Pictures of wiring, ab763 reverb parts

[überfuzz]

I've been asked to check an amp, ab763, that has a noisy tremolo channel. When I had a look at it I saw that the reverb circuit is wired more or less like a birds nest. By poking around here I can induce or lower noise. So, the idea is to rewire this part, before I start looking into bigger maneuvers. Does anyone have a nice picture of the reverb tube socket and the wiring of the reverb transformer of a ab763? Preferably a picture of an amp with a hum and noiseless tremolo cahannel. :-)

(I'm not really sure of what wires to twist and not.)

Edit: I forgot to mention that I'll check the socket it self, to make sure there's no glitch there.

[überfuzz]

The tremolo channels' noise increases with the volume. It's not very load, but still it's not in the realms of OK noise. It sounds as if you're gently blowing into a microphone.

Any suggestions of what to check would be appreciated. I'll try to get time to tinker this weekend.

[martin manning]

Google "AB763 gut shots"

[überfuzz]

- What can I say, my first language isn't English. I'll try to google that.

[martin manning]

It's a bit of an art, I think. Don't forget to search for images. This link looks like it has what you are looking for: Blackface Fender gut shots anyone? - The Gear Page

[überfuzz]

Thank you for the links!

Not sure though, most of the amps I found pictures of are wired in the same manner as the one I wanna fix. Apparently the filter caps was recently replaced in it. Old once retired in a spectacular fashion, short circuit, load hum etc. This made me suspect there's parts busted as a result of a bad filter. Any suggestions on hardware; caps, resistors, etc to check?

[martin manning]

Which model AB763 is this? Twin Reverb? Deluxe Reverb? You've already got the problem isolated to the trem channel, so that's a start- it rules out everything that is common to both channels.

[überfuzz]

Yes I know. The thing is, I don't have any oscilloscope to check for gnarlys. I just thought someone might have an idea of what could have gone bad. Like a standard thing, don't forget to check the ??? if the filter has gone.

Oh, by the way. It's a Twin Reverb.

[martin manning]

It's probably not as easy as that, but you should be able to zero in on the source of the noise. So, it has a hiss that increases when you turn up the trem channel volume. That suggests that the problem is in the preamp. If you pull that tube does it go away? Have you tried swapping the preamp tubes to see if the noise goes to the other channel?

[überfuzz]

Well as I said the filter caps have been replaced. I replaced some stuff in the tremolo circuit because it was tapping. I think they vent south because of the stress when the filter broke down. Not sure, just a guess. Anyhow it was easy to pinpoint, the tapping sound, and new caps there did the trick. Almost...

The issues now, hiss, yes maybe that's a way of describing it. A low keyed hiss, goes up with the volume of the VIBRATO channel. It also follows the tremolo volume. So far I've only done the first aid, like you suggested, swopped tubes around, pulling and pushing around wires, poking on solder joints etc. If I don't get to a oscilloscope I can post a file with the sound.

[martin manning]

Okay, so what did you learn? Since it's the Vib (that's the same as trem to me) channel that hisses, and that volume control causes the hiss to increase, I was suggesting that the problem is in the vib channel preamp. Does the hiss stop if you pull (remove) it's preamp tube? If not, what if you pull the reverb driver? Reverb recover? You should be able to isolate which part of the circuit is the source of the problem this way. BTW, is this hiss occurring with nothing plugged in to the input jack?

[überfuzz]

BTW, is this hiss occurring with nothing plugged in to the input jack?

I seem to recal the noise being there with noting plugged in. So its safe to pull a tube and run it?

[martin manning]

Sure, no problem.

[überfuzz]

Hi again!

I had some time over to tinker with the amp. If I pull v4 the noise goes, but so does the volume of channel 2. This makes it hard to tell if the noise is gone or if it's just hard to hear. Pulling the reverb tank connections doesn't affect the noise.

I've been looking at the scheme, ab763, and I have some thoughts. Somewhat in order of to do list...

1. The amp is quite over-driven on channel 2. If I lower the gain of the stage by placing a 12ay7 or 12at7 in the socket. Would that put any stress on the circuit, damage it in any way?
2. The ground scheme seems to mix cathodes, like the once in tune v4, with preamp grounds. I could give the cathodes of v4 a separate ground wire. Could this affect the noise level of this stage?
3. The cathodes share the same resistor/capacitor. What if I gave them separate cathodes? 1,5k/25pF, could that affect the noise?

Oh, a thing I just thought of. What if nr 1 does the trick. Should I redesign the stage, giving it new grid and cathode resistors to lower the amplification of the stage..?

[martin manning]

So, if you remove V2 the noise is still there? If so then you know the problem is with V4's circuit. The tube itself could be the cause, but you already said you tried swapping tubes. Look for bad solder joints and re-flow them. Are the 100k plate resistors old carbon composition types? These are known to generate noise of the type you describe, so you could try replacing them, preferably with metal film types for the lowest noise. The shared cathode resistor and cap probably are not at fault, they are not in the signal path. It's ok to try different tube types, but I don't think that will cure the noise problem.