component (50 or 60 Hz) to the rectified waveform, so easier to filter.

All looks good at low cost.

Cheers, John

[Phil Allison[_3_]]

"John L Stewart"

And the rectifier system is symmetrical (FWB), so makes better use of
the HV winding when compared to a full wave CT, something called Utilty
Factor. As well, symmetry contributes no Power Frequency component (50
or 60 Hz) to the rectified waveform, so easier to filter.

All looks good at low cost.

** Why not leave out the 6X4 completely and save even more cost - it does
nothing really.

Simple fact is, the previously mentioned Chinese gent had a nice PT on hand
with a 6.3 volt heater winding ( no 5V) plus
a 230V secondary with no CT and needed to use a tube rectifier for
appearances sake to mimic the Champ schem.

So this cunning dodge with the 6X4 was his way out.

Reminds me of that famous Confucius saying.:

" Man who go to bed with stiff problem,
likely to wake up with solution in hand ".



.... Phil

[Roger Jones]

On Jun 1, 3:57*am, "Phil Allison" wrote:
** Hi all,

customer brought me his Fender Champ clone, 5E1 version.

http://www.webphix.com/schematic%20h...heaven.com/fen...

There were a couple of changes including a "triode /pentode" switch and the
use of a 6X4 rectifier in lieu of a 5Y3. *The amp was fairly well built on
an aluminium chassis by a local Chinese gent using a mix of Chinese,
Japanese and Russian made parts. The 6V6 was in fact 6n6c with blackened
glass, like these:

http://www.decware.com/newsite/images/6p6s.jpg

Closer inspection showed something odd about the 6X4 wiring, the plates both
went direct to ground and there was a standby switch that linked a 56 ohm 5W
to the cathode. The other end of the 56 ohm went to the negative of a bridge
rectifier made from four 1N4007s.

At idle, the amp drew about 43mA from the 300V HT and the voltage on the
cathode of the 6X4 was -7 volts.

So the only function of the 6X4 was to create a warm up delay and confuse
the owner.

Cute eh ?

... *Phil

Neat idea for an "iffy" tube... or is that the 6X5? IIRC, one of these
is prone to heater cathode insulation failure, as may be the EZ80 and
EZ81 but you hear less about these two. Anyway, this scheme puts the
minimal voltage across h-k.
I have to modify a Stewart-Warner table radio containing 6X5 before it
catches fire!
Cheers,
Roger

[GRe]

"Roger Jones" wrote in message
...
On Jun 1, 3:57 am, "Phil Allison" wrote:

Closer inspection showed something odd about the 6X4 wiring, the plates
both
went direct to ground and there was a standby switch that linked a 56 ohm
5W
to the cathode. The other end of the 56 ohm went to the negative of a
bridge
rectifier made from four 1N4007s.

At idle, the amp drew about 43mA from the 300V HT and the voltage on the
cathode of the 6X4 was -7 volts.

So the only function of the 6X4 was to create a warm up delay and confuse
the owner.
?
Cute eh ?

... Phil

Neat idea for an "iffy" tube... or is that the 6X5? IIRC, one of these
is prone to heater cathode insulation failure, as may be the EZ80 and
EZ81 but you hear less about these two. Anyway, this scheme puts the
minimal voltage across h-k.
I have to modify a Stewart-Warner table radio containing 6X5 before it
catches fire!
Cheers,
Roger


Although Vk-h is minimal during normal operation, immediatly after power-on
the cathode to heater voltage is high and NEGATIVE (heater potential near
GND assumed).
For 6X4 the paper spec.(RCA) is 450Vk-h max. with cathode either pos. or
neg. in respect to heater.
Not so for EZ80/EZ81, Vk-h spec.(Philips) is 500V max. but only for cathode
positive in respect to heater.

Latter spec's are for tubes at operating temp., what else, so the question
is what negative Vk-h is allowed with a cold cathode or better a cathode
that is just starting to be warm?

Gio.

[Patrick Turner]

On Jun 2, 11:37*pm, "Phil Allison" wrote:
"John L Stewart"



And the rectifier system is symmetrical (FWB), so makes better use of
the HV winding when compared to a full wave CT, something called Utilty
Factor. As well, symmetry contributes no Power Frequency component (50
or 60 Hz) to the rectified waveform, so easier to filter.

All looks good at low cost.

** Why not leave out the 6X4 completely and save even more cost - *it does
nothing really.

Indeed, why not? One would have never seen such tricks in 1960.
But the 6X4 gives delayed turn on, something that has become
fashionable over the last 20 years.

Simple fact is, the previously mentioned Chinese gent had a nice PT on hand
with a 6.3 volt heater winding ( no 5V) plus
a 230V secondary with no CT and needed to use a tube rectifier for
appearances sake to mimic the Champ schem.

So this cunning dodge with the 6X4 was his way out.

Point taken. Another common trick these daze for where there isn't a
tranny available for a tube project, and only the junk at Jaycar, is
to use a pair of 240V:30V trannies, each with 30V tapped.
One is used just to get filament heating, arranged as 6.3V or 12.6V or
18.9V or 25.2V, and the second tranny is backwards off the first
tranny sec to get 240V HT which gives about 300Vdc under load, quite
OK for a preamp where trannies are small and losses are low. Also OK
for an old radio where there is room on the chassis or a low power
geeetar amp. The first tranny can be higher VA rated than the second
one.

Reminds me of that famous Confucius saying.:

" Man who go to bed with stiff problem,
*likely to wake up with solution in hand ".

I thought it that one came from Con Fuctos, an old greek guy, who
found the price of a root had gone up.

Patrick Turner.


... * Phil

[Nick Gorham]

On 02/06/12 01:16, Phil Allison wrote:
"GRe"

BTW, maybe the only intended function of the circuit was to create a warm
up delay but in that case a waste product of the circuit is that it adds
tube rectifier impedance.

** The paralleled 6X4 plates act like a 150 ohm resistor - along with the 56
ohm 5W plus the resistance in the transformer secondary of about 180 ohms.
All helps to reduce the ripple voltage on the HT.

But there is no sag under load as the current draw is constant.

Why would you assume the current draw is constant?

The power that drives the loudspeaker has to come from somewhere.

Just because a class A output stage constantly draws current, that
doesn't mean it draws constant current.

--
Nick

[Phil Allison[_3_]]

"Nick Gorham"

Phil Allison wrote:


But there is no sag under load as the current draw is constant.

Why would you assume the current draw is constant?

** Oh dear ......

Having a steady current draw is characteristic of all class A stages.


The power that drives the loudspeaker has to come from somewhere.


** Oh dear - we have ourselves a * live one * here folks....


Just because a class A output stage constantly draws current, that doesn't
mean it draws constant current.

** How utterly asinine.

Go look the topic up sometime - ****wit.



..... Phil

[Nick Gorham]

On 07/06/12 14:01, Phil Allison wrote:
"Nick Gorham"

Phil Allison wrote:


But there is no sag under load as the current draw is constant.

Why would you assume the current draw is constant?

** Oh dear ......

Having a steady current draw is characteristic of all class A stages.


The power that drives the loudspeaker has to come from somewhere.


** Oh dear - we have ourselves a * live one * here folks....


Just because a class A output stage constantly draws current, that doesn't
mean it draws constant current.

** How utterly asinine.

Go look the topic up sometime - ****wit.

You do realize the Fender Champ had a single ended output stage?

--
Nick

[John L Stewart]

Originally Posted by Nick Gorham

On 02/06/12 01:16, Phil Allison wrote:
"GRe"

BTW, maybe the only intended function of the circuit was to create a warm
up delay but in that case a waste product of the circuit is that it adds
tube rectifier impedance.

** The paralleled 6X4 plates act like a 150 ohm resistor - along with the 56
ohm 5W plus the resistance in the transformer secondary of about 180 ohms.
All helps to reduce the ripple voltage on the HT.

But there is no sag under load as the current draw is constant.

Why would you assume the current draw is constant?

The power that drives the loudspeaker has to come from somewhere.

Just because a class A output stage constantly draws current, that
doesn't mean it draws constant current.

--
Nick

No surprises here, the total plate current draw in SE or PP for a Class A power amp is not constant. As the OP level increases, so does the total plate current. Something called rectification. I will leave it to the curious to research that one!

The positive going plate current excursions are larger than the negative going so average plate current goes up, but not a lot. The difference is the cause of the 2H in the OP of a SE amp, whether triode or pentode.

For example, the published specs for a SE 6L6 with supply at 250 volts show no signal plate current at 75 mA while at max OP it increases to 78 mA. Similarly there are many other examples. Space charge tubes go the other way, but that is another story.

So in general, a class A power amp don't need a well regulated supply. But it does need a good PS bypass cap, that will help determine the LF operation limit.

Cheers, John

[Phil Allison[_3_]]

"John L Stewart"

No surprises here, the total plate current draw in SE or PP for a Class
A power amp is not constant.

** Wrong.

As the OP level increases, so does the
total plate current.

** Wrong.


For example, the published specs for a SE 6L6 with supply at 250 volts
show no signal plate current at 75 mA while at max OP it increases to 78
mA.


** That is only a 4% increase and would be much less with some NFB applied.

****ing nit picker.

[Phil Allison[_3_]]

"Nick Gorham"
Phil Allison wrote:
"Nick Gorham"

Phil Allison wrote:


But there is no sag under load as the current draw is constant.

Why would you assume the current draw is constant?

** Oh dear ......

Having a steady current draw is characteristic of all class A stages.


The power that drives the loudspeaker has to come from somewhere.


** Oh dear - we have ourselves a * live one * here folks....


Just because a class A output stage constantly draws current, that
doesn't
mean it draws constant current.

** How utterly asinine.

Go look the topic up sometime - ****wit.

You do realize the Fender Champ had a single ended output stage?


** Duhhhhh !!

[flipper]

On Thu, 07 Jun 2012 12:48:45 +0100, Nick Gorham
wrote:

On 02/06/12 01:16, Phil Allison wrote:
"GRe"

BTW, maybe the only intended function of the circuit was to create a warm
up delay but in that case a waste product of the circuit is that it adds
tube rectifier impedance.

** The paralleled 6X4 plates act like a 150 ohm resistor - along with the 56
ohm 5W plus the resistance in the transformer secondary of about 180 ohms.
All helps to reduce the ripple voltage on the HT.

But there is no sag under load as the current draw is constant.

Why would you assume the current draw is constant?

The power that drives the loudspeaker has to come from somewhere.

It comes from the plate. Or, to be more precise, it comes from the
power supply and goes to the speaker *instead* of being dissipated on
the plate. Say you have 12 Watt idle plate dissipation. At 5 Watt Po,
including losses, you then have 7 Watt plate dissipation. You simply
alter 'where' the same amount of power goes.

The extreme case of feeding a square wave makes it easy to visualize.
In that case, if the tube were perfect with 0 transition times and
able to saturate to 0 V, plate dissipation would be 0 because at
maximum current the plate is pulled down to 0 Volts and at maximum
voltage plate current is 0 mA (cut off) and, in both case, 0 times
anything is zero. But it's the opposite across the load. Max current
is max voltage drop across it for maximum power, with 0 power the
other half cycle since current is 0. For the hypothetical 12 Watt
plate make that 300 V B+ at 40 mA. Max across the load is 300 V at 80
mA (twice idle) for 24 Watt and half that is 12 Watt to the load with
0 plate Watt. Turn off the square wave and you have 12 plate Watt (300
V at 40 mA) with 0 Watt to the load. It simply 'shifts' where the same
12 Watt goes.

(In reality the tube doesn't have 0 transition times and can't pull to
0 V so there is still plate dissipation, and correspondingly less to
the load, but the theoretical point remains. Technically that's Class
C and it should be obvious why RF uses it for increased power
efficiency).

Look on the bright side, it 'costs nothing more' to crank up the
volume because you're either going to burn it on the plate or burn it
as sound.

The black cloud behind that silver lining is you're burning 12 Watt no
matter what, including when doing nothing.

Just because a class A output stage constantly draws current, that
doesn't mean it draws constant current.

It does by saying "Class A" because that's how Class A works.

[Patrick Turner]

On Jun 7, 11:01*pm, "Phil Allison" wrote:
"Nick Gorham"



Phil Allison wrote:

But there is no sag under load as the current draw is constant.

Why would you assume the current draw is constant?

** *Oh dear ......

Having a steady current draw is characteristic of all class A stages.

The power that drives the loudspeaker has to come from somewhere.

** Oh dear *- *we have ourselves a * live one * *here folks....

Just because a class A output stage constantly draws current, that doesn't
mean it draws constant current.

** How utterly asinine.

* *Go look the topic up sometime - *****wit.

.... *Phil

Yes, pennies take awhile to drop.

But a class A SE output tube often does have a slight *increase* in
Idc drawn from the PSU between 0.0 Watts and up to where clipping is
just beginning to occur when THD may be 5% and mainly 2H, depending on
type type and the the RL value. In fact, the old fashioned way of
measuring 2H was to measure the Idc increase and apply a simple
formula which radio engineers in 1950 were expected to remember. I
don't remember it.

But in all class A amps, the Pda at idle is when the maximum amount of
heat in Watts is liberated from the tube, and this Pda becomes minimal
when the tube is asked to produce audio power, so an EL34 in triode
may dissipate 24Watts at idle, but when producing 7 Watts at clipping
the tube dissipates approximately 17 Watts, so it runs cooler. The 7
watts of audio power is mainly heat in the voice coil of the speaker,
which is often usually only 3% efficient. During the process, the
power drawn from PSU varies from 24 Watts at idle to maybe 25Watts at
clipping because of the 2H distortion, ie, rectification effects which
need to be understood by study elsewhere. But the change of Ia between
idle and clipping is usually so little, the Ia is said to be constant.
In an SE pentode class A amp the Ig2 can increase to complicate
matters. If lots of NFB is applied, then rectifying effects diminish
and hence 2H current also is reduced so Ia remains more constant up to
clipping. Beyond clipping, Ia changes considerably.

If the SE amp is heavily over driven as musicians like to do, then the
grid cap charges up and tube begins working in class C and it makes
square waves. Maybe you get slightly more PO, when THD is about 30%+.
PP AB amps at clipping or beyond can draw maybe 4 times the current
from a PSU, a huge Ia increase, far more than any amp kept working in
pure class A.
With sine wave production on class A with THD 5%, pentodes give up
to 45% efficiency, and triodes maybe 33% depending on set up. 50% is
the maximum theoretical efficiency of any class A amp, ie, if you have
100W drawn from a PSU at idle, then the pure class A audio power can't
be more than 50W. A number of amp designs have been set up to draw say
25Watts from a PSU at idle, when maybe 12 Watts of class A could be
possible, and then as the audio power increases the Idc is increased
from the PSU to say a maximum of 100W, and then 45W of class A is
made. Such amps are siad to have "sliding bias". A number of solid
state amps employed such techniques to allow low current bias and low
amp heat for most ppl most of the time who in fact rarely need more
than a couple of watts for hi-fi listening. Many claims have been made
about such amps. When the sliding bias circuit malfunctions, they can
make lots of smoke.

Patrick Turner.

[Nick Gorham]

On 08/06/12 08:00, Patrick Turner wrote:
On Jun 7, 11:01 pm, "Phil wrote:
"Nick Gorham"



Phil Allison wrote:

But there is no sag under load as the current draw is constant.

Why would you assume the current draw is constant?

** Oh dear ......

Having a steady current draw is characteristic of all class A stages.

The power that drives the loudspeaker has to come from somewhere.

** Oh dear - we have ourselves a * live one * here folks....

Just because a class A output stage constantly draws current, that doesn't
mean it draws constant current.

** How utterly asinine.

Go look the topic up sometime - ****wit.

.... Phil

Yes, pennies take awhile to drop.

Ok, simple question, if the current in the output stage of a single
ended amplifier is constant, then why does bipassing the cathode
resistor make a difference?

--
Nick

[Phil Allison[_3_]]

"Nick Gorham"
Patrick Turner wrote:
"Phil Allison
"Nick Gorham"



Phil Allison wrote:

But there is no sag under load as the current draw is constant.

Why would you assume the current draw is constant?

** Oh dear ......

Having a steady current draw is characteristic of all class A stages.

The power that drives the loudspeaker has to come from somewhere.

** Oh dear - we have ourselves a * live one * here folks....

Just because a class A output stage constantly draws current, that
doesn't
mean it draws constant current.

** How utterly asinine.

Go look the topic up sometime - ****wit.


Yes, pennies take awhile to drop.

Ok, simple question, if the current in the output stage of a single ended
amplifier is constant,


** Not what was ever claimed - you illiterate, bull****ting ****.



.... Phil

[flipper]

On Fri, 08 Jun 2012 12:16:44 +0100, Nick Gorham
wrote:

On 08/06/12 08:00, Patrick Turner wrote:
On Jun 7, 11:01 pm, "Phil wrote:
"Nick Gorham"



Phil Allison wrote:

But there is no sag under load as the current draw is constant.

Why would you assume the current draw is constant?

** Oh dear ......

Having a steady current draw is characteristic of all class A stages.

The power that drives the loudspeaker has to come from somewhere.

** Oh dear - we have ourselves a * live one * here folks....

Just because a class A output stage constantly draws current, that doesn't
mean it draws constant current.

** How utterly asinine.

Go look the topic up sometime - ****wit.

.... Phil

Yes, pennies take awhile to drop.

Ok, simple question, if the current in the output stage of a single
ended amplifier is constant, then why does bipassing the cathode
resistor make a difference?

The "constant current" being referred to is RMS. Instantaneous is not
'constant', except at idle, or else there would be no 'sine wave', or
any other signal.

Point of fact, though, you do not 'have to' bypass Rk but doing so
increases gain because it removes the negative feedback caused by the
current induced voltage across Rk.

Either way, more or less gain, Class A simply 'shifts' power from the
plate to the load.

[John L Stewart]

Originally Posted by Phil Allison[_3_]

"John L Stewart"

No surprises here, the total plate current draw in SE or PP for a Class
A power amp is not constant.

** Wrong.

As the OP level increases, so does the
total plate current.

** Wrong.


For example, the published specs for a SE 6L6 with supply at 250 volts
show no signal plate current at 75 mA while at max OP it increases to 78
mA.


** That is only a 4% increase and would be much less with some NFB applied.

****ing nit picker.

Refer to p551 in RDH4 for a discussion of rectification effects in amplifiers. There is a graphical method of analysis given.

FE Terman 4th Ed & many other texts cover rectification in amplifiers by way of Fourier Series. As the signal level increases all the even order (2nd, 4th, 6th....Etc) in the OP increase. That includes the zero:th order, the DC component.

Ordinary NFB has no long time effect, the cct is AC coupled.

Cheers to all, John

[Patrick Turner]

On Jun 10, 8:44*pm, John L Stewart
wrote:
'Phil Allison[_3_ Wrote:





;957993']"John L Stewart"-

No surprises here, the total plate current draw in SE or PP for a
Class
A power amp is not constant.-

** Wrong.
-
As the OP level increases, so does the
total plate current.-

** Wrong.

-
For example, the published specs for a SE 6L6 with supply at 250
volts
show no signal plate current at 75 mA while at max OP it increases to
78
mA.-

** That is only a 4% increase and would be much less with some NFB
applied.

****ing nit picker.

Refer to p551 in RDH4 for a discussion of rectification effects in
amplifiers. There is a graphical method of analysis given.

FE Terman 4th Ed & many other texts cover rectification in amplifiers by
way of Fourier Series. As the signal level increases all the even order
(2nd, 4th, 6th....Etc) in the OP increase. That includes the zero:th
order, the DC component.

Ordinary NFB has no long time effect, the cct is AC coupled.

Cheers to all, John

Indeed John. I might predict the graphical method of working out the
change in Ia dc from PSU at RDH4 pg 551 is maybe of interest to
someone somewhere, not sure who, but I've never tried to predict
rectifier effect. I prefer to just set up the class A tube, and then
measure what happens. Usually one never ever has to worry about
rectifier effects in SE class A tubes because average PO 1/10 of
clipping PO, and where there is a Rk&Ck bias network, the Ek hardly
moves at all with music when its level doesn't cause clipping.
But in class AB amps, rectifying effects are so great the use of Rk&Ck
biasing is unwise if high PO is envisaged as for geeeeetar amps. For a
hi-fi amp where 1/3 of the maximum AB PO is pure class A and which
covers all of what someone listens to except for the wildest of drum
beats, then Rk&Ck biasing is OK, especially where one uses say 470uF
or 1,000uF for Ck instead of the usual 47uF used in 1960.

Patrick T.

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John L Stewart- Hide quoted text -

- Show quoted text -

[Roger Jones]

On Jun 4, 4:12*am, "GRe" wrote:
"Roger Jones" wrote in message

...
On Jun 1, 3:57 am, "Phil Allison" wrote:











Closer inspection showed something odd about the 6X4 wiring, the plates
both
went direct to ground and there was a standby switch that linked a 56 ohm
5W
to the cathode. The other end of the 56 ohm went to the negative of a
bridge
rectifier made from four 1N4007s.

At idle, the amp drew about 43mA from the 300V HT and the voltage on the
cathode of the 6X4 was -7 volts.

So the only function of the 6X4 was to create a warm up delay and confuse
the owner.
?
Cute eh ?

... Phil

Neat idea for an "iffy" tube... or is that the 6X5? IIRC, one of these
is prone to heater cathode insulation failure, as may be the EZ80 and
EZ81 but you hear less about these two. *Anyway, this scheme puts the
minimal voltage across h-k.
I have to modify a Stewart-Warner table radio containing 6X5 before it
catches fire!
Cheers,
Roger

Although Vk-h is minimal during normal operation, immediatly after power-on
the cathode to heater voltage is high and NEGATIVE (heater potential near
GND assumed).
For 6X4 the paper spec.(RCA) is 450Vk-h max. with cathode either pos. or
neg. in respect to heater.
Not so for EZ80/EZ81, Vk-h spec.(Philips) is 500V max. but only for cathode
positive in respect to heater.

Latter spec's are for tubes at operating temp., what else, so the question
is what negative Vk-h is allowed with a cold cathode or better a cathode
that is just starting to be warm?

Gio.

You're right... thanks for the insight. Indeed, the transient h-k
voltage is very high! Another great idea perishes in the light of
day.
It seems that there's just no work-around for those 6X5 time-bombs!...
except good, fast fuses!
Cheers,
Roger