Reverb Thumping

[Gibsonman63]

I rewired my 1972 Super Reverb to AB763 specs using cloth covered solid core wire like they did in the 60's. Every thing is working fine except for the reverb. It works well until I turn the reverb up to 5 or 6 then I get a low end thumping sound. I have plenty of reverb before I get there, but I would like to have the amp where I can turn the reverb all the way up without creating the sound even though I won't ever play it that way.

Suggestions?

[selloutrr]

are you sure the tank is plugged in to the correct in/out?
next would be tube
try moving around your wire dressing

[Drumslinger]

i've never run into this before so i'm curious to find out what the issue is. let us know what you find Gibsonman.

[selloutrr]

impedance issue? is it the correct tank for the transformer?

[Gibsonman63]

Thanks for the replys, guys.

It is the original reverb tank. The amp was working before I rewired it, but it had that ice-pick treble attack and didn't have the harmonics and warmth of my older Fenders. There is definitely some Mojo to be found in the wiring.

I swapped some tubes around, but no difference.

I double checked against the schematic and everything matched the schematic for the reverb driver and recovery circuits. I increased the cathode bypass capacitors for the driver and recovery circuits, but I can't hear any difference and the problem persists.

I am well beyond the usable range of the reverb when the reverb starts thumping, so the next plan is to study the reverb mix circuit and try to dial it back a bit.

[Firestorm]

Make sure you didn't inadvertantly swap the two leads on the reverb pot. Aside from that, take a look at where those leads are dressed. They can interact with leads running from the tone controls; Fenders of that vintage have leads that are way too long anyway.

[Structo]

If you started with the reverb tank connections reversed, the reverb won't be very strong, so if you tried to fix that without reversing the cables then that may be the problem.

[Gibsonman63]

I definitely have the leads the right way around on the reverb.

Here's a couple of pics in case anyone spots something obvious. It is sounding great at the reverb levels I play at. I get a pretty lush reverb on about 2. The thumping starts at about 6 1/2 but by then it's overwhelming even for surf.

[Gibsonman63]

... and a "before" just for fun.

[selloutrr]

i wonder if one of the leads that was twisted has a bur or is almost making contact to ground and when that line is loaded it's close enough to jump voltage but not make contact giving you a thump.

I'd try poking around with a chopstick and seeing if you can create the noise. then address that area.

If no luck try turning the lights off and looking for a spark when it happens.

it might not hurt to double check continuity from point o point and ground.

is the driver tube a 12AT7 or 12AX7?

[Structo]

Maybe it is more of a motorboating type thing that usually has to do with capacitance, I remember something about those ceramic disc caps having something to do with a similar problem, unfortunately I can't remember much about it as it was 25 years ago.

Yeah, chopsticks can sometimes localize the problem.

[Dr-Joned]

I'm with Structo, as it sounds like LFO, or low frequency ocillation.

[selloutrr]

The good news it's only a handful of parts to test. It could be a resistor that's gone out of spec, probably a ceramic capacitor, or bad tube or wire dressing.
you can replace the ceramics with orange drops. this used to be know as a studio mod. used to deal with noise and make circuits more reliable.

http://www.geofex.com/ampdbug/mtrboat.htm

[Firestorm]

Looks to me like you've replaced the .003 coupling cap from the reverb recovery stage with a .02. That'll motorboat like crazy.

[xtian]

Looks to me like you've replaced the .003 coupling cap from the reverb recovery stage with a .02. That'll motorboat like crazy.

You have the eyes of a freaking eagle. Where do you see that?!?