Peavey VTM60 tube head lost volume

[aaronsg71]

hi, im new here and i have a vtm 60 head. i recently had a volume drop and the gain isnt as good as it was. all the tubes are new, the compression dip switch isnt working either as it had before, it is stuck in the on position. i dont know anything about repairing what might be wrong, what should i check for? any help would be awesome, thx. aaron.

[Enzo]

Isolate the problem.

First, plug a spare cord from FX send to FX return. ANY help?

No?

Plug the guitar - or other test signal - into the FX return jack. Still weak, or does it sound clear and strong?

Plug the guitar into the regular input, and run a cord from FX send to some other amp. How does that FX send sound coming from the other amp? Does it sound OK or does it have the problem?

[aaronsg71]

i will try this possibly later today or tomorrow, my amp is at my practice place, thx! oh, the only thing i ran through the fx loop was a small volume box, which allowed me to use as a master volume so i could crank the amps master volume up.

[aaronsg71]

ok i tried plugging a chord through the fx send and into the return, no difference. also tried my guitar into the send, got nothing, i previously ran a cable from the back of the head's booster output into my Randall head's guitar input, it worked as far as sound but was kinda crappy sounding. the only dip switch that isnt functioning is the compression, it is stuck in the up, on position, i push it to the off position and doesn't do anything. i just looked at your information again, i dont think i tried the fx send to my other amp, crap! sry

[Gtr_tech]

You prolly have a ribbon cable problem. Very common on these and similar PV amps. The heater current causes the connectors to burn and they go high resistance. Also common is cracked solder joints at the board on the headers.

My advise? Have a tech eliminate those connectors and solder the cables direct to the board on *both* ends. You won't have that trouble again....

[aaronsg71]

Ok then I will have those looked at and get back with you! Thx alot!

[Enzo]

Um, I suggested plugging the guitar into the return, not the send. The point was that would send signal directly to the power amp, not through the preamp. This would tell us if the power amp has the issue or if it is OK and the preamp has the issue.

[aaronsg71]

ok, guitar through the return, i will try that sorry., thx

[Enzo]

WHuch number switch is the compression - the function not working?

[aaronsg71]

the comp is the 3rd dip switch, after the gain 1 and 2, it worked until i had this volume problem, so right now it is on, the way i like it, but it doesn't work when i turn it off. http://www.peavey.com/assets/literat...s/80300704.pdf

[Guitarist]

I've got that same amp. It came from a pawn shop and looked thrashed so I had to buy it...but had 100% dried out/finished power supply caps.
Thus I started with: 2 of Illinois Capacitor Axial Electrolytic 220uF @ 300V cap for the first bank of filtering. But as GtrTech says, taking it to a forward thinking tech may be smart.
You can see how moving the ribbon connector over two pins exposes the heater pins allows for a wire upgrade.
That amp has an AWESOME design.

How does that diode compressor work?

[aaronsg71]

really, i dont know about caps, etc...diodes? i havent been able to get my heasd looked at yet.

[Enzo]

The diodes are noise suppressors. A sort of poor man's gate.

Think of your signal on a scope - the waveform fills the screen top to bottom. Now stop playing, nothing left but the background hiss and noise. That stuff doesn't fill the screen, just a narrow band across the center. Let's say the signal goes up to 30v peaks, but the noise burbles along at nothing over a half volt. If we could cut out the half a volt eithe side of zero from the center, the remianing waveform would be most of the original, just a little glitch as it crosses zero from one polarity to the other.

Enter the diodes. Diodes don't conduct until the voltage acros them gets over a certain amount - about a half a volt. So from zero, as the signal starts upwards, normally that would pass right on through. But if we make it go through a diode, then the first half a volt of the signal rising from zero won;t conduct through. Once we get to that level, all the rest passes easily. And another diode facing the reverse direction does the same thing for the opposite polarity.

SO now we have done it, we have prevented the first half a volt from passing on through the amp. A relatively small portion of the signal, but it is pretty much the entire amount of hiss and noise. Voila, instant noise gate.

The downside is the missing part results in essentially a little crossover distortion. That is why the noise diodes are only in the circuit on the overdrive channel. It would be a lot more moticable on the clean channel.

[Guitarist]

renewing and improving

I get your drift Enzo. They could've better named this than compression then I traced what seems to be happening.

I'm still debugging the amp. I think the small 400V caps have gotten overworked over 30 years and some of the resistors look so tiny! I'll check the heater joints now.

I want to try ValveWizard's bootstrapped cathode follower (split loaded plate resistors preceding -->cathode following) for more gain and clipping. He said 47K will distort more. But that page is gone presently. Does the CF side clip more with an 12ax7 than a 12au7? In case one had a 12dw7 about. I also realized that the 12ax7 has only 10K of bandwidth vs other tube types which are far extended, according to a chart I viewed.

Anyone else notice that the VTM 60/100 is very similar to the SLO? But definitely not built like one though
I hope the loop plays well with a Lexicon rack reverb, wish I could go parallel somehow...

As the OT is smaller I may pull the KT66s now and try 5881 or 6v6.

[Gtr_tech]

No way 6V6s will hang in that amp. Don't bother unless you like fireworks. These would be a decent choice tho: Electro-Harmonix 6CA7 EH