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

[clintrubber]

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.

No idea how long ago, but I'm sorry to hear that.
Many Classic amps indeed (IIRC the earliest had solid state preamps & tubed power stages (and/or hybrid power stages: stacked BJTs & tubes, like MusicMan did before/after/during - but not sure all pre-tweed Peavey Classics had those hybrid power stages)

 

[radardoug]

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

Then I would be acting like a moron.

JR

Fair enough. But this really is a bad design. Its like they thought the amps would never need any work done on them.
I was going to take a picture but didn't, wish I had now!
So to do anything on this amp, you have to completely disassemble it. The reverb plug wont fit out through the chassis hole so you have to do it all with the amp by the case, or completely remove the reverb pan.  Peavey did use polarised plugs, three of them, same body but not interchangeable. It takes half an hour just to get to what you want to do. Ten minutes to fit the resistors. Another half an hour to re-assemble. And then you just have to hope you got it right. Otherwise you would have to run it in peices on the bench, which would be very difficult.

 

[clintrubber]

FWIW,
saw that later Classic 50 amps (sch dated 2014)
were given a screen-R each (1k 5W)

 

[trobbins]

Fair enough. But this really is a bad design. Its like they thought the amps would never need any work done on them.
I was going to take a picture but didn't, wish I had now!
So to do anything on this amp, you have to completely disassemble it. The reverb plug wont fit out through the chassis hole so you have to do it all with the amp by the case, or completely remove the reverb pan.  Peavey did use polarised plugs, three of them, same body but not interchangeable. It takes half an hour just to get to what you want to do. Ten minutes to fit the resistors. Another half an hour to re-assemble. And then you just have to hope you got it right. Otherwise you would have to run it in peices on the bench, which would be very difficult.

You are modifying the amp - not fixing it.  So a critique about how long it takes to modify is not really valid for original design.  Many many guitar amps don't allow adjustment for ageing output stage valves - they simply apply the proactive maintenance mantra of replace the valves before they degrade.  If you can afford to buy the amp you can afford to proactively replace the tubes.

That said, then yes I too would insert cathode sense resistors, but with a sensing terminal strip, so you don't then have to pull apart the amp to probe.  But there are bias sense adaptors, so there is a choice on expending effort.  And this whole pathway requires 'technical' awareness - not something every guitarist has.

If modding to eek more life out of mixed valves, then as PRR suggests, increase the cathode sense to alleviate the imbalance, or split the bias to two adjustable voltages.

The real worry imho about mismatched bias currents in the output stage is that it may be a sign of a valve on the way out - which then moves attention to how well the amp can survive a failed valve.  Fixed bias usually means more chance of the output transformer, or the power transformer being pushed in to thermal damage from overcurrent - which opens up another technical front of whether more modifications are need to improve survivability of a valuable amp.

Am I missing something - but I don't see any output stage heaters in series ? -  maybe preamp dc fed heaters from 27V rail?

 

[CJ]

if you have a quad and one tube is drawing more, that is a problem because it will hog current from the partner and wear quicker.

but if you have two power tubes, then a little mismatch is no big deal, the output transformer will take care of the waveform.

OT - watch out for the small bottle GT5881's, put a set in a Marshall 4100 and two ate the load,

glass helps to cool the tubes  thinking that the smaller envelope might have something to do with it,

another thing to look out for is a 250 volt cap in certain JMP900 amps that feeds the neg voltage diode, 4100 series, it has been know to fail which causes a cookout in the back yard, replace with 600 or better.

 

[clintrubber]

Am I missing something - but I don't see any output stage heaters in series ? -  maybe preamp dc fed heaters from 27V rail?

Apart from the added screen resistors, that's indeed another modification shown by a 2014 Classic 50 schematic:
the 'unique' heater arrangement of the earlier models has now been changed into tubes V3-V7on 6.3Vac, and V1 & V2 in series on -23 ... -27Vdc.

  (FWIW, apologies I don't post or forward the schematic: I kindly received it from the friendly people at Peavey support, contact them so they have view on the distribition)

 

[JohnRoberts]

  (FWIW, apologies I don't post or forward the schematic: I kindly received it from the friendly people at Peavey support, contact them so they have view on the distribition)

Always worth repeating, Peavey has a well respected and helpful service department.

When in doubt ask them, they will generally be aware of known issues.

JR

 

[clintrubber]

Always worth repeating, Peavey has a well respected and helpful service department.

When in doubt ask them, they will generally be aware of known issues.

So true, nothing but good experiences with them. Quick response and knowledgable. And their filing cabinet is well maintained.