Princeton Reverb - Reverb Noise

[pompeiisneaks]

If you have a multimeter with hz display you can put it on ac and see what frequency the noise is, it will bounce around but you should see it around 100 hz or 120hz if in North America.

Not sure if I fully understood, but did you say with the reverb pan 1 meter away you still have hum? then it's not likely to be the pan. Do you have any led or fluorescent lights near the area you're working? You could try shutting them off to see if it impacts the hum? Sometimes shop noise can cause crazy noise with noisy LED/Florescent lights.

If the 1 m distance was understood correctly, it's likely not a problem with the reverb pan, as that would make it go dead silent because it's so far away from the sources of noise.

Not sure what else to try, though, at this point.

~Phil

[Bombacaototal]

If you have a multimeter with hz display you can put it on ac and see what frequency the noise is, it will bounce around but you should see it around 100 hz or 120hz if in North America.

Not sure if I fully understood, but did you say with the reverb pan 1 meter away you still have hum? then it's not likely to be the pan. Do you have any led or fluorescent lights near the area you're working? You could try shutting them off to see if it impacts the hum? Sometimes shop noise can cause crazy noise with noisy LED/Florescent lights.

If the 1 m distance was understood correctly, it's likely not a problem with the reverb pan, as that would make it go dead silent because it's so far away from the sources of noise.

Not sure what else to try, though, at this point.

~Phil

Hi Phil, thanks for the reply, and yes you understood correctly, I tried the pan 1meter away from the amp and also wrapped around copper foil and there was still hum. Also no LED/Florescent lights on the space I am working on the amp.

I will try the multimeter at Hz display the check the frequency but I am pretty sure it is 100hz.

I will move the grounds of the supply caps around today. I have a couple of ideas on what to try and I will report back.

[pompeiisneaks]

If you have a multimeter with hz display you can put it on ac and see what frequency the noise is, it will bounce around but you should see it around 100 hz or 120hz if in North America.

Not sure if I fully understood, but did you say with the reverb pan 1 meter away you still have hum? then it's not likely to be the pan. Do you have any led or fluorescent lights near the area you're working? You could try shutting them off to see if it impacts the hum? Sometimes shop noise can cause crazy noise with noisy LED/Florescent lights.

If the 1 m distance was understood correctly, it's likely not a problem with the reverb pan, as that would make it go dead silent because it's so far away from the sources of noise.

Not sure what else to try, though, at this point.

~Phil

Hi Phil, thanks for the reply, and yes you understood correctly, I tried the pan 1meter away from the amp and also wrapped around copper foil and there was still hum. Also no LED/Florescent lights on the space I am working on the amp.

I will try the multimeter at Hz display the check the frequency but I am pretty sure it is 100hz.

I will move the grounds of the supply caps around today. I have a couple of ideas on what to try and I will report back.

You could also try adding more capacitance by putting another cap across the filter for that node to see if that helps, it may just be very susceptible to the power supply noise?

~Phil

[Bombacaototal]

I measured with the multimeter and cross referenced with some YouTube audio and it is a 50Hz hum only when I have the reverb on the circuit (noise audio attached)

[pompeiisneaks]

Oh yeah that's def 50 cycle hum. that means something in the main input power supply is leaking into it. heaters, or just general reverb transformer placement etc all may be culprits.

~Phil

[Bombacaototal]

Oh yeah that's def 50 cycle hum. that means something in the main input power supply is leaking into it. heaters, or just general reverb transformer placement etc all may be culprits.

~Phil

Got it, so ground loop discarded?

How can I check for heater leakage? Because the issue seems to be happening on the reverb recovery tube

My donor chassis is a fender reverb deluxe reissue and I haven’t moved anything around, ie the reverb transformer is as per stock amp in between reverb driver and recovery. I have added the copper foil A4 sheet in between the recovery tube socket and the transformer and made no difference. It is worth mentioning that the triode of the reverb recovery is not the one next to the reverb transformer but the one on the other side.

Today I moved a lot of the supply grounds and made little difference and also re checked all the soldering on the reverb driver socket and all seems fine.

[Bombacaototal]

Here is a close up pic of the reverb recovery socket.

I marked in green a couple of wires (red and purple) which are the ground and the supply from the large supply capacitors under the board that go to rectifier, standby, etc. Could this be the cause?

The reverb recovery plates are the purple wire on the socket and the ones when I ground make the hum disappear

[pompeiisneaks]

You can use chopsticks to move the wires around and see if moving them up/down or moving the heater wires near them farther away causes the hum to get worse/better. If so then you'll want to find an optimal path for routing those wires so they don't get near signal wires OR use shielded wires to see if that blocks the hum.

~Phil

[Bombacaototal]

I moved those 2 cables completely out of the way. They are now routed via under the board.

I have also poked and moved the heaters up an down but didn’t make much difference

There is still hum, but it seems to me it’s higher pitch now, is it 100hz (attached)? I think so, therefore back to ground loops

I will re-do the alligator grounding test to make sure it is still happening at between reverb recovery and the reverb pot.

[pompeiisneaks]

Okay I stand corrected, I listened to that hum and it sounds like the same type of hum, so I put it in a spectrum analyzer and its 100hz. See here:

first file you attached:

sound1.png

Second:

sound2.png

Sorry for being wrong yesterday. So it's filtered hum, not wall power hum.
Phil.

[Bombacaototal]

Okay I stand corrected, I listened to that hum and it sounds like the same type of hum, so I put it in a spectrum analyzer and its 100hz. See here:

first file you attached:
sound1.png
Second:
sound2.png

Sorry for being wrong yesterday. So it's filtered hum, not wall power hum.
Phil.

Thanks Phil ,that is very helpful indeed. The second recording I had the mic right at the speakers versus the first which was much further, which I think explains the difference between the high end hiss on the two recordings and subsequently on the charts.

So back to the drawing board, I think I will re-do the grounding scheme completely, have a few ideas in mind.

I will report back

[Bombacaototal]

Bearing in mind my circuit has everything disconnected besides the reverb circuit and the power section I tried a couple different grounding schemes one of which was the Ceriatone SSS. I have too many supply caps overall to go full on Fender style. There is still hum

I also changed the orange drop decoupling cap of the reverb recovery once again and made no difference.

Overall I have changed at least once every component including wires between the reverb recovery triode and the reverb pot. I think failed component can be discarded

I did the alligator grounding a lot of times along the way during a lot of the experimenting and the issue always stops by grounding pin 6 of reverb recovery onwards and every connecting point until the reverb pot.

Besides the grounding loop is there any other possibility of issue which would create the 100hz?

I honestly ran out of ideas, will take a break from this amp for now I think

[Bombacaototal]

So the 50Hz AC is changed to 100Hz DC by the rectifier and then the filter capacitors will smooth the DC.

We know that the rectifier is working fine as there is no issue with the reverb pot at Zero.

I will try again feeding the reverb recovery plates from the tonestack supply to double check the filter cap is not the issue. Then we can once and for all rule this one out too.

But by the looks of it can only be a ground loop somewhere which will also be 100Hz

I have a 4th reverb pan arriving today and will give that one a try too as the 3 previous ones I tried dind't make difference to the hum.

[Bombacaototal]

My grounding scheme is fairly similar to the attached.
1) My input jack is isolated and therefore I have a chassis ground (called PRE-AMP ground) which has:

• a) input jack
  b) Pre amp ground from the board (tonestack and reverb circuit)
  c) buss bar which only has the pots
  d) Pre amp supply filter caps (tonestack and reverb recovery + mixer)
  e) Reverb driver filter cap (I use a dedicated supply for the reverb driver as I prefer running it at way lower voltage than screens). This is part of my cap can which has the tonestack and I therefore cannot isolate its ground

2) RCA jacks are grounded to the chassis and have the reverb transformer wire on the send (black) rca.

3) Speaker jacks are grounded to the chassis and have the OT ground. Similar to the attached I also have a wire running to the Power tubes ground

4) For the Power Tubes ground I have:

• a) Power tube cathode with the bias testing point ground (1R resistor)
  b) Bias Pot ground
  c) PI and Screens filter cap ground

5) I guess the main difference is that I have another ground point called PT Ground which has:

• a) Power Transformer CT (red/yellow wire)
  b) bias supply
  c) Large power supply caps ground

Another difference is that I have a FX Loop, which is grounded separately and has the cathode of the PI (like Dumble). I have had the fx loop and PI cathode grounded at the Power tubes ground at some point with the fx loop jacks isolated and made no difference.

SOURCE: http://el34world.com/charts/grounds.htm

[pompeiisneaks]

Yes, hoffman amps stuff is outstanding for their known working designs, built by tons of people, (myself included).

As seen on this, the reverb grounds are separate even from all other parts of the grounding. You may want to try grounding reverb stuff quite a ways away from both power sections and input.

Other than that, I'm at somewhat of a loss myself.

~Phil