Ampeg V4 VT22 Loud Noise in Preamp

[Helmholtz]

This tells us that the noise is either generated by V3 or its plate resistor - or between V1 and V2 and V3.
Does any of the controls influence the noise?
Did you try a different V201?

What kind of noise is it anyway?
A scope pic could help.

[misterc57]

This tells us that the noise is either generated by V3 or its plate resistor - or between V1 and V2 and V3.
Does any of the controls influence the noise?
Did you try a different V201?

What kind of noise is it anyway?
A scope pic could help.

V3 has one plate resistor at pin 6, 220K, which is in spec. There is no pin 1 plate resistor. Also V3 is supposed to be a 12DW7, but is instead a 12AT7.

With all tubes inserted; tapping with a chopstick anywhere around V1 thru V3, the noise is a loud snap, and after some consistent tapping it becomes a loud buzzing oscillation which also spikes the current draw on my variac.

what is V201?

[misterc57]

forgot to mention that I can get noise with both vol pots at zero, but it is possible that I am hearing a louder noise with the vol pots turned up, hard to tell, seems more sensitive with the vol pots turned up.

[Helmholtz]

Acc. to schematic V201 is a 6K11.

Your symptoms suggest a bad/intermittent contact: Tube socket, bad solder joint, cracked trace, bad ground connection etc.
Can't help with that (remotely that is).

[misterc57]

what a bugger this amp is. so V201 was also noisy when touched, I reflowed all the pins and surrounding pins and no more noise there.

The tapping still creates noise in the preamp and occasionally goes into oscillation. Seems to get worse as the amp warms up.

Some new observations.

When flipping the standby switch the amp sometimes immediately goes into oscillation. If I wait another minute, and do the same there is no oscillation.

While tapping on the board and hearing the loud snaps, I noticed an arc inside the V1 tube.

[Helmholtz]

How does the oscillation show? What frequency?

Could be a microphonic tube. Tap the tubes with your finger.

[misterc57]

How does the oscillation show? What frequency?

Could be a microphonic tube. Tap the tubes with your finger.

Since I reflowed all the preamp tube pins tapping a tube does not usually cause the issue. I have to aggressively tap the board. If I turn the volume up, the board becomes more responsive to making noise and oscillation.

Since the oscillation cause the current to spike I am reluctant to keep the amp on and have no time to take a scope picture. Suggestions welcome on how to do this.

Maybe I should swap all 4 preamp tubes for a quick test.

[Helmholtz]

Not clear. Can you hear the oscillatiion?
If not, how do you know its oscillation?

What do you mean with current spike?

Do you have a bulb limiter?

[misterc57]

Not clear. Can you hear the oscillatiion?
If not, how do you know its oscillation?

What do you mean with current spike?

Do you have a bulb limiter?

Maybe I am using the wrong terminology. By oscillation I mean a loud constant buzzing noise and the amperage meter on my variac spikes (pegs to its limit).

I do have a bulb limiter.

New and good news. I swapped all 4 preamp tubes and no longer hear loud snaps or constant buzzing noise. However the noise is was replaced with a metallic clang every time the board is touched or tapped with a chop stick.
I then started to swap tubes one at a time. In the end I am left with two of the original tubes and two replacements. No more noise of any kind!

Hopefully it stays that way.

Thank you so much for your help. Your comments helped me get to this point.

[Helmholtz]

The advantage of a bulb limiter over a variac is that it automatically limits the current to the amp.
E.g. a 100W bulb limits the current to less than 0.8A, allowing to do some measurements and scoping without blowing a fuse or causing further damage.
Of course all internal amp voltages will be low on a bulb limiter and some failures might not show.