Reverb Hum in a Princeton Reverb Build

[Johnrcurry]

I am trying to track down an annoying reverb hum in a Princeton Reverb kit build. The amp was built from a Weber 6a14 kit using the schematic and layout provided by Weber (which I have attached for reference). I used the brass grounding plate and the non-isolated RCA jacks included with the kit. The hum is not present with the reverb turned to zero or switched off with the footswitch. it is progressively louder as the reverb knob is turned up. It is present and behaves this way with the volume at zero and nothing plugged in. Here's what I've tried so far to fix it:

1. I wrapped the two wires from the reverb pot with a wire and grounded it at the pot as an attempt at shielding. (The wires were already twisted together from the original build).
2. I replaced the RCA jacks with jacks that are isolated from the chassis and ran a grounding wire to a spot near the input.
3. I replaced the wire from pin 2 of V3 to the reverb pedal jack with a shielded wire grounded at the jack end.
4. I replaced the wire from the reverb pedal jack to the reverb output jack with a shielded wire grounded at the reverb pedal jack ground.

These changes made no difference in the hum.

I used a signal tracer to try to figure out where the hum is being introduced into the signal and determined that it first shows up on pin 6 of V3. From there it carries through the circuit to the speaker. I tried several different 12ax7 tubes in the V3 position with no change.

I would appreciate any ideas as to where to go next with my troubleshooting.

Thanks

(I've cross posted this in the Weber amp building forums as well)

[Chuck H]

In my experience hum that occurs with the channel volume down but the reverb volume up is usually EMF from proximity of the tank to the PT. You should try relocating the tank and see if you can find a quiet spot. Hell, you can flip the tank opposite it's usual orientation or mount it diagonally. Any place that is quiet. Just scootch it around in there and see if anything changes.

Otherwise...

Since you've already installed isolated jacks it's usually best to lift the ground on one side or the other and just use the chassis ground nearest the jack that you choose. You might even try one jack first and then the other because sometimes it can make a small difference. The shield will still be grounded because it has continuity between the other jack ground and the tank. Grounding both has the potential to create a ground loop. Especially if you're doing any circuit grounding on the jacks.

[Enzo]

I agree it is likely the reverb pan picking up the field around the power transformer. Unplug the cables from the jacks on the chassis, still hum? Or try this: remove the pan from the cabinet and get it as far from the chassis as the cords will reach. does that reduce the hum?

Make sure the output end of the pan is as far as possible from the PT, maybe turn it around? Also make the open side face away. A steel plate across the open side of the pan, at least on the output half can help.

[Johnrcurry]

Thanks for your replies, Chuck and Enzo. With the tank and chassis out of the cabinet I've tried moving the tank in various orientations relative to the chassis and it does not make any difference in the hum. I've also lined the cardboard covering the bottom of the tank with shielding tape, again with no improvement.

Chuck, I'm not sure I understand what your suggesting as far as the grounding of the jacks. As it is configured now I have the four RCA jacks on the back panel isolated from the chassis. Each has a solder lug attached. Those four lugs are linked by a ground wire which is connected to the front panel near the input jacks. Are you suggesting that I disconnect either the input or output jack from that ground wire?

Thanks

[Johnrcurry]

Enzo, I forgot to add: When I unplug the cables from the jacks on the chassis, the hum remains. The sound changes somewhat. . . as if there is an element of hiss added to the hum, but the hum remains with no discernible change in volume.

[Chuck H]

Are you suggesting that I disconnect either the input or output jack from that ground wire?

Yes.

Why four RCA jacks? You only need two for a reverb.

[Chuck H]

Enzo, I forgot to add: When I unplug the cables from the jacks on the chassis, the hum remains. The sound changes somewhat. . . as if there is an element of hiss added to the hum, but the hum remains with no discernible change in volume.

I think you must have created a ground loop with your wiring in the chassis itself. A schematic would be very helpful right now.

[Enzo]

If the hum remains with the cables removed, then leave them off as we continue. No need for the pan if it is not involved.

The cardboard will have zero effect on hum ever.

Isolating the panel jacks is not helping. In fact, the return jack would rather be grounded to the bottom of the return triodes cathode resistor, not the input jack.

Why four RCA jacks? You only need two for a reverb.

Two for reverb pan, two for foot switch.


Also, we have cut this in half. The drive side of the reverb has nothing to do with the hum.

[Chuck H]

Two for reverb pan, two for foot switch.

Ah. Right. I forget that the vintage amps are done that way. When I build I use regular 1/4" phone jacks for the foot switch.

[Johnrcurry]

Thanks guys. Chuck, the schematic and layout are attached to my original post. I will experiment with the grounding of the reverb input and output as you and Enzo suggest and report back. Enzo, when you say that the return jack would rather be grounded to the bottom of return triode's cathode resistor do you mean the spot where pin 3 of V3 is grounded through R33 and C18 on the schematic? Does the fact that the hum first shows up in the circuit at pin 6 of V3 suggest anything about its origin/source?

[Johnrcurry]

Enzo, I forgot to add: When I unplug the cables from the jacks on the chassis, the hum remains. The sound changes somewhat. . . as if there is an element of hiss added to the hum, but the hum remains with no discernible change in volume.

After some further poking around I need to revise this answer. The hum is reduced with the cables unplugged from the jacks, but it is replaced by a hiss.

[Enzo]

OK, that means you have two sources of hum. Something in the reverb circuit, and also some from the pan and cables. Leave the cables out and fix the amp part first. Tack a short across an RCA plug and stick it in the return jack on the amp. Does that kill the hum?

R15, R33, C18 all go to ground, that point would be best.

Does the fact that the hum first shows up in the circuit at pin 6 of V3 suggest anything about its origin/source?

Actually the hum is apparent on the reverb control. You reported that turning reverb to zero stops the hum. So pin 6 might be the first place YOU see it, but it must be there earlier for that control to affect it.


Have we determined if the hum is 60Hz or 120Hz?

[Johnrcurry]

Thanks guys. Here's the latest . . . . Chuck, when I disconnect the ground to the reverb input jack it kills the hum, but the reverb does not work. When I disconnect the ground to the reverb output jack the hum gets a lot worse.

Enzo, with the reverb cables disconnected from the chassis the hum is much reduced, but there is a pronounced hiss. When I short the reverb output jack it kills the hiss and the hum is perceptible, but barely.

[Enzo]

OK< we are troubleshooting, not fixing. With no cables you get hiss, but that his is not present when the cables are connected? Then hiss is not a problem, it is just an artifact of the test procedure.

Is the pan you are using really a 4AB3C1B? That specifies that the input jack be isolated. The output jack is grounded to the pan frame. Make sure the input jack is not grounded in the pan itself. Modern pans sometime have a little piece fo circuit board and you can solder a jumper to make each end grounded or not. The cable from the amp should have the shield grounded at the amp end. The shield is NOT connected to the pan fram, but ONLY to the RCA shell.

I would wager that might be where your ground loop is forming.

[Johnrcurry]

Thanks, Enzo. Yes, with cables I get hiss, but it is not present when the cables are connected.

The reverb pan is an Accutronics 4AB3C1B. Checking for continuity between the RCA outer shell on the output side, there is continuity to the pan frame. On the input side, there is not continuity between the RCA outer shell and the pan. So it seems to be behaving as you indicate it should.