Pavey VTM 60 preamp hum

[vintagekiki]

Cathode to grid is showing 1.5v on both sides

Voltage U_R19 are OK
1)



The Peavey VTM120 has 4 PCBs that are interconnected with a lot of connector into one unit.
Popping / crackling is a malfunction of a mechanical nature, which occurs due to bad contacts in (interconnect) connectors, tube sockets, jacks with switches ... ...
By measuring DCV, it is difficult to determine something concretely, only by the elimination (part by part) can determine assembly where popping / crackling occurs.

[wormdirt]

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)

I grounded pin 2 and it's dead silent

What's this tell me?

[Enzo]

Means the source of the noise is BEFORE pin 2 in the circuit.

[wormdirt]

Voltage U_R19 are OK
1)



The Peavey VTM120 has 4 PCBs that are interconnected with a lot of connector into one unit.
Popping / crackling is a malfunction of a mechanical nature, which occurs due to bad contacts in (interconnect) connectors, tube sockets, jacks with switches ... ...
By measuring DCV, it is difficult to determine something concretely, only by the elimination (part by part) can determine assembly where popping / crackling occurs.

This morning I tested all the ribbon connectors in and out of circuit. They all check out with no noticable burns or breaks

[wormdirt]

Means the source of the noise is BEFORE pin 2 in the circuit.

So with not preamp tubes, except v4 , and the grid grounded test, I can assume the noise is in between the signal coming out of v3 and the grid of v4

[Helmholtz]

I don't like shorting the input grid of a LTPI with a wire (and not a cap), as the grid sits on positive voltage (here around 75V) and shorting it to ground totally screws PI operation. It essentially puts the PI input triode into cutoff.

And I don't agree that PI voltages look right. Cathode voltage seems too high and plate voltages should be lower as well.
What is PI supply voltage?

[vintagekiki]

So with not preamp tubes, except v4 , and the grid grounded test, I can assume the noise is in between the signal coming out of v3 and the grid of v4

When you remove only V3 do you have noise?
Between V3 and V4 you have R14, R15, R21, C7, C8 and jack J7 (via connector J3).
In what condition is the cable shield between J3 - J3?

[wormdirt]

When you remove only V3 do you have noise?
Between V3 and V4 you have R14, R15, R21, C7, C8 and jack J7 (via connector J3).
In what condition is the cable shield between J3 - J3?

What would be the difference between v1 v2 v3 being pulled and just v3?

Shield has continuity across it.
Instead of pulling and checking each component it might be better to just start replacing everything between v3 and v4

[Enzo]

Most times I do not remove a part to check it.

[g1]

I don't like shorting the input grid of a LTPI with a wire (and not a cap), as the grid sits on positive voltage (here around 75V) and shorting it to ground totally screws PI operation. It essentially puts the PI input triode into cutoff.

Agree. wormdirt maybe double check grounding pin2 of V4 through a cap, like .1 or .047 or similar.

[vintagekiki]

What would be the difference between v1 v2 v3 being pulled and just v3?

The question is not answer to the question.

When remove V3 and no noise, communication between preamp and PI is interrupted, it indicates that noise is generated in preamp (V1, V2)
Different description of malfunction are mixed. Please what exactly is about: noise, hum, popping, crackling ... ...

[SoulFetish]

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.

thats a great point.

[Helmholtz]

Shield has continuity across it.

Not sure what that means.
Shield needs to have continuity to ground.

A shield acts a capacitor shorting electrical interference to ground.

[wormdirt]

Not sure what that means.
Shield needs to have continuity to ground.

A shield acts a capacitor shorting electrical interference to ground.

It has continuity to ground and continuity across it, I just pointed out the less obvious thing to check I guess

[wormdirt]

The question is not answer to the question.

When remove V3 and no noise, communication between preamp and PI is interrupted, it indicates that noise is generated in preamp (V1, V2)
Different description of malfunction are mixed. Please what exactly is about: noise, hum, popping, crackling ... ...

Yes the sound persists.
The noise is this
https://youtu.be/2oPOBuYCBRk