Fender Blackface Deluxe Reverb Hum/Buzz problem

[Westrek]

I've got an Blackface AB763 Fender Deluxe Reverb on my bench with a quite baffling 120 cycle hum/buzz problem.

With all the control pots in the full ccw position, the amp outputs a not overly loud, but unacceptable hum/buzz. Raising the volume pot on the vibrato channel increase the volume of the noise, but approaching the midpoint of the pot, the noise is almost nulled out. Continuing cw the noise is further increased.

Same test with the Normal channel, (all amp controls ccw) the noise increases in a linear fashion, but is nulled out in the full cw position. This is the main clue to the problem I believe, but I don't know how to interpret it. The amp actually sounds fine and in the case of the Vibrato channel, setting the volume to mid point, one would never know there was a problem. Pulling the PI tube kills the noise.

The power supply capacitors have been recently replaced with the blue sprague's and the rest of the electrolytics are new. Coupling caps are orange drop, with the exception of the tone stack caps which are Sozo film. The power tube bias is correct as well as voltage measurements at all of the test points. Fully disconnected the reverb recovery circuit at V4 with no improvement.

All the tubes have been swapped and/or replaced. I've made improvements to the grounding scheme as suggested on this site and others, but with no improvement.

I'm sure I'm missing something here...any input would be appreciated! Thanks Mike

[Enzo]

Isolate the problem. Apparently the hum exists with all controls down? Fine, then ignore the preamp for now. Pull the phase inverter tube, does the hum stay or go? Stay? Then the power tube stage is the issue. Do both pins 3 and 4 of every power tube socket have good B+? 400v more or less? And clean? And on pin 5 verify both tubes have good CLEAN bias voltage.

Oh yes, verify the phase inverter was a 12AT7.

There are several wires to ground from the eyelet board, are all intact?

The pots and jacks rely on good connection to chassis. I like to loosen all the nuts, poke at the star washer behind the panel to move it a little so the teeth get a fresh contact. I use my little pointy tool. The snug the nuts back up. This makes sure the pots and jacks are well grounded.

As to the control that has least hum in center, either the pot is picking up the hum, or you have two sources of hum at counter phase.

[Chuck H]

When you say all the tubes have been swapped or replaced, does this mean it was done in an attempt to mitigate the problem? Or is the tube swap/replacement something that was done for maintenance before the problem? I mention it because I've had plenty of noisy preamp tubes new out of the box. So much so that I buy extras when I'm building just to cull the noisy ones. And I buy from the known vendors.

Otherwise I might suspect some untested problem with the power supply. First, can we assume you replaced the bias filters along with the power rail filters??? After that there's the resistors in the circuit and the grounds. Check for grossly drifted resistance in all HV related resistors (inclusing PI plate loads) and recheck all grounds for the HV rail filters for relative goodness and accuracy.

EDIT: Oh, and what Enzo said also - just before I pressed enter

[Westrek]

Isolate the problem. Apparently the hum exists with all controls down? Fine, then ignore the preamp for now. Pull the phase inverter tube, does the hum stay or go? Stay? Then the power tube stage is the issue. Do both pins 3 and 4 of every power tube socket have good B+? 400v more or less? And clean? And on pin 5 verify both tubes have good CLEAN bias voltage.

Oh yes, verify the phase inverter was a 12AT7.

There are several wires to ground from the eyelet board, are all intact?

The pots and jacks rely on good connection to chassis. I like to loosen all the nuts, poke at the star washer behind the panel to move it a little so the teeth get a fresh contact. I use my little pointy tool. The snug the nuts back up. This makes sure the pots and jacks are well grounded.

As to the control that has least hum in center, either the pot is picking up the hum, or you have two sources of hum at counter phase.

Yes, the hum happens with all controls down, but turning up the volume pot on either channel dramatically increases it.

Pulling the PI stops the noise.

Voltage readings at pins 3-4 on the power tubes and pins 1,6, and 3 on the preamp tubes are within 10% of specifications.

At the paralleled main filter caps, it has a sharp sawtooth wave of around 3 vac.

All the grounding is solid, but I did not check the pots and jacks, but will per your suggestion.

I've attached a recording of the noise. It's a bit exaggerated as I held my phone a bit close to the speaker.

Thanks for the suggestions!

[Westrek]

When you say all the tubes have been swapped or replaced, does this mean it was done in an attempt to mitigate the problem? Or is the tube swap/replacement something that was done for maintenance before the problem? I mention it because I've had plenty of noisy preamp tubes new out of the box. So much so that I buy extras when I'm building just to cull the noisy ones. And I buy from the known vendors.

Otherwise I might suspect some untested problem with the power supply. First, can we assume you replaced the bias filters along with the power rail filters??? After that there's the resistors in the circuit and the grounds. Check for grossly drifted resistance in all HV related resistors (inclusing PI plate loads) and recheck all grounds for the HV rail filters for relative goodness and accuracy.

EDIT: Oh, and what Enzo said also - just before I pressed enter

Yes, the power supply caps are new Blue Sprague's. I keep JJ tubes in stock, so I was able to try several different units. All the plate load resistors have been replaced with modern metal film types.

Thanks for your response! Mike

[Enzo]

If pulling the PI stops it, then the power tube readings are extra.

3v ripple on the main filter cap is about right.

We didn;t ask, but is the hum 60Hz or 120Hz? That is a clue to its source. 120Hz is power supply ripple hum. 60Hz is elsewhere. Oops, bias supply ripple is 60Hz too.

Each channel has its own input tube, but the second section of each shares a cathode resistor. SO pull one of the tubes, then replace it and pull the other. Either one stop the hum? Or pull both, same question?

I don't recall, does the reverb control affect the hum?

The reverb recovery tube uses one half for reverb. But the other half is in the Vibrato channel output,, and it shares a cathode with the reverb recovery. Pull that tube. It ought not affect the Normal channel, but the hum? (Pulling that tube should kill the Vib channel signal.)

The two tubes with shared cathode resistors means the channels can interact. So we need to isolate.

[Westrek]

The hum is 120 hz. Fortunately, my Tek 2236 has a freq counter display. I did scope the bias supply. Looks clean with .023 vac of ripple.

Pulling V4 eliminates the noise for the most part. The reverb pot will increase the noise with V4 installed. In previous experiments I have disconnected the reverb side of V4 by removing the grid input resistor and the B+, with no improvement.

With V4 removed, the Normal channel functions ok, but still has some of the noise when at or near full gain.

I keep thinking this is a grounding issue, but I've measured all the grounding points in reference to the PT center tap chassis connection and they all measure .01 ohms.

Thanks again for your thoughts and suggestions. I will keep hammering away at this problem and report back.

[Enzo]

Here is a trick for those without counters:

Touch your scope probe with your finger. The result on the scope will be 60Hz. It will likely be a very ugly waveform, but 60Hz based. I set the scope for one full cycle on screen, or maybe two. Now I scope the circuit hum. If I see the same number of cycles on screen, it is 60Hz. If I see twice as many cycles on screen, it is 120Hz. It takes less time to do than to describe.

[Chuck H]

Here is a trick for those without counters:

Touch your scope probe with your finger. The result on the scope will be 60Hz. It will likely be a very ugly waveform, but 60Hz based. I set the scope for one full cycle on screen, or maybe two. Now I scope the circuit hum. If I see the same number of cycles on screen, it is 60Hz. If I see twice as many cycles on screen, it is 120Hz. It takes less time to do than to describe.

That makes for a faster visual reference than the way I've been doing it. Which is to either set my meter so I know mathematically what it's telling me and then count OR set my sig gen to 60Hz for a visual. Even though my signal generator is right there on the bench my finger is STILL more accessible

[alexradium]

The hum is 120 hz. Fortunately, my Tek 2236 has a freq counter display. I did scope the bias supply. Looks clean with .023 vac of ripple.

Pulling V4 eliminates the noise for the most part. The reverb pot will increase the noise with V4 installed. In previous experiments I have disconnected the reverb side of V4 by removing the grid input resistor and the B+, with no improvement.

With V4 removed, the Normal channel functions ok, but still has some of the noise when at or near full gain.

I keep thinking this is a grounding issue, but I've measured all the grounding points in reference to the PT center tap chassis connection and they all measure .01 ohms.

Thanks again for your thoughts and suggestions. I will keep hammering away at this problem and report back.

Try feeding the preamp tubes heaters with 6VDC,even a switching supply is ok,and report if it changes some way.

[Chuck H]

Try feeding the preamp tubes heaters with 6VDC,even a switching supply is ok,and report if it changes some way.

Do you suspect 120Hz on the filament supply?

[Enzo]

I suspect the D node filter cap ground return is sharing conductor with the main filter ground return.

[Chuck H]

...and recheck all grounds for the HV rail filters for relative goodness and accuracy.

I suspect the D node filter cap ground return is sharing conductor with the main filter ground return.

This is what I was thinking from the start. The ab763 amps do actually ground the preamp filters on the preamp end of the amp via a relatively long, but independent lead. If the earlier filters are grounded there also or the preamp filter is grounded with the main filter this might cause the problem. If you look at doghouse wiring on an image search you'll find A LOT of variations done during recaps. Not all are correct.

[Randall]

I did something this week while re-capping a AA165 Pro Reverb. While pulling the old 20uF filter caps at the same time, which I don't usually do, a yellow wire came out that could have gone to two different eyelets. I spent quite a while flipping that chassis back and forth tracing out that wire to make sure it went back to the right one.

[Chuck H]

I spent quite a while flipping that chassis back and forth tracing out that wire to make sure it went back to the right one (eyelet).

It would be fun to have the audio of that