Peavey VTM 120 Repair Troubles

[The Dude]

I'm asking about the control grids- not screen grids. Trying to determine if bias -V is dropping or a coupling cap is leaking.

[Jynk_O]

I'm asking about the control grids- not screen grids. Trying to determine if bias -V is dropping or a coupling cap is leaking.

Gotcha! Control grids fall to about -50.2v from -53.5v

[The Dude]

So, all voltages are dropping slightly. Probably because of the extra current when the tubes red plate. More current = less voltage. The amp might be going into oscillation. Do you have a scope? If not, it could be a bad tube. You might try tube substitution, one at a time, first since it's an easy step.

[Jynk_O]

Tried all the combos, and trading out for another to no avail. The redplating took one tube from the original matched quad because I didn’t catch it and it was glowing for a few minutes, so I’ve put 6 new tubes total in here

[Jynk_O]

I haven’t mentioned it because I assume it’s a given by design, but all the voltages are fully stable during standby, and the warmer the tubes the more frequent the drops until it finally redplates one side

[The Dude]

I may have forgotten or missed it, but have you ever checked bias current? Maybe the thing is just running away if you're way over biased.

[Jynk_O]

I may have forgotten or missed it, but have you ever checked bias current? Maybe the thing is just running away if you're way over biased.

Is there a way to measure directly on a fixed bias? Using the output transformer resistance method, I got 52.3ma. Using a bias cheap probe, it just shows 0ma

[Jynk_O]

Really wanted to finish this myself since I’ve put a lot of time (and money) into it, but I may just go ahead and take it to a tech. Feel like I’ve tried about everything I can think of or scavenge the web for, and am just frustrated at this point

[Baronkarza]

Is there a way to measure directly on a fixed bias? Using the output transformer resistance method, I got 52.3ma. Using a bias cheap probe, it just shows 0ma

Hi.

Can you easily cut the traces ?

If yes:

You can put a 1R ohm resistor ( 1% 2W), across pin 8 to ground to easily measure the bias of one or each tube.

You can also modify the circuit for adjustable bias.

You can put a grid stopper ( 5k6 1W) from the grid leak (47k) to pin 5 ( grid) of every 6L6 to prevent oscillation.

If im not wrong.

[Helmholtz]

Is there a way to measure directly on a fixed bias? Using the output transformer resistance method, I got 52.3ma. Using a bias cheap probe, it just shows 0ma

See post #11.Tube problems show best with cathode current.
You'll need to measure DC mVs across the cathode resistors.

[Enzo]

The 47k already is a grid stopper. They are mounted right at the socket on the pc board. The grid return resistors are 220k and are on the preamp[ board with the phase inverter.

A one ohm resistor in the cathode lead is a simple and time tested method of checking tube current. HOWEVER, it is not as simple as just adding them in on this amp. These sockets are on a pc board. Peavey ran a common trace from pin 8 to pin 8 all down the row. They also wired the flyback diodes and the hum balance cap to this trace. SO to run a resistor to ground, we'd need to keep the trace continuous but isolated from the pins 8. Not technically daunting, but really overly complex to implement on the board.

The layout image on the factory drawing file does show trace work, so you can see what I mean.

[Baronkarza]

The 47k already is a grid stopper. They are mounted right at the socket on the pc board. The grid return resistors are 220k and are on the preamp[ board with the phase inverter.

.

Sorry. My mistake.
I've missed totally R24 and R25...

[Helmholtz]

The layout image on the factory drawing file does show trace work, so you can see what I mean.

I see.

Regarding the OT resistance method to measure plate idle current, it's not necessary to have all tubes in. With only one PP pair, B+ will increase somewhat, but tube currents will essentially stay the same.

As the problem is intermittent, it's most probably caused by a bad contact/connection (e.g. tube socket) or a faulty tube.

[g1]

You said you replaced the power tube sockets. If you are not 100% sure of your soldering ability, you may want to resolder them.

[Jynk_O]

You said you replaced the power tube sockets. If you are not 100% sure of your soldering ability, you may want to resolder them.

In this case, I’m sadly pretty confident- I dropped it off with some great techs and leave it to them to get the final pieces together. They said their preliminary fears w/o looking may be it actually is a PT this time, but I’ll update with any info for folks with similar issues in the future!