Duo Medaliste Reverb Hum

[lowell]

This amp has an annoying hum when the reverb is on. I've narrowed it down to a certain area but cannot figure out what to do. If I disconnect the .001 coupling cap from the plate of V4 (reverb recovery) there's still the hum. Oh and the hum is low, like 60hz. I then grounded the bottom of the 470k and the hum is gone. I checked the DC resistance of V2 pin 5 grid to ground and it's ~500k when the reverb is on. So the grid DOES have a ground path when the reverb is on.

So I'm confused here because with the .001 coupling cap disconnected there's still hum. This would tell me that the source of the hum is after that cap.

On the other hand, grounding the 470k (thus reducing the overall value of the grid leak path for V2) gets rid of the hum... so where the heck is this coming from??

[Enzo]

How about from the wire leading away from that 470k, and trailing all the way down to the foot switch? That ought to make one fine antenna. Unplug the footswitch, any help? And inspect the wire dress from that resistor to the jack.

[lowell]

Wire dress looks good to me. It's shielded cable too. Yes I unplugged the footswitch completely (leaving the reverb on) and still hum. Maybe this is an inherent design issue?

[Chuck H]

If lifting the .001 cap doesn't stop the hum, the hum is not from the reverb recovery. But notice that the reverb isolation (1/2 V2) grid has several paths to ground depending on operation. Since grounding the bottom of the 470k resistor at the switch gets rid of the hum I'm going to guess that it stabilizes a ground loop on that V2 grid. You can try adding a cap to eliminate one of the 0V references for that V2 grid. Worth a try. Probably any handy value you have is fine (.022, .033, .047, etc)

[lowell]

Hi Chuck, good idea, didn't fix it unfortunately.

[Chuck H]

Shucks. I thought I was on to something. I'm going to to look at the schem again but don't hold your breath.

Have you tried a different V2? I've had a few hummy tubes. Though I would think that would be constant. Since grounding th bottom of the 470k resistor stops the hum it's probably not just a hummy tube. Pretty easy to stick one in there though.

[lowell]

Yes they are all new tubes Chuck. I tried swapping it with one in another position too.