Slightly odd output stage configuration. Thoughts ?

[Arfa Daily]

A muso customer has asked me to carry out some tonal mods to his Peavey
Classic 30. All pretty straightforward cap changes and so on. However, he
also asked me to look into fitting a pentode / triode switch whilst I was at
it. At first glance, this didn't look like a problem. The output stage is a
fairly classic AB class using 4 x EL84 tubes as two paralleled pairs. Anodes
of each pair commoned. Cathodes all commoned and decked. Grids of each pair
commoned via individual 47k stoppers. But then, things get a little odd
around the screen grids. One tube of each pair, has its screen grid fed by a
100 ohm 5 watt resistor off the "screen" supply rail, whilst the other of
each pair, has its screen grid fed *direct* from the "screen" supply rail.

I don't think I've seen this done before. I've seen one screen resistor
feeding both tubes of a pair, or one resistor per screen, but not just one
tube having a screen feed resistor. Apart from anything else, I wouldn't
normally have considered it very good design practice to have no current
limiting at all in place. Also, it will mean that the screen voltage will be
higher on one tube of the pair, than the other. I doubt that it would have a
significant effect on the operation of the stage, but just interested as to
whether anyone else has come across this configuration, and knows the design
reasoning behind it. Anyone got any opinions about just strapping the
screens to the anodes on each pair via a switch, to implement a triode mode
?

Arfa

 

[Meat Plow]

On Fri, 25 Feb 2011 17:00:41 +0000, Arfa Daily wrote:

A muso customer has asked me to carry out some tonal mods to his Peavey
Classic 30. All pretty straightforward cap changes and so on. However,
he also asked me to look into fitting a pentode / triode switch whilst I
was at it. At first glance, this didn't look like a problem. The output
stage is a fairly classic AB class using 4 x EL84 tubes as two
paralleled pairs. Anodes of each pair commoned. Cathodes all commoned
and decked. Grids of each pair commoned via individual 47k stoppers. But
then, things get a little odd around the screen grids. One tube of each
pair, has its screen grid fed by a 100 ohm 5 watt resistor off the
"screen" supply rail, whilst the other of each pair, has its screen grid
fed *direct* from the "screen" supply rail.

I don't think I've seen this done before. I've seen one screen resistor
feeding both tubes of a pair, or one resistor per screen, but not just
one tube having a screen feed resistor. Apart from anything else, I
wouldn't normally have considered it very good design practice to have
no current limiting at all in place. Also, it will mean that the screen
voltage will be higher on one tube of the pair, than the other. I doubt
that it would have a significant effect on the operation of the stage,
but just interested as to whether anyone else has come across this
configuration, and knows the design reasoning behind it. Anyone got any
opinions about just strapping the screens to the anodes on each pair via
a switch, to implement a triode mode ?

Arfa

Several amp and tone stack mods. Variable cathode bias, OD mod on CH2, OT
replacement, speaker replacement.

blueguitar.org/new/articles/blue_gtr/amps/peavey/c30_origmod.pdf



--
Live Fast, Die Young and Leave a Pretty Corpse

 

[Phil Allison]

"Arfa Daily"

A muso customer has asked me to carry out some tonal mods to his Peavey
Classic 30. All pretty straightforward cap changes and so on. However, he
also asked me to look into fitting a pentode / triode switch whilst I was
at it. At first glance, this didn't look like a problem. The output stage
is a fairly classic AB class using 4 x EL84 tubes as two paralleled pairs.
Anodes of each pair commoned. Cathodes all commoned and decked. Grids of
each pair commoned via individual 47k stoppers. But then, things get a
little odd around the screen grids. One tube of each pair, has its screen
grid fed by a 100 ohm 5 watt resistor off the "screen" supply rail, whilst
the other of each pair, has its screen grid fed *direct* from the "screen"
supply rail.

I don't think I've seen this done before.

** Peavey have been doing that since their first 4 tube ( 6L6GC) models.

See Peavey Deuce from 1975:

http://www.el34world.com/charts/Schematics/files/peavey/peavy_deuce-b.pdf

Strangely, the 6 tube models have 6 screen resistors.

http://www.el34world.com/charts/Schematics/files/peavey/Peavey_Mace_Deuce.pdf

I doubt that it would have a significant effect on the operation of the
stage, but just interested as to whether anyone else has come across this
configuration, and knows the design reasoning behind it.

** Same question has puzzled me for ages - I suspect it could be an error
that has just got repeated over the decades. No-one but Peavey knows for
sure.

Anyone got any opinions about just strapping the screens to the anodes on
each pair via a switch, to implement a triode mode ?

** It better be a damn good switch, insulation and voltage rating wise -
most makers who have such triode/pentode switching use a very hefty slide
switch.

Also, the power output will of course drop in triode mode and each EL84 will
dissipate more heat under overdrive. Combined with their use of a
"concertina" phase splitter, I suspect that overdrive clipping will not be
symmetrical.

Not recommended.


..... Phil

 

[Arfa Daily]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:8squrgFdptU1@mid.individual.net...

"Arfa Daily"

A muso customer has asked me to carry out some tonal mods to his Peavey
Classic 30. All pretty straightforward cap changes and so on. However, he
also asked me to look into fitting a pentode / triode switch whilst I was
at it. At first glance, this didn't look like a problem. The output stage
is a fairly classic AB class using 4 x EL84 tubes as two paralleled pairs.
Anodes of each pair commoned. Cathodes all commoned and decked. Grids of
each pair commoned via individual 47k stoppers. But then, things get a
little odd around the screen grids. One tube of each pair, has its screen
grid fed by a 100 ohm 5 watt resistor off the "screen" supply rail, whilst
the other of each pair, has its screen grid fed *direct* from the "screen"
supply rail.

I don't think I've seen this done before.

** Peavey have been doing that since their first 4 tube ( 6L6GC) models.

See Peavey Deuce from 1975:

http://www.el34world.com/charts/Schematics/files/peavey/peavy_deuce-b.pdf

Strangely, the 6 tube models have 6 screen resistors.

http://www.el34world.com/charts/Schematics/files/peavey/Peavey_Mace_Deuce.pdf


I doubt that it would have a significant effect on the operation of the
stage, but just interested as to whether anyone else has come across this
configuration, and knows the design reasoning behind it.

** Same question has puzzled me for ages - I suspect it could be an error
that has just got repeated over the decades. No-one but Peavey knows for
sure.

Yes Phil. I too wondered whether it was an error. At first glance, I thought
that the two screen grids were commoned, and then taken back to a single
resistor, but when I looked at the print layout diagram a bit more
carefully, I could see that what appeared to be the case was that one screen
went to the 'cold' side of the resistor, and the other went to the 'hot'
side. A squint at the schematic then confirmed this ...

Anyone got any opinions about just strapping the screens to the anodes on
each pair via a switch, to implement a triode mode ?

** It better be a damn good switch, insulation and voltage rating wise -
most makers who have such triode/pentode switching use a very hefty slide
switch.

Yes indeed, my thoughts as well. Anode supply is quoted at 330v nominal, so
a switch rated to 250v ac, as most are, should be good for that. At the end
of the day, many tube amps use the same switch for the HT standby switching,
as they do for the mains, and lots of them run with an HT of much more than
330v.

Also, the power output will of course drop in triode mode and each EL84
will dissipate more heat under overdrive. Combined with their use of a
"concertina" phase splitter, I suspect that overdrive clipping will not be
symmetrical.

I hadn't considered that there might be any changes to the phase splitter
output as a result of such a change. What leads you to think that this might
be the case ?

Not recommended.

Well, not something that I would normally recommend either, and the owner is
happy to live without the mod. He just thought that he would like to give it
a try if it was practical, whilst I was in there doing the other work.
Apparently, he would like to use it in 'triode' mode when he is at home
practicing. It's not actually a hard mod to carry out, and would be
reasonably easy to implement with the current print layout. I think what I
will probably do is to just 'hang' the mod in temporarily to see how it
performs, before doing any chassis drilling for switch fitting. If it seems
to work ok, then I'll finish it off. If not, I'll just recommend against it.

.... Phil

Thanks for your comments Phil. Useful

Arfa

 

[Arfa Daily]

"Meat Plow" <mhywatt@yahoo.com> wrote in message
newsan.2011.02.25.18.42.19@lmao.lol.lol...

On Fri, 25 Feb 2011 17:00:41 +0000, Arfa Daily wrote:

A muso customer has asked me to carry out some tonal mods to his Peavey
Classic 30. All pretty straightforward cap changes and so on. However,
he also asked me to look into fitting a pentode / triode switch whilst I
was at it. At first glance, this didn't look like a problem. The output
stage is a fairly classic AB class using 4 x EL84 tubes as two
paralleled pairs. Anodes of each pair commoned. Cathodes all commoned
and decked. Grids of each pair commoned via individual 47k stoppers. But
then, things get a little odd around the screen grids. One tube of each
pair, has its screen grid fed by a 100 ohm 5 watt resistor off the
"screen" supply rail, whilst the other of each pair, has its screen grid
fed *direct* from the "screen" supply rail.

I don't think I've seen this done before. I've seen one screen resistor
feeding both tubes of a pair, or one resistor per screen, but not just
one tube having a screen feed resistor. Apart from anything else, I
wouldn't normally have considered it very good design practice to have
no current limiting at all in place. Also, it will mean that the screen
voltage will be higher on one tube of the pair, than the other. I doubt
that it would have a significant effect on the operation of the stage,
but just interested as to whether anyone else has come across this
configuration, and knows the design reasoning behind it. Anyone got any
opinions about just strapping the screens to the anodes on each pair via
a switch, to implement a triode mode ?

Arfa

Several amp and tone stack mods. Variable cathode bias, OD mod on CH2, OT
replacement, speaker replacement.

blueguitar.org/new/articles/blue_gtr/amps/peavey/c30_origmod.pdf

Hi Meat. Yes, it is Blue Guitar mods that he wants to do. He is not
interested in the changes to the tone stack. He wants the output bias
changing from fixed grid to cathode auto, he wants the C4 cap value change
in the overdrive channel, and he wants the input simplification mod, where
all the C-R garbage between the I/P socket and the first 12AX7 grid is
removed, and a simple series resistor and grid return resistor are put in
their place, a la typical Fender and Marshall I/P schemes.

Arfa

 

[Phil Allison]

"Arfa Daily"

"Phil Allison"
Anyone got any opinions about just strapping the screens to the anodes on
each pair via a switch, to implement a triode mode ?

** It better be a damn good switch, insulation and voltage rating wise -
most makers who have such triode/pentode switching use a very hefty slide
switch.

Yes indeed, my thoughts as well. Anode supply is quoted at 330v nominal,
so a switch rated to 250v ac, as most are, should be good for that.

** Nope.

Switches ( and relays) rated for AC operation are not suitable for DC
voltages above about 24 to 30 volts at the same current. DC arcs are tough
SOBs to break.

At the end of the day, many tube amps use the same switch for the HT
standby switching,

** All *sensible* designs switch the AC secondary of the transformer.
Those that switch the DC rail directly often arc and burn - even though
the current is generally only 100mA or so.

Fender ( for example ) did this in many older models, but chose a particular
switch that had a very large opening clearance to survive the inevitable
arcing - ESPECIALLY if the standby switch was ever operated under full
power.

Some Boogie models made the massive blunder of having the standby switch
parallel two electros ( one charged, one not) when it was closed. Splat.

Also, the power output will of course drop in triode mode and each EL84
will dissipate more heat under overdrive. Combined with their use of a
"concertina" phase splitter, I suspect that overdrive clipping will not
be symmetrical.


I hadn't considered that there might be any changes to the phase splitter
output as a result of such a change.

** Please try reading what is there.

The PS dos not itself change ( ??) but the job it has to do does - cos
the required grid drive to the EL84s is much higher in triode mode.
Concertina ( single triode ) PS circuits have less output and clip
asymmetrically compared to twin triode PS circuits.

Not recommended.

Well, not something that I would normally recommend either, and the owner
is happy to live without the mod. He just thought that he would like to
give it a try if it was practical, whilst I was in there doing the other
work. Apparently, he would like to use it in 'triode' mode when he is at
home practicing.

** Wot a fuckwit.

It only drops the power by half or 3 dB.


..... Phil

 

[Phil Allison]

"Arfa Daily"

** Same question has puzzled me for ages - I suspect it could be an error
that has just got repeated over the decades. No-one but Peavey knows for
sure.

** Here is my analysis:

The low value of 100 ohms in series with G2 has no significant effect on the
operation of a 6L6GC.

Peavey designs have a large value resistor ( 300 or 400 ohms 10W ) in the
feed to all G2s - unlike other amps that generally have a filter choke with
low DC resistance.

So, the 100 ohm resistors are there for factory test purposes only - so
staff can check if valves are drawing G2 currents within pre-determined
limits. G2 current levels mimic cathode currents for a given valve type and
brand.

By measuring the voltage drop across the 300 or 400 ohm resistor AND each
100 ohm in the G2 feeds - correct operation can be established.

Dunno about others - but I routinely check the voltage drop across each G2
resistor when servicing amps and replacing valves. It should be similar for
all valves in an output stage.


..... Phil

 

[Phil Allison]

"Arfa Daily"

He wants the output bias changing from fixed grid to cathode auto,

** That will either lose you a lot of output power or cook that puny power
tranny ( by going close to class A operation) if you don't do it the smart
way.

Not recommended either.




...... Phil

 

[David Nebenzahl]

On 2/25/2011 4:12 PM Phil Allison spake thus:

Also, the power output will of course drop in triode mode and each EL84 will
dissipate more heat under overdrive. Combined with their use of a
"concertina" phase splitter, I suspect that overdrive clipping will not be
symmetrical.

If you wouldn't mind indulging my relative ignorance for a minute, that
"concertina" stage business got my curiosity up. So I found the two
triode (12AX7) stages before the EL84s, and it looks to me like the
first stage (V3A) is a phase inverter, with the other triode acting as
*another* phase inverter cascaded from the first stage, presumably with
unity gain. Is that correct?

So it seems this setup would be quite problem-prone, with possible phase
distortion and phase imbalance (or crossover distortion) effects. So why
would anyone use such a phase inverter, when there are much better ones
available?


--
The phrase "jump the shark" itself jumped the shark about a decade ago.

- Usenet

 

[Phil Allison]

"David Nebenzahl"
Phil Allison spake thus:

Also, the power output will of course drop in triode mode and each EL84
will dissipate more heat under overdrive. Combined with their use of a
"concertina" phase splitter, I suspect that overdrive clipping will not
be symmetrical.

If you wouldn't mind indulging my relative ignorance for a minute, that
"concertina" stage business got my curiosity up. So I found the two triode
(12AX7) stages before the EL84s, and it looks to me like the first stage
(V3A) is a phase inverter,

** With an imagination like that - you should be writing fiction for a
living.

A " phase inverter" ( or phase splitter) has equal and opposite polarity
outputs.

with the other triode acting as *another* phase inverter cascaded from the
first stage, presumably with unity gain. Is that correct?

** Nope.

V3A is a voltage amplifier ( gain of about 50 before NFB) while V3B is the
"concertina" phase inverter. Concertina circuits always have equal plate and
cathode resistors and in operation, the voltage between the plate and
cathode grows and falls with audio drive like a concertina being pumped in
and out.

The cathode of V3A acts as a negative ( opposite phase) input for feedback
from the output transformer secondary.

Because of the easy drive requirements of an EL84, such a concertina circuit
is able to send them deep into overdrive - while maintaining good symmetry.
Not so for 6L6GC or EL34 stages which need several times more drive voltage.


..... Phil

 

[Arfa Daily]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:8srb56FhrbU1@mid.individual.net...

"Arfa Daily"

He wants the output bias changing from fixed grid to cathode auto,


** That will either lose you a lot of output power or cook that puny power
tranny ( by going close to class A operation) if you don't do it the
smart way.

Not recommended either.




..... Phil

Well, that one is a well known and documented mod for that amp, so I'm not
too concerned about doing it. However, I take (and somewhat agree with) your
point. Personally, I would prefer to make the fixed grid bias variable
within 'blues to rock' limits, and give him another knob to twiddle with,
but he is adamant that he wants that particular cathode bias mod. I did
offer to make that switchable for him, but he just wants it changing and
leaving at that. You can only advise these people, but in the end, it's his
amp, and if he's prepared to pay the money to have it done at his own risk,
then that's fine by me ...

Arfa

 

[Phil Allison]

"Arfa Daily = MORONIC TROLL "

He wants the output bias changing from fixed grid to cathode auto,


** That will either lose you a lot of output power or cook that puny
power tranny ( by going close to class A operation) if you don't do it
the smart way.

Not recommended either.


Well, that one is a well known and documented mod for that amp, so I'm not
too concerned about doing it.

** ROTFLMAO !!

Wot a fucking moron the stupid pommy cunt is.

Only posts questions he already THINKS he knows the answers to.

Just so the pommy cunt can smugly contradict anyone who dares to reply.

Wot a fucking MORON !!!!!!!!!!!!



...... Phil

 

[Arfa Daily]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:8sroeoFpkvU1@mid.individual.net...

"David Nebenzahl"
Phil Allison spake thus:

Also, the power output will of course drop in triode mode and each EL84
will dissipate more heat under overdrive. Combined with their use of a
"concertina" phase splitter, I suspect that overdrive clipping will not
be symmetrical.

If you wouldn't mind indulging my relative ignorance for a minute, that
"concertina" stage business got my curiosity up. So I found the two
triode (12AX7) stages before the EL84s, and it looks to me like the first
stage (V3A) is a phase inverter,


** With an imagination like that - you should be writing fiction for a
living.

A " phase inverter" ( or phase splitter) has equal and opposite polarity
outputs.


with the other triode acting as *another* phase inverter cascaded from
the first stage, presumably with unity gain. Is that correct?

** Nope.

V3A is a voltage amplifier ( gain of about 50 before NFB) while V3B is
the "concertina" phase inverter. Concertina circuits always have equal
plate and cathode resistors and in operation, the voltage between the
plate and cathode grows and falls with audio drive like a concertina being
pumped in and out.

The cathode of V3A acts as a negative ( opposite phase) input for
feedback from the output transformer secondary.

Because of the easy drive requirements of an EL84, such a concertina
circuit is able to send them deep into overdrive - while maintaining good
symmetry. Not so for 6L6GC or EL34 stages which need several times more
drive voltage.


.... Phil

So is it the fact that the circuit relies to some extent on NFB for correct
symmetrical overdrive operation, that leads you to believe that this might
be upset by operating in a pseudo-triode mode ?

Interesting analysis. I'll report back when I've tried it

Arfa

 

[Phil Allison]

"Arfa Daily is total FUCKWIT TROLL "

So is it the fact that the circuit relies to some extent on NFB for
correct symmetrical overdrive operation, that leads you to believe that
this might be upset by operating in a pseudo-triode mode ?

** Giant HUH ??????????

How does anyone get THAT nonsense from anything I posted ???

Hey Daily - you are a fucking IDIOT !!

GO DROP DEAD



..... Phil

 

[William Sommerwerck]

** ROTFLMAO !!
Wot a fucking moron the stupid pommy cunt is.
Only posts questions he already THINKS he knows the answers to.
Just so the pommy cunt can smugly contradict anyone who dares to reply.
Wot a fucking MORON !!!!!!!!!!!!

<hands on hips; huffy>
You are SO out of line, mister!
</hands on hips; huffy>

I guess you have never been unsure about something, and asked for
confirmation. How nice it is to be omniscient!

 

[Meat Plow]

On Sat, 26 Feb 2011 02:25:00 +0000, Arfa Daily wrote:

"Meat Plow" <mhywatt@yahoo.com> wrote in message
newsan.2011.02.25.18.42.19@lmao.lol.lol...
On Fri, 25 Feb 2011 17:00:41 +0000, Arfa Daily wrote:

A muso customer has asked me to carry out some tonal mods to his
Peavey Classic 30. All pretty straightforward cap changes and so on.
However, he also asked me to look into fitting a pentode / triode
switch whilst I was at it. At first glance, this didn't look like a
problem. The output stage is a fairly classic AB class using 4 x EL84
tubes as two paralleled pairs. Anodes of each pair commoned. Cathodes
all commoned and decked. Grids of each pair commoned via individual
47k stoppers. But then, things get a little odd around the screen
grids. One tube of each pair, has its screen grid fed by a 100 ohm 5
watt resistor off the "screen" supply rail, whilst the other of each
pair, has its screen grid fed *direct* from the "screen" supply rail.

I don't think I've seen this done before. I've seen one screen
resistor feeding both tubes of a pair, or one resistor per screen, but
not just one tube having a screen feed resistor. Apart from anything
else, I wouldn't normally have considered it very good design practice
to have no current limiting at all in place. Also, it will mean that
the screen voltage will be higher on one tube of the pair, than the
other. I doubt that it would have a significant effect on the
operation of the stage, but just interested as to whether anyone else
has come across this configuration, and knows the design reasoning
behind it. Anyone got any opinions about just strapping the screens to
the anodes on each pair via a switch, to implement a triode mode ?

Arfa

Several amp and tone stack mods. Variable cathode bias, OD mod on CH2,
OT replacement, speaker replacement.

blueguitar.org/new/articles/blue_gtr/amps/peavey/c30_origmod.pdf



Hi Meat. Yes, it is Blue Guitar mods that he wants to do. He is not
interested in the changes to the tone stack. He wants the output bias
changing from fixed grid to cathode auto, he wants the C4 cap value
change in the overdrive channel, and he wants the input simplification
mod, where all the C-R garbage between the I/P socket and the first
12AX7 grid is removed, and a simple series resistor and grid return
resistor are put in their place, a la typical Fender and Marshall I/P
schemes.

Arfa

Looks like a good bit of work in store for you. The hard part is to end
up with tone that doesn't sound like ass.



--
Live Fast, Die Young and Leave a Pretty Corpse

 

[Phil Allison]

"William Sommerwanker "

** ROTFLMAO !!
Wot a fucking moron the stupid pommy cunt is.
Only posts questions he already THINKS he knows the answers to.
Just so the pommy cunt can smugly contradict anyone who dares to reply.
Wot a fucking MORON !!!!!!!!!!!!


I guess you have never been unsure about something, and asked for
confirmation.

** That is NOT what the CUNT did !!

You illiterate, fuckwit bloody TROLL.

 

[Phil Allison]

"Meat Plow is a Lying Retarded PIG "

Could refit with two EL34s.

** Make any issues worse, not better.

My Musicman 112 combo uses a pair of Mullards
at around 700v plate.

** And only half that voltage on the screens.

Not relevant at all - fuckhead.

Now how about you post something that MAKES SENSE.

But you cannot.

And you keep proving it.

My god you are retarded, fucking jerk.



..... Phil

 

[Charles]

Until P.A. develops a reasonable command of English (sans potty words), I
humbly suggest that he be ignored.

 

[Meat Plow]

On Sun, 27 Feb 2011 08:09:44 +1100, Phil Allison wrote:

illiterate, fuckwit bloody TROLL.

Yes you are. You can't wait for someone to reply to
your trolls so you can attack them. I take that back,
your not a Troll. Trolls have some scruples. You wouldn't
recognize scruples if they booted your weasel nuts up your
fucking greasy, shit encrusted throat.





--
Live Fast, Die Young and Leave a Pretty Corpse