Freaky Reverb Hum / Deluxe AA763

[Amp Kat]

I built this AA 763 Deluxe Reverb amp that has the Vibrato circuit only and it sounds great until I turn the Reverb up. This chassis is a 5C5 chassis tweed so it's all cramed but it has no hum until I turn the reverb pot and the volume drops and the hum gets loud after about 4. The reverb works but when it starts humming it also starts farting and gets a little crackly. All tubes are fine and I've checked wiring till blue in the face and everything is good. The reverb tranny is by itself on the opposite side of the power,choke and OT. What are some things I can check to find what's causing the hum and drop of volume ? Could be cancellation causing the drop or RF but not sure if that's what it is.

[MWJB]

Are all 5 preamp tubes mounted in the chassis (as opposed to a sub chassis for the reverb mounted in the side of the cab)?

[Amp Kat]

Yes I got the conversion sockets from Weber so all tubes are mounted in the chassis. I did move the pan all around and it had no effect on the hum. Might can take a shot of it tonight and post as it's not you're normal follow the directions build as you probably guessed already. ha !

[tubeswell]

bad/semi-Microphonic reverb tube??

[Amp Kat]

Nope ,checked all the tubes well and they are Telefunken smooth plates and one known good RCA 12AT. Even went as far as to put them in my Lee Jackson and tested the reverb and it was perfect.

[tubeswell]

Then I wonder if some signal wire somewhere is picking up feedback coupling. Are any of the reverb grid signal wires (on the recovery side) shielded? Might pay to shield them if they aren't. (Or maybe the shielding isn't working on your reverb cables?)

[Amp Kat]

That could be possible and nothing is shielded and I'm just using regular RCA cables (red and white) to the tank but I did think about shielding from that 10pf network back to the tube grid. Do you guys think that if it was cancelling from phase that it would give that sympton or feedback in general ?

[Amp Kat]

Ok I shielded the network back to the grid and we'll see what happens tonight when I test again. One more thing is I have the input and output transformer wires going thru the same tiny hole in the chassis. Do you thing that could do it ?

[tubeswell]

I have the input and output transformer wires going thru the same tiny hole in the chassis. Do you thing that could do it ?

Dunno but I always put them through different holes

[Amp Kat]

Ok I'm gonna try some stuff as I got tons of room in this cabinet to work with so if I have to remove a tranny and mount it to the cab it's no problem. I'll let you know what happens. Was just looking for what it could be. Played it last night and the tone and sound is just really nice but it still drops out and gets blocking distortion when the reverb hits 4.

[tubeswell]

it still drops out and gets blocking distortion when the reverb hits 4.

Ah well if its blocking distortion then try lowering coupling cap values and/or adding /increasing grid stopper somewhere (depending where the blocking distortion is)

[Amp Kat]

Well you were on it Tubeswell and it was the 500pf coupler that it didn't like. After shorting that cap the reverb quit humming and quit acting like a volume pot. I scoped it and it was crushing the signal but now the reverb is almost to strong so I may try a reistor there are maybe a different type size and cap.
It sounds really nice and I may have some sound bites soon.