Pavey VTM 60 preamp hum

[wormdirt]

Do you with the elimination method (# 18) determine which tube make sound posted in your video?

I don't know what you mean #18 the link you posted is about solder joints. But the sound is only made with a tube in v4 no other tubes but v4 and the power tubes were in the amp when I made the video

[vintagekiki]

It is logical because the V4 (12AX7 preamp bd) transfer sound from the preamp V1-V3 (12AX7 preamp bd) to poweramp V1-V4 (6L6GC output tube bd).

#18 is link for #18 post
https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/917077-pavey-vtm-60-preamp-hum?p=918220#post918220

Read carefully the procedure described in #18 how remove tube(s) to determine tube where noise is generated.

Question
When remove V3 or V2 or V1 whether noise stop?

[wormdirt]

I've done that, the noise does not change with any external forces, guitar plugged in, cable into the return jack, or when any of the preamp tubes are pulled, except for V4. The the offender has to be in the preamp between after v3 and before v4, correct?

[vintagekiki]

Check interconnect connector J3, and is it the shield (ground) from the cable J3 correct.
Change the V4 (12AX7) with a new (not like new) tube to hear whether the noise has stopped.
If by replacing the tube (s) noise still exists, it is the thermal noise that is generated in some of the anode resistors that need to be replaced.

[g1]

Change the V4 (12AX7) with a new (not like new) tube to hear whether the noise has stopped.
If by replacing the tube (s) noise still exists, it is the thermal noise that is generated in some of the anode resistors that need to be replaced.

He already said he replaced the tube and the anode resistors.

wormdirt you could try grounding the grid (pin2) of V4. Noise still there?
You changed the plate resistors, but how about the cathode resistor? There is a 470R and two 10K's in the cathode circuit. Also all their solder joints.
And how about the power supplies to the PI and the power tubes, all clean? (check with scope)

[glebert]

My Peavey Triumph has similar ribbon cable board connectors. I once wasted time trying to fix a problem that turned out to be having the connector mated incorrectlyand being off by 1 pin. Might be worth a quick look to make sure.

[Enzo]

glebert, I certainly have done that same thing. I bet most of us have. DOn't feel bad.

[glebert]

glebert, I certainly have done that same thing. I bet most of us have. DOn't feel bad.

Oh, it takes something much stupider than that to make me feel bad

[SoulFetish]

Got it.
for posterity I'm getting
350v at the plates
55v at the grids
77v at the cathodes
This is for v4

thats wrong.
There’s no way you’re 12AX7 as a grid bias of -18V to cathode. With a 470R cathode resistor
The two triodes would be drawing like 38 mA combined.
check the grid to cathode voltage not the grid to ground voltage and report back

[Enzo]

Please remember when measuring long tail pair PI grids, your meter distorts the readings. measure across the resistor itself or measure from cathode to grid if you want accuracy. Measured to ground, it will always show way less grid voltage than is true.

[vintagekiki]

https://atomiumamps.tumblr.com/post/88403615731/peavey-vtm-60-pretty-common-problem-of-cracked

... Peavey VTM-60. Pretty common problem of cracked solder on the control PCB, since the only thing supporting it are the pots and jacks on the front panel, which themselves are only connected to the board by their solder pads ...

https://forums.peavey.com/viewtopic.php?f=19&t=26189
^{VTM60 VERY weak! Thoughts, advice?}
https://www.flickr.com/photos/3rdof3/5446749165/in/photostream/lightbox/
^{VTM 60 Preamp Schematic}
https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/24475-peavey-vtm-60

[vintagekiki]

He already said he replaced the tube and the anode resistors.

Maybe I'm boring. I know from practice that when tell someone to replace a tube, he replaces it with the first one (old long ago replaced) that he has in the drawer.

When said to replace tube it means replace tube with new tube (not like new = old long ago replaced)

#29
Good news is that by trying every 12ax7 in my drawer I found that there is some difference in sound between different tubes, but this probably due to varying gain factors or quality

[wormdirt]

Maybe I'm boring. I know from practice that when tell someone to replace a tube, he replaces it with the first one (old long ago replaced) that he has in the drawer.

When said to replace tube it means replace tube with new tube (not like new = old long ago replaced)

I understand but I've only ever had a few 12ax7 go microphonic, and my drawer includes new JJs as well as some NOS EHX tubes with a smattering of random known working chinese tubes. I don't make a habit of saving bad tubes and if 20 tubes do the exact same thing then that is a big enough sample set for me, no offense

Please remember when measuring long tail pair PI grids, your meter distorts the readings. measure across the resistor itself or measure from cathode to grid if you want accuracy. Measured to ground, it will always show way less grid voltage than is true.

Thanks for the knowledge I'll do this

[wormdirt]

Cathode to grid is showing 1.5v on both sides

[Helmholtz]

PI grid voltages must be the same as the voltage at the lower end of the cathode bias resistor R19 as the grid leak resistors R18/R20 don't drop voltage.
Thus it is typically not necessary to measure DCV at the grids.