Noise when housing a Princeton Reverb into 19" Rack Case

[smudge]

Hi guys

I'm new here and have searched and read a lot. Could you guys at all help me? I'm sorry my post is a bit long.

I've taken an existing Fender Princeton Reverb Reissue circuit board and tried to house it in a 2ru 19" rack case. I've also slightly modified the circuit by removing the tremolo circuit and changing the phase inverter to a LTPI (like an AB763) rather than the version the Princeton normally uses. I've also plugged in a SS rectifier.


The problem I have is a hum/buzz (completely unbearably loud ) as soon as the reverb pot is turned up. Hum increases in proportion to increase in the reverb pot.  I'm almost certain the problem is in the reverb recovery part of the circuit (as I've searched here and realised it most commonly is the problem) but perhaps its in the stage after it (ie V3B after the 3.3m/10pf mixer)?

After so much testing and tweaking, the only thing that completely kills the hum is disconnecting pin 1 (plate) from the V3A reverb recovery tube. Obviously the reverb no longer works if I do this, but the hum is gone. I also note grounding the reverb socket also kills a fair bit of hum, but nothing like completely removing the plate wire at pin 1 of V3A does.

I've tried the following to no avail:

1) grounding pin 2 (grid) (ie path from the reverb pan to reverb recovery triode) to see if this kills the noise. It does lightly but not really by very much at all.

2) I've gone through every possible format for isolating one or both reverb pan sockets, coupling one or both to the case, using a separate ground wire there etc. Nothing seems to work.

3) I've removed tube 2 (reverb driver) totally and the reverb transformer and the pan. No change with any of these and the noise continues. Ive tried different pan cables as well. Also tried another tube. No change

4) changed the 0.003 coupling cap between the V3A and reverb pot. No change.

5) Removed r38 (220k grid leak resistor from pan's output) and moved it onto the pan output socket and case. No change.

5) No amount of chopsticking does anything except that going near v3B's grid or the junction between the 3.3M/10p or the 470k resistor between the reverb pot and the 3.3/10P makes noise as the hand gets closer or when the wooden chopstick actually makes contact with these components. I've tried to ensure these parts are connected and seem to get continuity to them. 

6) Tested all grounds. They all seem to measure about 1ohm regardless of what I do. I realise ideally should be 0 ohms but even the case lid tests about 0.5ohm on its own when I measure two points across it.

I get the impression the problem is between the V3A's plate and the reverb pot since the pot will completely turn up and down the noise.

I also note however that disconnecting either pins 1 or 2 (wiper) of the reverb pot makes the noise even more unbearable. This made me wonder if the noise is after the reverb recovery and in V3B instead.

Given this is the actual fender PCB that I've mounted, I think that making a mistake with the wiring shouldn't be a problem.

The only other thing I can think of is that a normal AB763 has a separate power node for the PI where as the Princeton Reverb pcb power supply shares all the preamp and reverb recovery plus PI etc all on one node.

I'm almost ready to give up on the project since I've run out of every idea.

Could anyone solve this riddle for me?

Thank you!

[Latole]

Noise when housing in a rack ;

Did you mean there are no noise outside the rack ?
Yes: what is in the rack ? Electronic, transformer
Where are reverb tank in the rack ? Close to a magnetic field.

Better answer need to see some good pictures

[smudge]

Ok I'll post some pictures tomorrow.


As for the questions- its a 2ru rack with all transformers mounted inside. The reverb pan is separate and will attach outside to a case.

I just simply got hold of existing Princeton PCB's (I'm guessing ripped from someone who wanted to gut out and do a point to point version of their amp) and installed them into the 2ru case so I cannot say whether it would have been noiseless outside of this.

I'll post those photo's tomorrow.

[sluckey]

I suspect V3 pin 7 is losing it's ground reference when you turn the reverb pot (possibly faulty pot). Here's a quick test. Temporarily connect a 220K between V3 pin 7 and chassis ground (OK to use gator clip test leads). This gives V3 pin 7 a dedicated ground return path just like the AB763 amps. Does that fix the hum/buzz? If so, your reverb pot should be replaced.

I noticed that R30 on your schematic indicates it has been changed to 1M. My undoctored schematic shows 470K. Does your amp actually have a 1M for R30? If so, why?

[smudge]

Thanks Sluckey!

Wow adding that 220k between pin 7 and chassis ground makes a huge difference. The hum and buzz is still there and still increases with the reverb pot, but adding this resistor instantly reduced the hum and buzz by at least 50% if not a bit more. So I'll replace the reverb pot.

Given such a big change with that resistor to ground, I realise you are right that there is a dedicated ground assisting the grid's reference point. There is still a fair bit of hum/buzz left however. Is there anything else I could do around this issue then? Like replace the reverb pot and also keep that 220k to ground there?

The doctored 1M for R30 is part of the suggested changes for converting a 65 PR to a 68 PR. I left it as 470k as I wanted to keep to the original 65 PR blackface specs anyway.

[sluckey]

You may not have really discovered anything here. I was hoping to totally kill the hum/buzz. The added 220K gives V3-7 a dedicated ground path but it also creates a voltage divider for the reverb signal which would reduce the reverb signal as well as any hum/buzz coming from the reverb recovery circuit.

Try this... Leave the added 220K but now disconnect the reverb pot wiper. Does this totally kill the hum/buzz?

[smudge]

Thanks Sluckey

So removing the middle wiper of the reverb pot makes the noise even worse. I note if I unattach the 220k from grid to ground the noise goes crazy loud, so putting it back in tames it a fair bit.

[sluckey]

Time to post some hi-rez pics.