AB763 Build hum issues in the vibrato channel

[ToneJunkie]

I built this amp about a year ago and am finally getting around to chasing down some hum issues it’s had since day one. I’ve isolated the problem to the reverb recovery / third gain stage of the vibrato channel tube.
When I pull all the vibrato channel tubes (V2, V3, V4) and play through the normal channel, the amp is very quiet and sounds great. As soon as I reinstall V4, I get a noticeable increase in buzz.
To narrow it down further, I temporarily removed the reverb recovery circuit and left only the V4B (third stage of the vibrato channel) wired up. With that setup, there’s still some hum—though not as much as when the reverb recovery is connected—so I feel like I’m eliminating some factors.
Should I expect to hear an incremental increase in hum with each additional stage brought into the circuit? Any insights or ideas on what to check next would be greatly appreciated.
Also, you’ll notice I used a non-Fender layout, which I suspect might be contributing to the issue.
 
Thanks in advance for any help!

[stratomaster]

What is the frequency of the hum? Mains or 2x mains?

[glass54]

Hi T/J,
Quite nice work 

Also, you’ll notice I used a non-Fender layout, which I suspect might be contributing to the issue.

Just for our sake (our Colleagues) could you do a schematic and layout diagram which you worked to, for us to follow.
It will be an easier task.
Thank you
Mirek

[ToneJunkie]

I changed the lead dress in that reverb recovery so the grid leak grounded at the reverb return jack.  It was originally grounded at the preamp star ground close to the input jack of the amp.  I also moved that wire as far away from my heater wiring as possible.  I think I fixed the problem.  I'm going to continue with the reverb driver now and see how it sounds!  I'm just using a AB763 deluxe reverb schematic but with the trem removed.  In this part of the circuit I separated the cathodes of V4A and V4B.  Thanks Glass54 and stratomaster for the reply!

[HotBluePlates]

I changed the lead dress in that reverb recovery so the grid leak grounded at the reverb return jack.  ...  I think I fixed the problem.  ...

Reverb Recovery should have the grid-leak, jack and cathode resistor/bypass all grounded together (or a direct wire to connect them).  In a perfect world, these also connect directly to the filter cap negative feeding this stage.

There is a very weak signal here, and mixed-grounding of those elements can allow long looping ground-paths that are susceptible to picking up ripple.

[tubeswell]

You will get more hum if the reverb pan’s output transducer coil (the one connected to the reverb recovery stage cable) is near any electromagnetic field (like a PT). If that’s the situation, try rotating the pan so the output transducer is as far away from the PT as possible.

[ToneJunkie]

Got it all together and it came out great as far as noise floor and tone.  I like the sound of a little darker reverb so I put a .0056uf cap over the grid leak of the reverb recovery for some high end roll off.  Sounds killer.  Thanks for everyone's help!

[stratomaster]

I came across a post a while back on another forum where they dug into simulations of a cap bypassing the reverb recovery grid leak.  The short story is that it creates a treble peak instead of roll off due to the inductance of the tank output coil.

I'd go for more tried and true methods of taming high end in a stage like a plate bypass or plate to cathode cap as these are unlikely to interact with the tank and the filter cutoff points are easy to calculate and compare.

[SEL49]

I like the sound of a little darker reverb so I put a .0056uf cap over the grid leak of the reverb recovery for some high end roll off.

Since you like a darker sounding reverb, you may want to increase the 500pF cap feeding into the reverb driver grid and let some more dark tones bounce around in the tank springs.