Fender vibrolux reverb issue

[Bigdrums]

Client brings me this amp with a claim of it having a "buzz" when low E string is struck. Even though not very noticeable, I could hear it. After tightening all the speaker and cabinet screws, I checked the bias at idle, then watched it when the guitar is being played. The dual bias probe which measures in mA, sweeps much further on one tube than the other. I realize this is not the intended way of using this meter, but it does correlate to the customers complaint. Struck hard on the low E string, the meters rise to 65 mA on one tube, but 100mA on the other. At idle they are 30 and 40 mA. So I put in a pair of closely matched 6L6's measuring 34 and 35mA at idle. But striking the low E string again one rises much higher than the other as before. To my understanding these should be about the same, at idle or when being played. I have checked the components values around the bias circuit and elsewhere but found nothing much to report out of balance. Any comments or suggestions?

'68 Custom Vibrolux Reverb Schematic.pdf

[Jon Snell]

I would look at the waveform from V6 anodes; they should be similar but 180° out of phase.

[Chuck H]

Jon got right to the same thing I see. Only I was going to suggest trying a different PI tube. Thinking on it though his suggestion of measuring first is a good idea in case it has to do with component values in the amp rather than the tube. So do both and measure with each tube. If both tubes demonstrate greater signal to the power tube with more current then check component values for the PI circuit.

[g1]

I don't expect a symmetrical signal from guitar, so I don't think this is necessarily a fault. If you can reproduce those current readings with a sinewave, you might have an issue.

[Bigdrums]

Update: I changed the pi tube. no difference. Swapped the power tubes, no difference. Swapped the bias probe sockets, the readings on the probe changed places. So I know its part of the amp that's the problem. Open it up and check the tube voltages. Both show 428.5 on the plate. The more responsive of the tube sides has 427.9 on the grid but the less side shows 428.6 on the grid. AH Ha. That explains the different bias probe readings. Now to find out why. Update coming. If that doesn't work I'll get the scope out and try looking at the waveform. Thanks

[Helmholtz]

The more responsive of the tube sides has 427.9 on the grid but the less side shows 428.6 on the grid. AH Ha. That explains the different bias probe readings.

I guess you mean screen voltages.
The difference is 0.7V or 0.16%. Can't make a difference in idle currents.
Mains voltage fluctuates all the time.
A change of 1V in mains voltage causes plate and screen voltages to vary by around 4V. Go figure.

[g1]

Swapped the bias probe sockets, the readings on the probe changed places.

I'm not sure I'm understanding this right. To me this indicates an issue with the probes.

[Chuck H]

I'm not sure I'm understanding this right. To me this indicates an issue with the probes.

THIS^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[Chuck H]

It's possible the anomalous readings are a red herring (jargon, sorry Helmholtz ) The problem may not be (probably isn't) anything to do with an imbalance in current. Try this... Check bias current on the tubes using the same bias probe instead of different ones. Then you know you have apples to apples.

[Bigdrums]

Chuck H: The probes I have are dual; 2 sockets, 2 meters. Well calibrated. Switching the sockets demonstrates that the meters are reading the same (when they switch places as well) as they should and that the difference in plate current is a issue with the amp.
The voltage readings I reported was to say the (screen grid) voltage is higher than the plate if only a little on the lesser responsive amp socket

[Chuck H]

Well I didn't intend to derail your thread. I'll just note that it's easy to interpret (or misinterpret) the quote in g1's post 7 above. And we can leave it aside now.

Have you tested the OT?

[Helmholtz]

Get out the scope and check PI outputs (as Jon Snell advised) as well as amp output at full power.

(I never returned a repaired amp without checking output signal/power.)

[g1]

the difference in plate current is a issue with the amp.

Only true if you verify they are getting equal input signals. You have to check the PI outputs as the others have stated.
As I mentioned earlier, a guitar does not put out a nice clean symmetrical waveform. For example, if the top half is bigger in amplitude than the bottom half, one of the power tubes would conduct more current.
You would have to use a sinewave to check the PI outputs.

[Bigdrums]

Well inputting a sine wave yields a much more even output from the 2 tubes on the bias probe so I guess I was barking up the wrong tree as they say. I will continue using the advice of all here.

[Chuck H]

FWIW there's phenomenon that happens with some hot strat pickups and BF style amps that causes the tonal problem you describe. I've read about it here and there but only experienced it on one occasion. On my last Fender-ish build I was using a 22uf first stage cathode bypass cap and a 1uf for the second stage after the tone stack. All was fine but the owner wanted more bottom end so I changed the second stage to a 22uf. Still fine, BUT... Then I did a pickup swap in my own strat and the result was a barking sort of rattle distortion on heavy bass notes. And it only happened with the neck pickup after the neck pickup swap. I didn't think this pickup was hotter per se but it did have more bass dynamics than the previous one. The amp owner didn't have the problem and wanted the extra bottom end so I left things as they were but made a mental note of it.

So if there's been any modification to the amp past or recent having to do with the cathode caps, and/or there's been any pickup or guitar changes recently this might be something to consider. The vintage tones are great as far as they go but now when we add things like hot pickups and pushing limits further it can go over the line.