[Peavey] Tube-amp Output TX unbalanced currents & missing screen resistors

Hello,

In short, the actual question is whether a quartet of powertubes with one current-greedy member
can result in undesirable effects (in a push-pull power stage) for the output transformer due to the unequal current draw?
I guess not, strong bass-notes might have a comparable effect, but just wondering...


The longer version, and something about screen-grid resistors:

I got hold of an old Peavey Classic 50 (the 410 version) with (4) EL84 / 6BQ5. Sovtek.
Knowing this amp is fixed bias ('twofold': negative grid voltage, and non-adjustable, no cathode resistors),
I wanted to know the currents per tube.

Using insert-adapters for measuring the cathode-currents, and re-measuring over time,
I found (3) stabilized around 25...27 mA each, and (1) of the quartet
persisted on drawing around 40...42 mA regardless of which socket it went.

(I didn't measure the plate voltages yet, but I understood the 25 mA is the intended ballpark figure in this amp.)

OK clear, not (longer) a fully matched quartet. So replace with a fresh matched quartet, case closed.

Amp sounds fine though, and as long as eventual damage is limited to just wearing out that 40mA tube faster
then the replacement is not needed any minute... as long as when it fails, it doesn't cause other trouble.

In addition, I saw not all power-tubes are treated equal in this amp, circuit-wise (see attached part of schematic).
To my surprise not all tubes have their own screen grid resistor.
There's only R46 & R55 for V4 & V5, but V6 & V7 don't have any 'of their own'. False economy ? Or OK ?

Thanks/Bye,

  Peter

 

The single small G2 resistor suppresses push-push MHz parasitics.

The G1 bias is adjustable with the R16 R18 ratio.

I'd really think about individual 100 Ohm cathode resistors. That would slightly tend to equalize currents, and give a handy point to check all four currents without tube yanking.

Hi & Thanks,

PRR said:

The single small G2 resistor suppresses push-push MHz parasitics.

Click to expand...

OK, I see, as a tube to tube isolation then, and with a (non shown) 400 Ohm common resistor upstream. Haven't seen it before like that, but if it works...  saw some unhappyness around the interweb about this approach, (but the interweb can provide that for any topic.)

The G1 bias is adjustable with the R16 R18 ratio.

Click to expand...

Considering to make it trimmable but realize it won't address any tube-unbalance within a quartet EL84 tubes.

I'd really think about individual 100 Ohm cathode resistors. That would slightly tend to equalize currents, and give a handy point to check all four currents without tube yanking.

Click to expand...

That's a nice suggestion. The insert-adapters I've been using loop the cathode-pin to a current-meter, but it's a bit of a pain.

Regards

that schematic might be drawn wrong, trace out the screen circuit and see,

seems strange to have two tubes with, and two without screen resistors,

maybe it is really one 100 ohm resistor feeding two tubes,

one tube being mis-matched might mean that the amp had some problems, maybe scope it for unstability when everything is on 11,


mismatch affecting the OPT is probably a non issue, hopefully they picked a core big enough just in case,

also, if it is stacked Lap 3, then you have a bit more natural gap built in,

Hi,

CJ said:

that schematic might be drawn wrong, trace out the screen circuit and see,

seems strange to have two tubes with, and two without screen resistors,

maybe it is really one 100 ohm resistor feeding two tubes,

Click to expand...

I guess/am fairly sure the schematic is correct, but it doesn't hurt to check, I have the layout.

Several other interweb-posts express the same 'concern/amazement'.
I've now attached a more complete schematic, showing that 400 Ohm common screen grid resistor.

one tube being mis-matched might mean that the amp had some problems, maybe scope it for unstability when everything is on 11,

Click to expand...

That'd be a good one. The 40mA current draw 'travels along' with the tube, wherever of the 4 sockets I put it in, so clearly that specific tube has left the close company of the others of the quartet. Most straightforward approach is of course to try a fresh matched quartet, but after this measuring I got curious at what was actually going on.

mismatch affecting the OPT is probably a non issue, hopefully they picked a core big enough just in case,

Click to expand...

The Classic 50 (tweed) has a bigger OPT than the 30, but not sure if that is enough of 'bigger'.  Good to hear though about the non-issue.

also, if it is stacked Lap 3, then you have a bit more natural gap built in,

Click to expand...

Sorry for asking, is Lap3 w.r.t . the material used or the (E-I ?) construction ?

Thanks/regards

BTW, note that it could be argued that those tweed Peavey Classic amps could be considered as an exercise in how far you can
push things.

The Boutique approach it sure isn't (better safe than sorry, but also perhaps a bit of overengineering for a reason,
or also some overengineering when stuff is not fully understood, so let's just throw it in, at the expense of the buyer).

On the other hand, what those Classic amps do is more courageous and/or probably pushing things too far, or just staying just safe enough.

I mean, putting heaters in series, hmmm.  :-[ 
Changing power-tube bias-current when the footswitched relay is activated, hmmm.    :

On the other hand, why not ? 1mA accuracy for bias-current is not needed. I thought to have understood the schematic for this range hasn't changed much over the last decades, just the place of manufacturing and the restyled amp-front (mine is the old style).

Judging from the date on that schematic, that was while i was working there so I knew the design engineer, and his boss (RIP), the actual tube expert. Jack Sondermeyer who lead that engineering group was a tube design engineer, before he worked at RCA doing applications engineering for the workhorse 2N3055 transistor.

I am not a tube guy but as I recall the classic series amps were pretty well regarded for their sound.  Resistors are cheap so I do not understand that much sharp pencil frugality.

JR

Thanks John for joining.


The friendly support people at Peavey told me my Classic 50  410 amp was from August 1998.

I understood from this page ( http://www.amptweaker.com/page/About-James-1.aspx ) that James Brown was involved (no pun),
he might be the person you had in mind, without doubt you have known him.

I agree, those amps have a decent reputation, they go back a long time, and are in essence pretty much unchanged
and still in production.

But indeed, beats me as well why they didn't spend a few resistors more.

Speaking of riddles and resistors... in this very same amp, the reverb footswitching section in the preamp uses
some parts (marked in the attached schematic) which makes one wonder why they aren't in fact superfluous.

OK, Q1 (the J231 JFET-switch) needs to be 'biased' (=defined), but why the need to feed it some part of signal of the
reverb recovery amp (C30, R43, R40) ? It ain't a JFET in an allpass-section of an analog phaser pedal,
so for lowering distortion it won't be...  some trick for lower switching-noise is the only I can think of.

More: Resistor R44 (1M) connects to the B++, for lack of another positive voltage. OK...
(note the supply rails for the reverb-opamp are -27V & 0V, with -14V in between 'for reference' - so nowhere else to go for some positive voltage than the Large One)

Note that my ramblings are not intended to criticise any former colleague, just that it's insightful to understand the intentions and practical thoughts for an amp this succesful in the market.


Regards / have a good evening all

clintrubber said:

Thanks John for joining.


The friendly support people at Peavey told me my Classic 50  410 amp was from August 1998.

I understood from this page ( http://www.amptweaker.com/page/About-James-1.aspx ) that James Brown was involved (no pun),
he might be the person you had in mind, without doubt you have known him.

Click to expand...

Yes, James is an old friend. He is better known for the 5150 he did with EVH, but Sondermeyer is who taught James about tubes.

I agree, those amps have a decent reputation, they go back a long time, and are in essence pretty much unchanged
and still in production.

But indeed, beats me as well why they didn't spend a few resistors more.

Click to expand...

Agreed Peavey can buy resistors very cheaply.  The holes in the PCB and labor to pop them is more expensive than the resistors. 

Speaking of riddles and resistors... in this very same amp, the reverb footswitching section in the preamp uses
some parts (marked in the attached schematic) which makes one wonder why they aren't in fact superfluous.

OK, Q1 (the J231 JFET-switch) needs to be 'biased' (=defined), but why the need to feed it some part of signal of the
reverb recovery amp (C30, R43, R40) ? It ain't a JFET in an allpass-section of an analog phaser pedal,
so for lowering distortion it won't be...  some trick for lower switching-noise is the only I can think of.

More: Resistor R44 (1M) connects to the B++, for lack of another positive voltage. OK...
(note the supply rails for the reverb-opamp are -27V & 0V, with -14V in between 'for reference' - so nowhere else to go for some positive voltage than the Large One)

Note that my ramblings are not intended to criticise any former colleague, just that it's insightful to understand the intentions and practical thoughts for an amp this succesful in the market.


Regards / have a good evening all

Click to expand...

I am not a tube guy, the j231 are not tubes.

The resistor ratios do not look right for distortion cancellation, but I would not expect spare parts for no reason.

Also I do not trust schematics 100% but that looks like a relatively modern (computer generated) schematic, so was used to feed the original PCB design at that time. So it has a better chance of accuracy, while decades later I wouldn't be so certain that all production changes were reflected into the published schematic.

JR

I've got a 30 on the bench at the moment, it had blown the ht fuse. Replace fuse, two tubes get red in the face and then one arcs internally! Love those fireworks! The series filaments had me fooled for a while. Swapping the tube positions still gives fireworks so I assume its bad tubes, all JJ's. I am getting a new set. Does anyone know if these amps tended to be tube eaters, or how the JJ's are generally? (EL84s)

looking at some more schematics with EL84's it is obvious that your schematic is correct as the Classic 30, Classic 50 Blues, etc all have the missing screen resistors,

so what happens with red plating is amp designers got into the power race and found that they could get a huge power boost by using fixed bias as opposed to cathode (self) bias. But this power boost had a price, that price being stability.

Tubes, like transistors, can go into thermal runaway, once the parts start to heat up, they conduct better, especially EL84's. In a push pull amp, once one side starts to run away, it hogs all the power so only half the tubes will red plate. In the case of the Peavey, with the missing screen resistor, half the power tubes will run away, but the one with the screen resistor may not, so you just get one cooked tube. This is probably you have one tube that is reading way over what the others read. You could probably get away with replacing one tube only.

Looking at old VOX circuits, we notice plate voltages far exceeding the Peavey plate voltages, however, the VOX amps ran cathode bias.  Since you have 320 instead of 420 plate volts, switching to cathode bias might not be acceptable as your amp will have limited power.  So maybe cobble in an extra set of screen resistors and consider upping the 400 ohm to 1 K ohm to make the amp bullet proof and prolong tube life.

Maybe add 10 ohm cathode resistors to make sure the tubes stay together. They could be 1 or2 watt resistors since the value is low.

Replacing the 47K grid stoppers with 1.5 K might open up the amp to give you more of the famous EL84 chime, as this is the value VOX used.

There might be some sonic differences between the tubes running the 100 ohm screens and the ones that don't, but you would need a scope to figure that out, it could be a blend of tones they were going for, but it was probably economics and space saving.

Lap 3 on the OPT means 3 lams go one way, and 3 go the other way, this helps to keep the core from saturating from DC imbalance as opposed to Lap 1 one will raise the perm and make it easier for the core to saturate with DC.

JohnRoberts said:

Also I do not trust schematics 100% but that looks like a relatively modern (computer generated) schematic, so was used to feed the original PCB design at that time. So it has a better chance of accuracy, while decades later I wouldn't be so certain that all production changes were reflected into the published schematic.

JR

Click to expand...

Hello,

The friendly people at Peavey support told me the schematic is unchanged (there are a few out there, the one from May 1994 is the latest I've seen), but can indeed well imagine some details have been changed for production over the years.

And their Classic tweed range is now made outside the US, so can imagine at least a redraw for that reason alone already.
Magazine-reviews mention circuit tweaks, but I haven't found details. (As said, I asked Peavey support, but same schematic I was told. OK, no problem, just curious.)

Regards

radardoug said:

I've got a 30 on the bench at the moment, it had blown the ht fuse. Replace fuse, two tubes get red in the face and then one arcs internally! Love those fireworks! The series filaments had me fooled for a while. Swapping the tube positions still gives fireworks so I assume its bad tubes, all JJ's. I am getting a new set. Does anyone know if these amps tended to be tube eaters, or how the JJ's are generally? (EL84s)

Click to expand...

Had understood amps from this range are a reliable workhorses, but since there are so many around I can imagine trouble-reports exist.
Had understood they're 'meant for Sovteks', FWIW.    Maybe that's where the fixed-fixed-biasing was tweaked for.

In the Classic 50 the tube pairs are no neighbours, I assume the same for the 30 & you'll already be aware of that.
Looking at the amp-rear, it's V7,6,5,4, of which V7 || V5, and V6 || V4.

No experience with JJ's here, good luck with the fireworks  : (hope it stops one hour into the new year ?!  )

Regards

CJ said:

looking at some more schematics with EL84's it is obvious that your schematic is correct as the Classic 30, Classic 50 Blues, etc all have the missing screen resistors,

Click to expand...

Powerstage-wise, thought to have understood the only difference between the 50 & 30 types is the OPT-size & plate voltages.

so what happens with red plating is amp designers got into the power race and found that they could get a huge power boost by using fixed bias as opposed to cathode (self) bias. But this power boost had a price, that price being stability.

Click to expand...

I could very well live with somewhat less power from this amp. It's not the main amp in the rig, added it for a two-amp broader sound, to work along a SF TR. So adding the 'missing' resistors  would sure have benefits. 

Tubes, like transistors, can go into thermal runaway, once the parts start to heat up, they conduct better, especially EL84's. In a push pull amp, once one side starts to run away, it hogs all the power so only half the tubes will red plate. In the case of the Peavey, with the missing screen resistor, half the power tubes will run away, but the one with the screen resistor may not, so you just get one cooked tube. This is probably you have one tube that is reading way over what the others read. You could probably get away with replacing one tube only.

Click to expand...

I tried to find logic in the travel along the 4 sockets of the current-hungry 40 mA tube, but the higher current draw seems  unrelated to whether a socket does or doesn't have the 100 Ohms screen grid resistor. Also 'paired' it with another brother, but same result (yep, now go and order new quartet, but hey, curious, hence this thread).

Looking at old VOX circuits, we notice plate voltages far exceeding the Peavey plate voltages, however, the VOX amps ran cathode bias.  Since you have 320 instead of 420 plate volts, switching to cathode bias might not be acceptable as your amp will have limited power.  So maybe cobble in an extra set of screen resistors and consider upping the 400 ohm to 1 K ohm to make the amp bullet proof and prolong tube life.

Click to expand...

I didn't see any B+ or B++ voltage values in the Classic 50 schematics (perhaps the Classic 30 schematic has, I'll check), but thought to have understood that the approach here was to use high plate voltages and low bias current (25 mA).
So the conversion to cathode bias you suggested might be a good one.

Maybe add 10 ohm cathode resistors to make sure the tubes stay together. They could be 1 or2 watt resistors since the value is low.

Click to expand...

Sounds good, inserting these. PRR suggested 100 here, for ease of measuring, giving some 2.5V - while the Vbias could be compensated for that, those 10 Ohms would do as well (for some 250mV) so have less shift.

Replacing the 47K grid stoppers with 1.5 K might open up the amp to give you more of the famous EL84 chime, as this is the value VOX used.

Click to expand...

That'd be nice, you feel the 47k are dulling this stage ?

There might be some sonic differences between the tubes running the 100 ohm screens and the ones that don't, but you would need a scope to figure that out, it could be a blend of tones they were going for, but it was probably economics and space saving.

Click to expand...

It'd indeed be very interesting to find some designer notebook pages with his(/their) thoughts here!

I guess that blend of tones could be auditioned in isolated parts by plugging the 16 Ohm speakerloads into the 8 Ohm socket and pulling either V6,V7 (for amp-with-screen-R), or V4,V5 (for amp-without-screen-R). 

Lap 3 on the OPT means 3 lams go one way, and 3 go the other way, this helps to keep the core from saturating from DC imbalance as opposed to Lap 1 one will raise the perm and make it easier for the core to saturate with DC.

Click to expand...

Thanks for the info, appreciated. My amp should have same type OPT 70500211 as here ( http://picclick.co.uk/Peavey-Classic-50-50-Output-Transformer-70500211-EL-322190851186.html ) ,
so guess it'll be Lap 1  (pic attached).


Bye/thanks

OK, so a new set of tubes arrived and I had to fix this thing. Man, whoever designed this was on some seriously bad drugs! I think this is the worst design I have ever seen, and I've seen a few. There are three boards which join each other with wire links along one edge, so making a U. This is fastened inside a steel U chassis. So to look at anything you have to totally dismantle it out of the U chassis.
That means you cant work on it when it is running.
Anyway, I fitted 33 ohm 5W resistors to all the cathodes. Ended up quite tidy. Re-assembled it, switched it on and.... no sparks!!
It seems to be very happy, no red-faced tubes.
PRR, if you know who the moron was that designed this, I would like to personally tell him so! I hope he is no longer designing.
Merry Xmas all!

radardoug said:

Anyway, I fitted 33 ohm 5W resistors to all the cathodes. Ended up quite tidy. Re-assembled it, switched it on and.... no sparks!!

Click to expand...

Good to hear those resistors helped. That's a nice  value in between the mentioned 10 & 100.

if you know who the moron was that designed this, I would like to personally tell him so!

Click to expand...

His name was mentioned above, if you ask JohnR ehh, nicely, perhaps he can bring you into contact... 

Have a good kerstmis everybody as well!

clintrubber said:

Good to hear those resistors helped. That's a nice  value in between the mentioned 10 & 100.
His name was mentioned above, if you ask JohnR ehh, nicely, perhaps he can bring you into contact... 

Click to expand...

Why would I do that to a friend...?

Then I would be acting like a moron.

JR

JohnRoberts said:

Why would I do that to a friend...?

Then I would be acting like a moron.

JR

Click to expand...

Hello John,

My sincere apologies if I gave the impression that I wanted you to enable the path for someone to tell 'strong stuff' to a friend of yours.
I chose the format of a joke to bring my message that the designer involved was known, but that the tone of the message had to be friendly, obviously.

Apart from that, I'd truly appreciate it if JB could be involved i this thread - if possible.
Obviously not to held 'accountable' or anything like that, sure not - but I'd sure would find it instructive to hear about the various thoughts behind this design, like for instance those additional components at the reverb-switching. As far as these would be appropriate to be shared of course.

Thanks & best regards,

  Peter

clintrubber said:

Hello John,

My sincere apologies if I gave the impression that I wanted you to enable the path for someone to tell 'strong stuff' to a friend of yours.
I chose the format of a joke to bring my message that the designer involved was known, but that the tone of the message had to be friendly, obviously.

Apart from that, I'd truly appreciate it if JB could be involved i this thread - if possible.
Obviously not to held 'accountable' or anything like that, sure not - but I'd sure would find it instructive to hear about the various thoughts behind this design, like for instance those additional components at the reverb-switching. As far as these would be appropriate to be shared of course.

Thanks & best regards,

  Peter

Click to expand...

No Problem... I just don't like taking shots at design engineers who are not present to respond.

James is running his own business these days designing guitar pedals (amptweaker.com). Every time I talk to him he is very busy, and I expect this time of the year to be busier than normal.

AFAIK James is well regarded in the industry for his body of work and does not need me to defend him. I am not a tube guy but as I recall the Classic series was well regarded back last century.

Inviting him here to defend ugly criticism of work he did for hire decades ago, is not my idea of a Merry Christmas.

If Radardoug want to reach out and insult him, James is not hiding, but very active in the business. Maybe attend NAMM and visit his booth. 

JR

PS: Jack Sondermeyer the lead engineer is now dead (RIP). IIRC  there was more than one Classic amp model . I think(?) James did the first one and a different engineer may have done the later derivative model (no I won't name him too). I was managing a different design group (mixer engineering) at the time so I only know what I saw walking around the lab (we shared a common workspace), and chewing the fat with them.

Hello John,

All clear, thanks for the response. Sure understanding people have other stuff to do than getting back to designs of many years ago.

The power-stage additions* are 'straightforward tube-tech', so the talk above was more like 'curious why' (about those 2 screen grid resistors), than 'how'. 

(* by coincidence, the amp-section of German magazine Gitarre und Bass had some recent talk about missing screen grid resistors in early Marshalls as well, and the potential consequences)

Enjoy kerstmis all!