Super Reverb- reverb hum

[Wes]

Pulling my hair out on this one.

'66 Super- sounds great after servicing, but the reverb has an awful hum as you turn it up.

I have:

-replaced the rca send, return, and footswitch jacks (they were awful loose)

-isolated said jacks and grounded them with the recovery tube's k grounds

-different wires, different tanks

-different tubes, cleaned sockets, and new components in circuitry

-insulated the runs to the reverb pot

-recapped the old, busted power supply

-cleaned as best I could under the main circuit board as well as under the doghouse board

-all voltages look good- no dc business where it shouldn't be


Now, when I got this amp-

Someone, before, had replaced the original tank with a 4FB- it has 1475 for input impedance where the 4AB that is recommended is 8.

With this FB tank- there is no hum, but of course the reverb is weak. With a new 4AB, or the tank I took from another '65 Super Reverb-
the reverb is strong, but you get the hums as you turn it up. As best I can tell- it is 120Hz.

One weird thing- As I was testing voltages across the board I noticed that touching my probe to the junction of the .1 cap, 1M resistor, and
the grid of V6B (PI) the hum mostly went away.

Also- at some point someone replaced the power transformer with a silverface PT. It is dated to '69. The only difference I can see is that
of course there is no center tap for the heaters- I just did the old 100r-100r faux center tap at the lamps terminals, and it seems that a 5U4
is now needed to get the voltages right.

Any suggestions would be greatly appreciated. I have found many threads dealing with reverb hum, but it seems most of the issues were
resolved with tanks and associated conductors.

[Chuck H]

It could be noise from transformer proximity or an incorrect grounding scheme. Try lifting one of the chassis jack grounds. If both tank jacks are grounded only one chassis jack ground should be used or you get a ground loop. I know that would typically be 60Hz but try it anyway. Did you try a different reverb driver and recovery tube? I've had a few noisy tubes right out of the box. On a new build it can throw you off thinking that the amp is humming. I expect it could be the same for a repair. Did you try rotating the tank or moving it around? For some amps there's only one (or none) quiet spot and IMHE new tanks are more sensitive to EM than the older ones.

[Wes]

Hey, Chuck, thanks for your reply.

I did isolate the chassis jacks/lift one. I tried different tubes in drive and recovery. I tried rotating the tank and got it as far away as 4-6 feet from the amp.

Nothing resolved the hum except a higher input impedance tank masking it.

[pontiacpete]

Although I've never seen one that caused hum, but did you try swapping out the reverb transformer?

[Chuck H]

Hum from grounding is usually 60Hz AFAIK. Filaments are at 60Hz and represent the highest current circuit grounded at the chassis. With 120Hz evident with the tank several feet out of the amp I'm at a loss unless there is a problem with the power supply filtering, the cable shielding or transformer EMF is somehow inducting onto a component in the amp.

Try to isolate the hum. Is the hum present anywhere on the driver tube? If the hum is ahead of the 500pf coupling cap it wouldn't be very evident on the dry signal. So, where is the hum originating?

[Chuck H]

One weird thing- As I was testing voltages across the board I noticed that touching my probe to the junction of the .1 cap, 1M resistor, and
the grid of V6B (PI) the hum mostly went away.

I've experienced this when grounding a ground loop hum to a third location. Are you able to confirm 120Hz? Also check to see that the reverb driver HV is coming from the screen node and not the PI node.

[g1]

Just to get some very basic things out of the way, the foot switch is disconnected? The hum/buzz is much more than in a typical Fender vintage amp?

One weird thing- As I was testing voltages across the board I noticed that touching my probe to the junction of the .1 cap, 1M resistor, and
the grid of V6B (PI) the hum mostly went away.

This could be significant, or it could be that it is killing all sound. You would need to try it while passing signal. I suspect the signal would be greatly decreased as well as the hum. If not and the signal stays the same yet hum decreases, then it warrants further looking into.

[TimmyP1955]

Good point about disconnecting the FTSW, especially if it's a repro - I've seen some that had the shielded wire on the Vib instead of the Verb - the verb hums badly in such a case.

[g1]

I've seen some that had the shielded wire on the Vib instead of the Verb - the verb hums badly in such a case.

Exactly what happened to me. Did some non-reverb work on the amp, a few weeks later the owner phones and complains about hum in reverb. "Is footswitch hooked up right?", to which he assured me it was. Got to his house, looked at the back, reversed footswitch connections and flipped the label on the new repro footswitch upside down so it at least read correctly .