Princeton Reverb 240Hz Hum

[dajazzman]

Hello,
I'm building a Princeton Reverb and I'm getting 240Hz hum when the reverb pot is turned up to 10 through the speaker. I believe it's coming through the "B" node that supplies the reverb driver. I can minimize the hum by adding capacitance to the "A" and "B" node, but then I have 60uf on the "A" and 40uf on the "B".
It's strange that I see 60Hz, 120Hz, very little 180Hz, and then 240Hz which is higher than the 60Hz and 120Hz.

I have chopstick all the wires, replaced all tube including the rectifier three times, removed the "A" and "B" sections off the cap can and substituted external caps. I checked it against the other Princeton I built, but on that one I see only 60Hz and 120Hz, and very little 180Hz and 240Hz.

The one difference in this amp build is that I isolated the reverb jacks and wire the ground of the reverb receive, foot switch, and Trem foot switch to the reverb recovery cathode ground which is my preamp ground. I removed that ground and took it to the chassis with no change. If I remove the reverb send cable or ground the reverb receive grid to V3A it stops the 240Hz. Grounding the grid on V2 does not, and that is what makes me think it's coming in on the "B" that feeds the primary of the reverb transformer. Plus minimizing it with added capacitance at the can cap.

I'm running out of ideas to try and would appreciate a new perspective. I've attached a picture. Some wires are off for testing purposes. The amp works and sounds good except for the 240Hz hum.

Jim

[sluckey]

Is the 240Hz in sync with the input AC power?

No pics in that zip file.

[dajazzman]

I can't see the 240Hz on my scope, but only on my spectrum analyzer phone app I don't know how to see if they are in sync. The SA shows 60, 120, no 180 and then 240 which is stronger than the 60 or 120.

I'm trying the attachment again.

[sluckey]

I can't see the 240Hz on my scope, but only on my spectrum analyzer phone app I don't know how to see if they are in sync.

You would trigger your scope on line to tell if it is synchronous. If you can hear it you should be able to see it on a scope.

[dajazzman]

I'm not sure if this is what you meant. I took ch 1 of scope to the line voltage coming in and ch 2 to the speaker terminal. I triggered off ch 1. It was a little hard to tell with the 240 mixed in with some 60Hz at the speaker, but when I turned the off the 240 with the reverb pot, the 60Hz looked to be 180 degrees out of phase with the incoming line voltage.

[dajazzman]

It also looked like the 240Hz was out of phase from what I could tell

[sluckey]

That's one way, but there is probably an easier way on your scope. There is probably a switch (or mode if using a newer digital scope) called trigger source. It will likely have several positions such as INT CH1, INT CH 2, EXT, and LINE. Set that switch or mode to LINE.

I'm not talking about phase. I'm talking about sync. If your 240Hz signal was just drifting or free running across the display while the scope is triggered on LINE, then it is not related to the 60Hz line AC. If your 240Hz signal was steady on the display, then it's in sync with the power line and is caused by the power line, probably filter caps.

[dajazzman]

The trigger source only had vert, Ch1, Ch2, and Ext. I had the trigger source on Ch1 which was connected to the incoming AC at the fuse holder. Ch2 was connected to the speaker terminals. When I set the time at 1ms, I can see the 240 and 60 and they are rock solid steady just 180 out.

I have removed the A & B portions of the cap can and sub in separate caps with no change. The C & D are still connected to the cap can.