Princeton Reverb - Reverb Noise

[tubeswell]

I think there is still a ground loop or cold solder joint somewhere

Ground loops are a nuisance in reverb circuits (because the S:N ratio is particularly important in the recovery stage). Where have to got the reverb recovery stage grounded?

(I presume you've re-flowed all the joints?)

My reverb recovery cathode is grounded at the buss bar which goes to a star ground washer near the input. It's supply is also grounded at the same place (ie star ground washer)

So are there any other signal grounds onto the same buss bar - between the reverb recovery ground and the supply filter cap ground for the reverb recovery? And what other grounds are they from?

[Bombacaototal]

I spent the whole weekend trying to troubleshoot the reverb.

After removing the cathode capacitor of the reverb recovery triode I have no more hiss (as expected as the gain is being reduced drastically). Now I am left with a 100hz hum which reacts according to the pot level, that meaning when reverb pot is at zero there is no hum, at maximum there is a lot. The intriguing part is that the hum starts becoming more evident from 2 o’clock onwards.

I did the grounding experiment again by using the alligator clips and by grounding the dwell pot, and the rca return jack (red) and also the pin 7 of the reverb recovery triode tube there is still the hum present.

By grounding pin 6, or the input on the reverb pot there is no hum at all.

I have resoldered every component from pin 6 onwards, I have replaced the tube socket of the reverb recovery, I have replaced the 100k plate resistor and a while back I had replaced the .001uF decoupling cap.

I have also tried a different tube on that position (currently running the low noise 7025) and also tried another reverb tank. The hum is still there.

Last thing I tried was feeding the plates from another supply cap (the one from the tonestack) given I use individual supply caps per “part” of the amp, and did not make any difference

Very last thing I tried was trying to ground the reverb driver cathode ground at the rca jack, given the rca jack is grounded locally at the chassis and no difference

I am lost, could it be the decoupling cap? I will make a recording tomorrow of the noise situation

[Bombacaototal]

Below is the audio of me turning the reverb pot to max and then back to zero a couple of times

The test is with the chassis out of the cabinet, the reverb tank about 1 meter apart and the speaker cabinet about 1.5 meters apart

I have disconnected the tonestack completely for this experiment (via a toggle switch after the deoupling cap of the tonestack recovery triode). So what you hear is only Reverb circuit into the PI

[Bombacaototal]

Ground loops are a nuisance in reverb circuits (because the S:N ratio is particularly important in the recovery stage). Where have to got the reverb recovery stage grounded?

(I presume you've re-flowed all the joints?)

My reverb recovery cathode is grounded at the buss bar which goes to a star ground washer near the input. It's supply is also grounded at the same place (ie star ground washer)

So are there any other signal grounds onto the same buss bar - between the reverb recovery ground and the supply filter cap ground for the reverb recovery? And what other grounds are they from?

My grounding scheme is as follows:
All pots go to the buss bar which is grounded at the preamp star ground
All cathodes from the pre amp, reverb driver and recovery at grounded at the pre amp star ground
Input jacks use isolation washers and are grounded at the pre amp star ground
Rca jacks are grounded locally by contact to the chassis
Cap can used to be grounded at the pre amp star ground but now is grounded locally
PI and screens supply caps and bias are grounded at the power tube star ground
The mains have a separate ground
PT and rectifier supply grounded together on yet another ground

[Bombacaototal]

I drew on the schematic the findings and what has been done so far.

The only thing I haven't done yet is to isolate the RCA jacks and ground it at the pre-amp star ground. But is it correct to assume that the issue is from PIN 6 onwards?

By the way I also tried the amp without the reverb pan and the hum as still there

[pompeiisneaks]

If I'm understanding the drawing correctly (ground = HUM and ground = NO Hum, meaning you grounded the signal with an aligator clip at that point) then that means either the third triode itself is microphonic, the power supply filter cap coming into that triode is not getting a good ground or is bad, or the 1.5k cathode resistor is bad.

That testing proves the noise does not exist before entry into that last triode, and it appears somewhere after the exit of it, so either the triode is creating it, or something in it's signal chain after is.

It could also be a ground loop coming in through the 1.5k cathode resistor, or the reverb pot though, have you tried lifting either and grounding it somewhere completely different?

Sorry if this has been covered previously in the thread, but you've limited the problem pretty clearly with the latest picture there to that triode or it's components.

~Phil

[Bombacaototal]

If I'm understanding the drawing correctly (ground = HUM and ground = NO Hum, meaning you grounded the signal with an aligator clip at that point) then that means either the third triode itself is microphonic, the power supply filter cap coming into that triode is not getting a good ground or is bad, or the 1.5k cathode resistor is bad.

That testing proves the noise does not exist before entry into that last triode, and it appears somewhere after the exit of it, so either the triode is creating it, or something in it's signal chain after is.

It could also be a ground loop coming in through the 1.5k cathode resistor, or the reverb pot though, have you tried lifting either and grounding it somewhere completely different?

Sorry if this has been covered previously in the thread, but you've limited the problem pretty clearly with the latest picture there to that triode or it's components.

~Phil

Hi Phil, yes correct regarding the ground = HUM and ground = NO Hum; I have grounded that point with the alligator clip and I am showing the result of the hum afterwards.

Thanks for confirming that the noise does not exist until after the PIN 7 of the reverb recovery. There are not many components on that part of the circuit so hopefully I am nearly there as far as finding a solution.

Regarding your suggestions:
1) As far as the triode being microphonic, I have tested a different 12AX7 and had the same result. I also replaced the socket as my initial thought was a bad socket connection. I guess we can rule this option out?
2) As far as the power supply filter cap, I have a dedicated supply cap to the reverb recovery and the dry wet mix (which is the other triode of that tube). Given the tonestack is working fine I removed the current supply cap lead feeding the reverb recovery triode and added a lead from the tonestack power supply filter cap. Unfortunately the hum was still there at the same level as before.
3) Bad 1.5K cathode resistor: this I haven't tried a replacement. I will attempt tonight and revert accordingly. I will also try grounding the 1.5K elsewhere as I haven't yet. The ground of the 1.5K of the reverb recovery is currently jumpered together with the ground of the cathode of other triode and both are grounded at the pre-amp star ground. I will remove this connection and ground the 1.5K in different places to see if it solves.
4) Ground loop from the Reverb pot: The pot is currently grounded at the buss bar. I will double check if the grounding of the pot is fine (ie solder it again and make sure there is no cold joint)

[tubeswell]

My grounding scheme is as follows:
All pots go to the buss bar which is grounded at the preamp star ground
All cathodes from the pre amp, reverb driver and recovery at grounded at the pre amp star ground
Input jacks use isolation washers and are grounded at the pre amp star ground
Rca jacks are grounded locally by contact to the chassis
Cap can used to be grounded at the pre amp star ground but now is grounded locally
PI and screens supply caps and bias are grounded at the power tube star ground
The mains have a separate ground
PT and rectifier supply grounded together on yet another ground

If you have a cap can and the preamp ground seperate to the output tube ground and separate again to the power supply ground, it will hum more.

A quick reference grounding article by RG Keen attached, is suited to cap-can amps.

Another proven ground system is the galactic ground (see Merlin Blencowe's article) http://www.valvewizard.co.uk/Grounding.html. I find this is really quiet, although you need to use seperate filter caps, which incidentally is quieter (than cap cans) in amps with reverb circuits.

[Bombacaototal]

Ran further trials yesterday with no avail

1) Replaced the cathode resistor of the reverb recovery and there is still hum
2) disconnected the cathode ground of the reverb recovery and with the alligator clips experimented grounding it at different ground points (FX Loop, Pre Amp grounding star, buss bar, power tubes grounding, power transformer grounding). Some had more hum than the others but there was always hum. I realised I didn’t try grounding it at the rca jack ground.
3) moved the cap can grounding back to the pre amp star grounding, and I think it hums slightly more now, but negligible. The cap can is only feeding the tonestack and he reverb driver
4) I made sure that the reverb pot has no cold joints and is properly grounded to the buss bar, but I haven’t tried moving it’s ground elsewhere

All tests were with the tonestack disconnected and therefore only reverb into PI. There are not many grounding points between these two parts of the circuit:
1) Reverb circuit:
a) Dwell Pot - pre amp star ground
b) reverb driver tube cathode - buss bar ground
c) rca black (send) - ground locally
d) reverb transformer ground - grounded at the rca black ground
e) rca red (return) - ground locally along with the 220k
f) reverb pan ground
g) reverb recovery cathode - moved it around now back at the buss bar
h) reverb pot - buss bar ground
i) dry wet mix cathode - pre amp star

I think the RCAs might be a potential candidate of interaction with the reverb recovery cathode but will be a massive work as they are now a bit under the board and with limited space to enlarge the holes which means I’d probably have to move them elsewhere.

Could the .001uF Orange Drop PS series after the reverb recovery triode be leaking DC? I have to get some new ones to try a replacement again

I run the alligator clips grounding experiment again with same results

[norburybrook]

Raphael

are you using a correct 12AT7 for the reverb driver, you mentioned a 7025 tube not sure if you meant for the driver.

I'd try a couple of different AT7's if so.


Marcus

[Bombacaototal]

Raphael

are you using a correct 12AT7 for the reverb driver, you mentioned a 7025 tube not sure if you meant for the driver.

I'd try a couple of different AT7's if so.


Marcus

Hi Marcus, thank you for the note. Yes the reverb driver is correct but I will try a couple of different ones to see if it makes any difference.

I am using the low noise 7025 on the reverb recovery as per suggested by sluckey

This reverb has been a real nightmare...

[norburybrook]

Raphael

are you using a correct 12AT7 for the reverb driver, you mentioned a 7025 tube not sure if you meant for the driver.

I'd try a couple of different AT7's if so.


Marcus

Hi Marcus, thank you for the note. Yes the reverb driver is correct but I will try a couple of different ones to see if it makes any difference.

I am using the low noise 7025 on the reverb recovery as per suggested by sluckey

This reverb has been a real nightmare...

as this is a long thread, did you try a different tank? I had a 'bad' tank on my build that woulds self oscillate and go into feedback!!!! I tested it with a know good tank and that told me where the issue was. A replacement tank sorted it out.

It's become my grab and go Jazz amp, I'll be using it today in fact with my Jazz duo

M

[Bombacaototal]

Raphael

are you using a correct 12AT7 for the reverb driver, you mentioned a 7025 tube not sure if you meant for the driver.

I'd try a couple of different AT7's if so.


Marcus

Hi Marcus, thank you for the note. Yes the reverb driver is correct but I will try a couple of different ones to see if it makes any difference.

I am using the low noise 7025 on the reverb recovery as per suggested by sluckey

This reverb has been a real nightmare...

as this is a long thread, did you try a different tank? I had a 'bad' tank on my build that woulds self oscillate and go into feedback!!!! I tested it with a know good tank and that told me where the issue was. A replacement tank sorted it out.

It's become my grab and go Jazz amp, I'll be using it today in fact with my Jazz duo

M

Marcus, do you mind checking which side of your reverb tank is grounded? I have tried with both send and return grounded (inside the tank) and just the return (output) grounded (as per stock TAD pans). I haven't tried the send (input) grounded only.

[pompeiisneaks]

have you tried moving the pan around too? One of my repair jobs was on a reeve's space cowboy that had a decent hum and I found it disappeared if I moved it out of the cabinet. So I spent a ton of time rotating it slightly, moving it back and forth inside the cabinet until I found a spot where the noise was almost completely gone. The reverb pan's can be pretty sensitive as well. (Marcus' suggestion is also a good possibility). I ended up putting some cardboard inside the reverb bag so that it helped shield it a bit as well.

~Phil

[Bombacaototal]

More trials yesterday and same issue

I think I need to take a break and come back later with fresh eyes. Probably need to rework the entire supply cap grounding and see if it lends at a better outcome.

Yesterday I tried 2 different 12AT7 and given I had no more of those I also tried a 5751 on the reverb driver and there was still hum

I tried 3 different reverb tanks and 2 different rca cables with the tank half meter to 1 meter away from the amp. I also tried wrapping the least noise of the 3 pans with copper foil. All of the experiments still had hum

I then went on to replace all hook up and coax cables from pin6 of reverb recovery until the reverb pot. Still hum.

I then replaced the belton socket again because I like torture? No because I am out of ideas...

Is there a way to find out if the noise I have is truly a ground loop, or would the sound of a leaking DC orange drop be different, or yet the sound of a cold solder joint would be different too?

Going back to my gator clipping experiment, if PIN 7 has hum and PIN 6 doesn’t, I can forget that everything behind PIN 6 even exists, correct? So the issue has to be between the socket and the reverb pot if it is a failed component or a cold joint. If it is a ground loop we have to add the power section to the equation. Mine sounds like a 100hZ...