Peavey 212 Classic VT Series

[tubeAMP]

greetings.  new to the forum.  I have an old original Peavey 212 Classic hybrid SS front end with 6L6GC output.  after playing for awhile the tone control cuts out.  jumps to a high frequency and stays there until next time after being off for awhile.  the control turns UP/DOWN but no change :loco
any thoughts
likes/dislikes this amp
whatever
I am familiar with working on tube amps, radios and some with transistor radios
first attempt at attaching schematic.  check it out

[tubeAMP]

by tone control I mean treble.  see in first section solid state
maybe R45 going short

[Enzo]

The schematic you posted is for a Classic VT.  Is that what you have?   Peavey also made Classic (and there were A and B series of those) as well as Classic VTX.

R45 going short?  No, resistors do not short.  They open or change value, but not short.

First suspect is the control itself.  If pot R47 goes open at the bottom end - the end that meets R48 - OR if the solder is cracked so it acts open, you get your symptom.

Then again... when this happens, do the bass and mid controls still have any effect, or are they dead too?

While you are at it, check the solder on all the other tone stack components.

[tubeAMP]

greetings Enzo.  thanks for your reply.  I see that this is an active forum.  great.  new here
that should be the correct diagram VT Series.  bought amp used back in the 70s.  got the print from the factory more recently but some time ago.  factory pretty good about sending a diagram.
from memory I thought TREBLE but checking today it is the MID that suddenly has no control.  plays fine for awhile then MID quits.  no control but no scratchiness.  the sound becomes treble although there is control with BASS and TREBLE.  I would guess that the MID pot is bonkers stuck at the high side but I am not a solid state person.  I know that tone stacks are tone stacks and pots are pots but maybe something kooky unknown to me going on.  any further circuit analysis appreciated

[ChewyNasalPrize]

I have a 79 Peavey Classic VT 212 with phaser with original footswitch and cover. <3)

I personally think it is a great amp for what you can get them for. I bought mine from a local guy about a year ago super cheap because it wasn't sounding great at the time but I put Winged C 6L6 tubes in it, had a tech check it out and replace filter caps and some other minor worn circuitry bits and basically brought it back up to new specs.

It also currently has an Eminence Swamp Thang and WGS Vet 30 in it and it sounds GREAT!  :dbtu:

Fixed up right, it is a reliable and awesome-sounding amp for not much $$- and one that nobody will try to steal from you.  :lmao:

[tubeAMP]

yes phase shifter temolo.  different speeds different effect.  great reverb.  select and combine switches.  all in the original foot switch.  check out the schematic attached to see how they do the switching with transistors.  mine is all original.  I have a lot of tubes to swap out.  not much difference really.  everything works fine except for the MID cutting-out

[ChewyNasalPrize]

The earlier versions had "tremolo" but later versions had "phase." You can kind of make the phase sound like tremolo at the right settings. Which does yours have?

Distortion channel at low volumes isn't all that but when the volume is up and your pushing the power tubes, it sounds really good.

If you haven't already considered it, think about upgrading the stock speakers. It will make a HUGE difference. I started with two WGS Vet 30s and was really amazed at the difference. Then swapped out one of the Vet 30s for a WGS Reaper 30 watt and it gave it a bit more of a vintage sound with more speaker breakup from the Reaper. Again, great sounding just different.

The current Eminence Swamp Thang / Vet 30 combo is just brutal. Super tight low-end resonance you really feel in the bones with no mush whatsoever still plenty of mid and sparkle and a little breakup from the Vet 30 when pushed.

I have another Swamp Thang I may put in there but that may make it more of a bass amp than a guitar amp! But would sound AWESOME!

:dbtu:

[ChewyNasalPrize]

Forgot to mention that I also like that I can run my Randall Commander head through the power-in of the Classic VT. Makes the Randall sound great with that tube power section.

[JHow]

I have one of these that I got for $20.  It had a bad cap (the last node 10-350V on the power supply) and the two 22K resistors before it were bubbling and making a sound like frying bacon.  I replaced those, and now it appears to have all functions, although some of the controls are noisy (i ran out of deoxit and am getting some more). It also had a hole in one of the speakers ( I have the original peavey square magnet ones) which got the white-glue-tissue repair treatment. I also removed the two-inch palmetto bug corpse from inside the cabinet.  The cabinet is actually solid pine wood, the baffle is chip-board.  It also looks really convenient to work on if you had to, the ics are all socketed, there are a lot of connectors, so you could easily remove the board with just a couple of screws and the pot nuts.  All in all a solid piece of gear, I think.

[tubeAMP]

thanks for the input guys.  it has phase shifter with speed intensity and color control.  high speed = tremolo.  all original with square magnet speakers.  no need to change them because you can use the 4ohm 8ohm output jack to any cab you want.  I have tried different speaker cabs with some noticeable difference.  the input/output loop is a nice feature.  I did play around with both once upon a time.
shorting power supply filter capacitors will fry any resistors in the circuit.  dry shorting electrolytic capacitors = high current.  better fried resistors than output transformer.  I have used coffee filter and Elmers glue to mend speakers.  Elmers is water soluble, flexible and strong.  coffee filter paper is tuff and flexible.  repairs still going strong.
the issue with my unit is that it suddenly goes into a treble sound with the MID pot losing its control.  almost like a heat component failure.  like electronic switching.  I am thinking now that maybe one of the small electrolytic capacitors is dried-out and grounding some signal in one of the op amps.  not knowing characteristics of op amps I am only guessing.  yes it does have the original capacitors in it...   
short op amp tutorial would be great :dbtu:

[Roly]

I am thinking now that maybe one of the small electrolytic capacitors is dried-out and grounding some signal in one of the op amps.

I agree with Enzo that you most likely have an intermittent open in the Mid pot path.  Caps are always getting fingered but the fact is that broken tracks and solder joints (in the case of controls due to the amp face-planting on the floor at some point) is more likely.

Looking at the circuit there is no cap that presents itself as a likely cause of such behaviour, and I will even go so far as to nominate either one of the solder joints on the Mid pot, or possibly a rivet that connects the pot track to the pot connection lug.

[tubeAMP]

so if R46 low side is going open neither C29 or C58 is never grounded.  that might account for the highs being amplified.  funny how the amp operates normally then suddenly has failure.  let it rest it goes back to normal then operating awhile fails again.  no static no noise more like an electronic component failing.  but I have been fooled before  :loco

[J M Fahey]

funny how the amp operates normally then suddenly has failure.

Well. that's why those failures are called "intermittents".

more like an electronic component failing

Not necessarily.
Amps (and circuits in general) are made out of *parts* and *connections* .

"Bad parts" explain only half of the problems.
All parts may measure perfect and the amp may still develop a problem.

[Roly]

no static no noise more like an electronic component failing

I don't think there is any basis for that conclusion at all.  I've had components such as transistors fail like they have been switched off, and others that have made all sorts of fuss as they flake out; and I can say exactly the same for dry joints, cracked PCB tracks, socket contacts, and so forth.

One of the reasons I suspect an open, perhaps thermal, in the Mid pot path is your observation that the level of trebles rises.  The Mid pot going open will reduce the tonestack insertion loss, in other words the overall signal level should rise.

[tubeAMP]

you would think that 1.6v at U5A would not be enough to cause thermal issues at R46 pot.  but it acts thermal by nature after running normally then 'yoink' high frequency time with no MID control.  plugged guitar out/in to every input, switched foot switches, change control settings.  same thing.  turn OFF amp come back later back to normal.
BTW diagram doesnt say about the two ground symbols.  one is chassis the other is BUSS for CT at 0v