Reverb Circuit Hum

[mwaller]

Hello Everyone-
I'm trying to get my AC10 SRT clone working, and the only remaining issue is a prominent hum that is associated with the reverb circuit. The volume of the hum is directly related to the position of the REVERB pot - if I turn the reverb pot to the OFF position, the hum almost disappears. If I turn the reverb up, the hum increases accordingly.

Here is the original VOX schematic:

AC10_SRT_001_zpsejn5ubuv.jpg

Since the original design relied on an obscure reverb tank make with phonograph cartridges, I chose to implement a 1-tube reverb circuit that I discovered here:

https://el34world.com/Forum/index.php?topic=7957.0

The tank and transformer are typical Fender-style parts, such as would be found in a Princeton Reverb.

Here is an annotated schematic showing how I connected them to the VOX circuit:

Reverb Circuit.JPG

For the life of me I cannot figure out how to get the hum to go away? Any ideas?!

[xtian]

First thing to investigate is the position and orientation of the reverb tank relative to the power transformer. Sometimes moving the tank to a different place in the cabinet will solve the problem. Sometimes you have to get some longer RCA cables and move the tank out of the cabinet entirely! (I'm looking at you, Jim Kelly!)

[mwaller]

Thanks for the comments!
The chassis is not yet installed in a cabinet, so there is actually quite a bit of distance between the transformer and the tank. I've also tried another tank and cables from another complete amp; same issue.

[earl2]

Can you show us how this sits in your whole circuit relative to that Vox schematic. I made one of these a year ago and appreciate the idiosyncrasies of that schematic!!

[pompeiisneaks]

Another common issue is the tank grounding. Sometimes you need to test by jumpering between the tank and the chassis to see if grounding it works (I.e. some tanks aren't grounded from chassis to the rca jack(s). If that resolves it I think it's just a matter of connecting the chassis of the reverb pan to the ground on ONE of the reverb rca jacks

~Phil

[mwaller]

Can you show us how this sits in your whole circuit relative to that Vox schematic. I made one of these a year ago and appreciate the idiosyncrasies of that schematic!!

The reverb circuit replaces the original V3 and all the components directly connected to it. C5b is the same C5b you see in the original schematic. I will see if I can mock up a hybrid schematic...

[mwaller]

Another common issue is the tank grounding. Sometimes you need to test by jumpering between the tank and the chassis to see if grounding it works (I.e. some tanks aren't grounded from chassis to the rca jack(s). If that resolves it I think it's just a matter of connecting the chassis of the reverb pan to the ground on ONE of the reverb rca jacks

~Phil

I will try this! Thanks!

[mwaller]

I just remembered... most of the hum is still present with the reverb tank disconnected. I'm not sure what to make of this observation...

[pompeiisneaks]

Then that means the source of the hum is the circuitry around the 1 tube reverb itself. Is there some stray high voltage or output transformer wiring near the tube? It could be injecting hum in that way, does it seem like 60hz or 120hz hum?

~Phil

[mwaller]

Then that means the source of the hum is the circuitry around the 1 tube reverb itself. Is there some stray high voltage or output transformer wiring near the tube? It could be injecting hum in that way, does it seem like 60hz or 120hz hum?

~Phil

The only transformer near the reverb tube is the reverb transformer itself...
Now that you mention it, I believe the hum is 120Hz... Thoughts?!
The hum seems slightly sensitive to the presence of my hand near the reverb tube. As I pass my hand over the open chassis, the hum gets a bit louder when I am near the reverb tube....

[xtian]

You didn't mention swapping the tube out. Did you try?

[Roe]

b.

Here is an annotated schematic showing how I connected them to the VOX circuit:
Reverb Circuit.JPG

For the life of me I cannot figure out how to get the hum to go away? Any ideas?!

Try to put the transformer and the tube after it on different power supply nodes

[mwaller]

You didn't mention swapping the tube out. Did you try?

Yes. I actually went so far as to remove the reverb tube altogether... the hum is still mostly there.
It would appear that hum is being injected directly from the reverb pot into the grid of V2b. The connection between the reverb pot and V2 is shielded, so I'm confident that the hum is originating from the pot itself or further upstream...

[mwaller]

[/quote]

Try to put the transformer and the tube after it on different power supply nodes
[/quote]

This does seem like a reasonable idea. I assume you mean that the triodes in V3 should be connected to different supply nodes?
Thanks!

[mwaller]

Try to put the transformer and the tube after it on different power supply nodes
[/quote]

This does seem like a reasonable idea. I assume you mean that the triodes in V3 should be connected to different supply nodes?
Thanks!
[/quote]
Darn it! I moved the recovery triode to another power supply node... Same hum...!
Other ideas?