Very odd behaviour of preamp tube in vintage Gibson amp ("dancing voltage")

[slidincharlie (Carlo P)]

Hi all.
I have this old Gibson GA-18T amp (which actually matches to a tee the GA-16T schematic attached).
It works well and sounds very good, except for a put-put noise when the tremolo is engaged beyond 5-6 on the depth control dial and at high volume. At lower volume and/or at lower depth settings the ticking disappears.
EDIT:
The volume level affects the ticking much more than the depth level.
While taking measurements in the tremolo department, which is driven by V2, I saw that the plate voltage at pin 1 has a strange behaviour (or so it seems to me): the values "dance" between about 160 and 190 V at a frequency of about 0.5 sec. (you may want to look at this very short video that I took: https://drive.google.com/file/d/1wJ_...ew?usp=sharing ).
According to the schematic that plate should be at around 100 V.
This happens with both the current, old 12AX7 tube and with a new JJ tube.
All other readings in the amp are on specs.
What is happening? Could this be related to the ticking tremolo?
--Carlo

[Helmholtz]

V2B is wired as low frequency oscillator (LFO).
The very low frequency output signal appears on its plate pin 1, so plate voltage isn't constant as long as the oscillator is running.
To check plate voltage the LFO should be turned off via the footswitch.

The put-put sound shows that your power tubes aren't perfectly matched.

[slidincharlie (Carlo P)]

Hi Helmholtz.
Re: oscillating voltage - you were right, that ended turning off the tremolo via the footswitch.
Re: ticking tremolo - It occurs also with a new matched pair of JJ tubes. Changing tubes affects the intensity of the ticking noise (stronger with the new tubes than with older tubes).
The volume level affects definitely more strongly the intensity of the put-put than the depth level. I have swapped a new 12AX7 in V1 to no avail.

[Helmholtz]

Does the put-put noise disappear when V1 is removed?

[slidincharlie (Carlo P)]

Does the put-put noise disappear when V1 is removed?

The put-put is still there without V1, BUT it is much weaker AND it remains the same whatever the volume knob position.
If I disconnect the .33 uF cap from the depth pot the put-put disappears (but I guess this is normal).

[Helmholtz]

Seems the noise at least partly spreads via the power supply.
Are your filter caps new?

Try tacking a good 22µ/400V cap across either of the 10µ caps.

[slidincharlie (Carlo P)]

Sorry, I forgot to mention that all filter caps are brand new (30, 16, 16 uF instead of the 20, 10, 10 from the schematic). Installing them did not affect the ticking noise at all.
The previous filter caps were 40, 20, 20 uF that I guess were installed by a previous owner to replace the original caps.

[Helmholtz]

Please still try the test.

[slidincharlie (Carlo P)]

I do not have any 22/400 or similar cap at home unfortunately. I will search for one at a local shop, hoping they stock them.

[slidincharlie (Carlo P)]

Try tacking a good 22µ/400V cap across either of the 10µ caps.

"across": do you mean in parallel? Sorry for the naive question.

[Helmholtz]

"across": do you mean in parallel? Sorry for the naive question.

Yes.
Also other cap values like 16µ or 32µ would do.
Observe polarity.
Remember, not every new cap is a good cap.

[The Dude]

...... I'll add to that- not every new tube is a good tube. In other words, just because you bought a new set of matched JJ's doesn't mean they are actually matched or even good tubes. I would check idle current of each tube and make sure.

[Helmholtz]

...... I'll add to that- not every new tube is a good tube. In other words, just because you bought a new set of matched JJ's doesn't mean they are actually matched or even good tubes. I would check idle current of each tube and make sure.

Agree and add that tubes that are matched at some specific test condition may no longer be perfectly matched in the actual amp.

But for now I'm trying to find out, why V1 adds to the put-put noise.

[slidincharlie (Carlo P)]

I did the filter caps test.
With a new (and cheap) 22uF/450V put across each existing 16 uF cap (total capacitance: 38 uF each, correct?) the put-put is still there, but definitely much weaker.
All existing filter caps were just-installed brand-new Atom Sprague and Fischer-Tausche caps ($$$).

[Helmholtz]

That confirms my theory regarding the power supply.
With the added caps the effect of the volume pot should be much weaker if any.

The remaining put-put noise will be due to some imbalance in the power stage (i.e. power tubes and maybe OT).
A bit of this noise might be normal with these amps.
Does it really bother you with a normal/sensible tremolo setting?