Pictures of wiring, ab763 reverb parts

[überfuzz]

Since this amp's a heavy weight we agreed to it that I could swing by the rehearsal studio and fix it. I was able to tinker with it today. It was gutted and everything was set to go. I drained the filter and vent ahead and replaced the plate resistors, both v4. Then when I wanted to test it. Noting, nada piña colada, etc. I became somewhat nervous, I guess you can imagine. Well it turns out the sneaky owner always pulls the fuse to prevent other guitar heroes from trying it. (Several bands in the same studio.)

So not really an update... My mate suggested that we should add a couple of beers to the IOY list. I'll update this thread as soon as I hear anything.

[überfuzz]

Just got the message. The noise is still there and I'm in the dark...

I think I'll redo the picking and poking, pulling tubes etc, just to see if I can come up with something, or if I missed anything. (Back when I was still in school I could just walk in and use the oscilloscopes, now I don't know. And the amp is so heavy I'd don't really want to carrying it around.)

Update, or status quo
It starts to get evident a minute or so after the amp is fired up. Apparently this issue is quite recent. Qoute: It might have been there but where low, not really noticeable... This amp used to be SOOOO quite.

[MWJB]

As far as I can tell (I may well be wromg), you have just replaced the plate resistors at V4.

So, why not replace all the plate resistors (they're all the same age, same state), you know the vibrato channel is noisy, bu you haven't identified exactly why, eliminate the plate resistors...all of them.

Have you tried 12AX7 selected for low noise yet?

Have you checked for voltage creep on the board?

Have you listed all dc voltages?

[überfuzz]

As far as I can tell (I may well be wromg), you have just replaced the plate resistors at V4.

So, why not replace all the plate resistors (they're all the same age, same state), you know the vibrato channel is noisy, bu you haven't identified exactly why, eliminate the plate resistors...all of them.

Well, I don't feel like doing any shotgun surgery just yet. We haven't really agreed on anything like that, me and my pal (the owner).

Have you tried 12AX7 selected for low noise yet?

Have you checked for voltage creep on the board?

Rebuff - Rebuff

I'll check these things, priority numero uno.

Have you listed all dc voltages?

Yes, their all within 10%. Oh the negative bias is a bit lower, I seem to recall finding -64VDC there.

[überfuzz]

Homework
I tried different tubes with no effect on the noise. Brand new tubes, old ones, doesn't matter. Furthermore, I couldn't find any significant DC voltage in the tone control. I got the same in both channels, in the realms of 0 to 0.3 mV.

News
The noise is quite low at first, cold amplifier. After 5 minutes idle it slowly starts to get evident. However, if the vibrato/tremolo is turned on and the intensity/speed knobs are turned up and down the noise jump starts. Even within a minute from fired up. Occasional the amp pops, pop pop pop etc, if the intensity is turned real high, 8 to 10.

I replaced most of the resistors in the tremolo circuit. (I didn't replace the two 1Ms and the 2.2M resistor.) I also removed a 0.01 tap fixing cap, no difference. Also, I resolder much of the tone controls, groundings and tube sockets.

Ideas
There must be some component in the tremolo that's gnarly and if it's pushed, warmed up, it starts to misbehave. If I'm right I need to find out if there's a component that's in the signal path, even if tremolo/reverb is turned OFF. Also, it's pushed harder when the tremolo is turned ON. Any ideas of what it can be, the black opto bug..? Suggestions are appreciated!

[martin manning]

Since you've replaced V4's plate resistors, and the noise seems to be affected by the intensity control, I'd try the following:

1) See if the 0.01 uF coupling cap from V4's (pin 6) plate is leaking. Measure the voltage at its junction with the 220k mix resistor (the end opposite the 100k plate load) with the intensity pot turned up. The voltage to ground there should be near zero. If it's not, replace the cap.

2) Disconnect the wire from the top of the intensity pot. This will prevent the trem oscillator from feeding noise into the signal path. If the noise goes away, then it's coming from the trem circuit. This doesn't seem likely, though, since there is only the intensity pot and the "roach" resistances to ground.

[überfuzz]

Hi again.

No luck so far. I tried swooping al caps, 0.01, 0.1, 0,047, that's connected to the junction you mentioned. And I tried the intensity pot thing you suggested.

The noise is still there...

Suggestions..?

[martin manning]

About all that's left now are V2's plate resistors.

[überfuzz]

They, the plate resistors, were actually on my to try list. I figured I'd try new there, if that won't do it I'll pass this amp on to an amptech.

This amp even haunts me in my dreams now. I dreamt I had to lift the board. Underneath I found some broken or bad connections that were inducing the noise.

[martin manning]

Don't give up just yet.

To review, I believe you said the noise is there with no instrument plugged in to the amp, right? You removed V2 and still had noise, which led us to focus on V4. But you also said that you could raise and lower the noise with the (vib channel?) volume control. That would implicate V2a, or something around it so I don't quite understand that.

Just so we're speaking the same language, on the attached schematic I labeled the tubes in chassis-position order and traced the signal paths on the schematic. Red is vib channel prmary, Blue is the reverb loop.

Pulling V2 should stop current flow through the V2 plate resistors, but V2's 0.02uF coupling cap is still connected from the V2b plate resistor to the V3 and V4b grids. I don't think you've checked it for leakage or replaced it yet.

[überfuzz]

Thank you very much for all help and encouragement. I just felt a bit crestfallen... I'll definitely check the things you said.

The thing with the tech I'm referring to is that he has got a working oscilloscope. Feels like I'm in great need of one with this amp... He's retired and don't do much repairs. I'll just tell him to do a trouble shoot and write down what he finds. Pay him some pocket change and do the tinkering needed my self.

Last time I picked and poked I could clearly here that the noise is more evident than it was the first time I checked the amp. The owner said he never uses the trem channel, but he uses the normal channel. I'm not sure if it's of any value, but I thought I mention it. Further, I think I mentioned that the filter caps vent south in a spectacular fashion, 6 months prior to this. Now when we talked about the amp, he said that it's possible that the noise was there right after the filter job. And that it has become worse since. (Some of the grid stop and screen resistors, on the power tube sockets, seems to have been changed along with in the filter job.)

Cheers for now!

[Euthymia]

Replace the plate resistors at V2. If it's still noisy when it's hot, go for the V2 cathode resistors. Last, try the cathode caps.

This sounds like a classic aged carbon comp resistor. If you want to be more specific, try just the 100K plate resistor going to pin 1 of V2. That's the most likely culprit.

[überfuzz]

Since this amp's keeping me awake at night I had a go today, too. I checked the plate resistors and the coupling cap at v2. Nope...

You say that I should try the cathode resistors if the noise is present when the amp is hot..? The noise is sort of vague until the amp is hot. I'd say it take some 3 minutes until the noise starts to get real evident. I can speed this up by raising/lowering pots in the tremolo circuit. Does this imply that the cathode resistors are likely to be gnarly?

This time around I happened to tinker in dead silence. When the amp was hot, noisy tremolo channel, I could hear the noise even with the volume pot at zero. (Possibly some leakage over to the normal channel. It may also be me getting an all to noise sensitive ear...)

Slight of topic!
I had a guitar today. The tone of this machine is breathtaking. And the reverb, divine, goodness gracious me. The noise has to go!

[überfuzz]

Sorry for spaming this thread. LOL

I just phoned the tech I mentioned. At first he was sort of reluctant, but when I started talking about the amp. He he. I told him that I tried changing the plate resistors because they were old carbon resistors. And then the coupling caps, I asked him what effect DC leakage would have. He said that the tubes after get their bias messed up and are likely to distort. (Wish now leads me to rule out any coupling caps as the reason.) By now he started the standard routine about how he started out back in the the early sixties, before the transistor. So eventually I got the, why don't you drop by and I'll see what I can find.

[martin manning]

Sounds good! With any luck you'll learn a few things too. I'll be interested in what you find out, and the solution if the problem is solved.