Struggling With Reverb Circuit Hum

[Cosmo]

Hi Guys. This is my first post with the Amp Garage.

I have a reverb circuit I built that has chronic hum. There is also excessive feedback, but I suspect it’s a result of the combo cab. I’ll check out the feedback problem once the hum is resolved.

The amp is of my own design, but it's 5E3 inspired. It's in a 2x10 combo cab with a pair of Weber alnicos. I've build a few of these amps and they sound terrific. However, this is the first time I've incorporated a reverb circuit. This build is for my son, and uses a donor Fender 35 cab and chassis. The reverb circuit is lifted from the AB763 Deluxe Reverb. New Accutronics 4AB3C tank and new Magnetic Components 40-18034 driver tranny. New tubes as well.

I've been struggling with this off and on for the past few weeks. My guess is the hum is being introduced by the tank and then amplified by the subsequent two gain recovery stages. The hum increases as the reverb level pot is turned up. Even with the pot turned off there is a very very slight hum still present.

I've ordered a new tank, tranny, cables, and tubes. As well, I've built a second board which made no difference. I don't know if I have a bad component or a problem with my design/construction. The voltages are all reasonable.

The trace below shows the hum at two points in the circuit. The green trace is the point where the signal is returned to the P/I. The yellow trace is after the .003 filter cap on the first gain stage plate. It’s barely visible on the yellow trace, but you can see how it’s amplified coming out of the second gain recover stage.

the cab was a complete strip and rebuild. I replaced the particle board baffle with ply.

Below are my layout diagram, pic of the actual circuit, and scope scan of the noise.

Any help is much appreciated. I do know how difficult, if not impossible, it is to debug something like this via the internet! I suspect the answer will reveal itself once I have the new parts (tank, tranny, tubes, etc.) to experiment with.

[selloutrr]

carbon comp resistors are slightly noisy.

Grounding? Loop?

I see the traces on the scope but no relativity to db or level. It's clearly louder after the cap but how much louder?


what type of noise are we talking about?

[thejaf]

try grounding the reverb return pot (100k) and the 220K grid load on the return stage directly to the grounds within the rest of the local circuit instead of their current configuration. See if that helps.

[DR.Z]

Cosmo, try using Isolation RCA jack on the return.
The Jacks come with a two piece plastic washer to isolate the body of the jack from the chassis.
Then on the jacks ground washer, solder a wire to your ground buss.

DR.Z

[tubelectron]

Cosmo,

1 - or the reverb recovery grid tube takes the hum : too long wires, unsheilded, wrong GND return (or absence of, or bad point, or multiple points giving hummy GND couplings - so as suggested by thejaf) - check it : I'm pretty sure that's here... Test : ground the recovery tube input : is the hum still here ?

1A - what says DR.Z is right, but I would rather put the insulation jack for the send (reverb drive), or even both (drive and recovery) and make correct equipotential GND wiring at the correct point.

1B - or the +HV supply of the reverb is badly filtered (taken at a wrong point, not much filtered).

2 - or the reverb is induced by power transformer : I doubt, because your amp is large and the reverb unit is far from the PWR trafo, but, maybe...

3 - shield the reverb can with an aluminium foil glued on the reverb carton underneath, and touching the reverb can on all it's lenght for perfect conduction and shielding : it reduces residual hum, particularly on stage situations.

4 - put the reverb can in a reverb padded cover. You can make very efficient reverb cover with old blue-jeans and wool pull-overs sleeves cut at the requested size : it's the solution that I use the most often today... You will forgot Larsen effect at any reverb setting !

A+!

[M Fowler]

Place the reverb tank on the bottom of cab the opposite side from the transformers in a tank bag.

If no change then try the tank attached to the side in a tank bag to shield from transformers.

Mark

[Cygnus X1]

Welcome aboard Cosmo!

I don't have much to contribute beyond what we have already touched on.
Just dropping in to say I think you have found the right place!

To the rest of the forum, once again...although I don't stop in and actually say it enough as I lurk...THANKS and stay safe.

[Cosmo]

Thanks guys for the warm welcome and all of the suggestions. I had little hope for this thread, but now feel differently.

Sounds like the general consensus is a ground problem. Certainly makes sense to me, as everything else looks reasonable. I was suspicious of grounding, but had no previous experience with reverb to work from. I'll work with your suggestions to better isolate the circuit and tank ground.

I have isolated the power supply to this sub-circuit with a 22uf filter cap and even added a choke. All of which made no difference.

Thanks again guys!

[M Fowler]

I haven't had grounding problems with reverb so I must not be with the norm.

I do not isolate my reverb RCA jacks at all. All hum that I have had came from placement of the tank but maybe I was lucky or not

Mark

[Cosmo]

I haven't had grounding problems with reverb so I must not be with the norm.

I do not isolate my reverb RCA jacks at all. All hum that I have had came from placement of the tank but maybe I was lucky or not

Mark

Mark, I have the hum problem even when I'm working with the amp on the bench and the tank is located a distance away. Once I get the hum resolved, I'll experiment with placement and wrapping suggestions. I don't know how Fender keep the tank quiet in the original amp. The 2x10 20 watt setup can send a lot of vibrations through the cab.

[M Fowler]

When you look inside the reverb tank are the very small transducer wires connected? I have had some that were broken off touching the pan. I was unable fix the last tank.

Do you have another tank that could substituted or another amp with reverb that your pan could be tested on to eliminate pan as being the source of the hum and then go onto circuit wiring.

[jjman]

I’ve had mystery buzz from the reverb of my DR at times. The most recent time I had it apart and with the tank far away and all, to no avail. I had it back together and tilted it back for whatever reason and the buzz faded out. I blame basement interference, wherever that comes from. Most hum would usually be internal.

[Cosmo]

Mark, the tank is new and I've gone over it with a magnifying glass. Looks good.

JJ, I have some wild scope traces that show how much EMF is floating around the house. It's scary! I build amps at my computer desk where I have a significant collection of electronic devices radiating into the room. I have to do my final testing away from all of that R/F noise.

This reverb tank is nothing more than a huge antenna tied directly into the signal path!

[Cygnus X1]

Perhaps I missed it.
Did anyone mention the epoxy trick yet?

I solved one recently by rotating the tank 180 degrees away from the power transformer.
It was a head, it came to me that way, and I made a bad assumption it was installed correctly when I got it.
Mark already covered this and Cosmo I know you've done it.
Just reiterating it in case someone else finds it helpful.
Sho' made me feel like a big dummy.

[tictac]

One source of noise in the reverb circuit can be the placement of the reverb tank.

If the output of the reverb tank is oriented towards the power transformer end of the chassis it can pick up noise from the power supply.

The reverb tank should be positioned so that output of the tank is on the input / preamp end of the chassis...

TT