Princeton Reverb - Reverb Noise

[Bombacaototal]

I built a AA1164 reverb on a head (not a combo) with a few mods (in red on the schematic below) and I am have some issues which I will detail below.

Issue: High noise floor with hiss and hum on the reverb circuit.
1. When turning reverb pot clockwise hiss and hum increases (ie sensitive to the pot level).
2. The noise does not change by increasing or decreasing the Dwell (send) pot.
3. The noise does not change if the send (white) and return (red) RCA jacks are disconnected from the inputs underneath the chassis.
4. The noise does not change if reverb driver tube is removed.
5. There is some small amount of noise with the reverb pot set at zero.
6. By removing the reverb circuit leaving only tonestack and PI there is no issue.

Troubleshooting:
1. Replaced reverb driver (12AT7) preamp tube to another one. No difference
2. Replaced reverb recovery and mixer (12AX7) preamp tube to another one. No difference
3. Replace reverb pan by two different ones: TAD and Groove Tubes. No difference
4. Moved the head around the house (maybe interference from something else). No difference.
5. The noise is the same regardless of the chassis being out of the headshell (away from the reverb pan) or inside (closer to the reverb pan). Conclusion is that there isn’t transformer interference with the pan.
6. Turned the reverb pan 180 degrees. Much more noise, much worst.
7. Tighten RCA cable connections. No difference
8. Replaced the RCA cables with different one. No difference.
9. Removed the RCA (red) return cable only, leaving the RCA (white) send. No difference.
10. Removed the 5uF cathode capacitor of V4b to lower the return gain. Made the noise worst.
11. Replaced V4 tube to 12AT7. Same noise but only at higher reverb pot level.
12. Removed V4 tube. No noise.
13. Moved the 220K from the RCA (red) return connector to the layout board. No difference.
14. RCA jack connectors are grounded to the chassis. Add another ground to the pre-amp GROUND BUSS. No difference (but I decided to leave it).
15. Replaced the .0012uF after the reverb recovery triode plate as maybe it is leaking DC. No difference.

Questions:
1. Is it fair to assume that the issue is not from the cable that connects the pre-amp to the reverb circuit because increasing the Dwell (send) pot does not change the amount of noise?
2. Is it fair to assume that the issue is not from the cable that connects the the reverb circuit to the PI given the noise sensitivity happens at the reverb pot?
3. Is it fair to assume that the issue is somewhere between the RCA return connector and the reverb pot?
4. If there was an issue on the dry path would the hum be sensitive to the reverb pot level?

[sluckey]

Use a gator clip test lead to ground the grid of the return tube to chassis. Do this directly on the tube socket. Be careful. Does the noise/hum go away?

Use a gator clip test lead to ground the wiper of the reverb level pot to chassis. Does the noise/hum go away?

[Roe]

the black- and silverface reverb circuits are somewhat noisy. I rebuilt mine, which helped somewhat

[Bombacaototal]

Use a gator clip test lead to ground the grid of the return tube to chassis. Do this directly on the tube socket. Be careful. Does the noise/hum go away?

Use a gator clip test lead to ground the wiper of the reverb level pot to chassis. Does the noise/hum go away?

I just executed the test.

1) by grounding the triode reverb return pin the noise reduced a bit - maybe 20%
2) by grounding the end of the .001uF cap which goes to the pot input lug there is almost no noise

[sluckey]

So now you know where the noise comes from. I would get a handful of 12AX7s and roll tubes looking for one that gives acceptable hum/noise. I would even try lower gain tubes such as 12AT7, 12AU7, 12AY7, 5751, etc. I've had the same issues with that fender reverb recovery circuit. Rolling tubes has always cured the problem for me. Well, I did one more thing too... remove the cathode bypass cap on that tube.

[Bombacaototal]

So now you know where the noise comes from. I would get a handful of 12AX7s and roll tubes looking for one that gives acceptable hum/noise. I would even try lower gain tubes such as 12AT7, 12AU7, 12AY7, 5751, etc. I've had the same issues with that fender reverb recovery circuit. Rolling tubes has always cured the problem for me. Well, I did one more thing too... remove the cathode bypass cap on that tube.

Thanks Sluckey. I tried a 12AU7 but it was too low gain for me. The 5751 worked well, maybe my second favourite after the 12AX7, but I still prefer the sound/feel of the higher gain. So in order to stick with the 12AX7 I will follow you suggestion and get a number of tubes and go through them one by one.

By the way, did you remove the cathode cap of both reverb recovery triode and dry/wet mix triode? I had tried just removing the cathode cap of the reverb recovery triode but for some reason I felt it made my amp noisier. I will try that again

[sluckey]

I only removed the cap from the driver and the recovery tube. Weaker reverb but less noise. FWIW, Fender used a low noise 7025 for the recovery in the more expensive AB763 amps.

[Bombacaototal]

I removed the cathode cap of the reverb recovery. Indeed an improvement as far as noise but as you said weaker reverb, and also the frequency reponse seems to have changed slightly. I still prefer the sound of the reverb with the cap but I cannot stand the noise floor, so will leave the cathode without the cap for now.

I also just bought a low noise 7025. Cannot wait to try it out. Hopefully that will do it! I will report back as soon as I have it.
https://www.watfordvalves.com/product_detail.asp?id=684

Many thanks again for the help.

[Bombacaototal]

As an after thought, I am pretty sure my amp has a grounding issue somewhere on the reverb circuit as the noise is somewhat similar to the one on the video below which I found on youtube (not my amp).
https://youtu.be/jDJS6JbZSnY

I am attaching an audio of my issue recorded with the amp cranked (in order to make the issue more obvious). I am turning the reverb pot towards the max and then back to zero. By the way the recording is with a 12AX7 and with the 5uF capacitor on the reverb recovery triode.

I am using isolation washers on the input jack and grounding it to the ground buss bar but if I remove the reverb circuit and leaving only tonestack and PI I have no noise. Therefore my issue is somewhere on the reverb circuit.

Could it be something on the grounding of the rca jacks? I have them grounded to the chassis and afterwards I added an extra lead going from the rca ground to the pre amp buss bar (which made no difference to the noise)

I wonder if I should add washers and isolate the rca jacks as I see no other places of potential issues. They seems tight to the chassis too but I will double check today.

[Roe]

noise decreases if you use seperate cathode resistors/caps, instead of shared

[Bombacaototal]

noise decreases if you use seperate cathode resistors/caps, instead of shared

Thanks Roe, I am actually not sharing any cathode resistors/caps. The only shared point between them is the ground.

[Bombacaototal]

Before attempting to add isolation washers to the RCA connectors I think I spotted what may be causing the ground loop hum.

I have connected the grounds of the RCA send and the RCA return (illustration below) even though they are grounded at the chassis. Then afterwards I added an extra ground lead to the main pre amp star ground (which made no difference as far as the noise floor). I assume what I did is the isolation washer approach. For RCAs grounded at the chassis I should not make all those ground connections, as each RCA should be grounded independently, correct?

I will separate the grounds and report back. One question is, given I moved the 220K to the layout board, I am currently grounding it at the pre-amp ground buss. Should I make sure it is grounded together with the RCA return jack or is it fine to leave it at the pre-amp ground buss?

[sluckey]

I would do it like this...

https://el34world.com/Hoffman/images/Img_8412.jpg

[Bombacaototal]

I would do it like this...

https://el34world.com/Hoffman/images/Img_8412.jpg

Thanks sluckey. My approach is slightly different as I have a rca send, a rca return both underneath the chassis and then on the backplate of the amp a single stereo jack for the reverb and tremolo.

I assume I just have the RCA send and the RCA return grounded at the chassis independently.

Also I moved the reverb footswitch right after the send 1MA pot, because it was feedbacking at the stock position and therefore no need for the connection between the footswitch and the reverb return. But then I will probably just ground the 220K to the RCA return ground. Correct? I will try and report back

[sluckey]

I would mount the 220K directly on the return jack. Just let the jacks use their own self grounding (no insulating washers) and don't run any extra ground wires to the jacks. Maybe that will work. It did for Leo.