Pictures of wiring, ab763 reverb parts

[überfuzz]

Ok thanks.

I wrote down some measurements of pin 1 and 6 of the preamp tubes because I knew I'd forget them as soon as I measured them. Problem is, it seems as if I forgot my notebook. I'll get it and post the values here. I wanted to see if any voltage is to high, causing the tube to be pushed to hard.

Could it be that the tube, possibly v4, is pushed to hard due to a bad plate resistor? I mean channel two is pushed to heavy break up, if the volume is turned up. Way more then any other twin reverb I ever heard.

So for now, bear with me. I'll get more info posted soon.

[überfuzz]

I just compared my values with the scheme, ab763, and all seems close enough.

Two things

1. v5 pin 1: 440V should be 300V. However, I'm not sure I measured while the tremolo was on. Is that pins voltage depending on the tremolo circuit..?
2. Loud hiss when I measured v4 pin 7. Something to be bothered about?

[MWJB]

1. You must measure it with the tremolo set as described in the schem, a small crocodile clip, or shorted RCA plug is handy to have around for this.

2. V4 pin 7 is a grid wire, this will be noisy for that reason alone, grid wires are.

Replacing V2 with a 12AT7 or 12AY7 isn't fixing the problem, it will change the tone of the amp, not for the better in most circumstances. You can't run without V4 either, this will mean that you have no reverb. You say that you have swapped about tubes for V2, have you actually obtained a brand new, lo noise 12AX7 tube with good gain? There are low gain 12AX7 tubes like Sovtek 12AX7WA, but these have poor tone, most vendors offer a "7025" spec 12AX7 tube (7025 was military designation for a low noise 12AX7, you will see that they are often specified in noise sensitive applications in Fender amps), that should be quiet AND have good gain. V4 also need a good tube in it. V2 in a AB763 is well known as being sensitive to noisy tubes.

You say you can lower noise by playing with the lead dress? AB763 are often built with solid core wire, it should stay in place when arranged for the least noise?

[MWJB]

...long shot, does the noise stay at a constant level, like "sssshhhhhhhh", or does it go up & down in volume of its own accord, like waves lapping on a beach, or gusts of wind? Does pulling out V1 affect the noise level of the vibrato channel?

[überfuzz]

Measurements
Where do I find info about how to measure the tremolo circuit then? I have crocodiles to use.

Preamp tubes
I think it was JJ ECC83S -> 12AX7 and JJ ECC81? -> 12AT7 in the preamp. Pretty sure, not 100% though.

Effecting the noise
The pedal connection had a bad connection. Fixed now, tapping made it go SPARK, but it didn't effect the noise...

The noise
It's a 'sshhhhh' kind of noise. It's constant if the volume is constant. The noise goes up if I raise the volume. The noise goes up and down with the tremolos oscillating volume as well.

Things not affecting the noise - Hope I got everything in the list...

• Disconnected or connected reverb tank.
• Guitar plugged or not, channel 1 and 2.
• Tube v1, v2, v3, v5, v6 pulled or not.

[MWJB]

Measure the plate (pins 1 & 6) and cathode voltages (pins 3 & 8) of V5, with the RCA footswitch jack shorted & report back. You are very likely to find that dc voltages throughout the amp are higher than those printed on the schematic...this is normal and may not be directly related to your issue.

However, if you still get the noise without v6 installed (e.g. v6 pulled) then the noise would appear to be coming from the power section? However, if this was the case then the problem would affect both channels...this latest info doesn't quite scan?

[überfuzz]

Yes, as i was saying the voltages are ok, slightly high throughout the amp as you're stating.

I don't quite follow you on the shorting of the pedal plugs. Isn't the pedal sufficient if I'm going to measure when they're shorted..? There is a VIB|REV pedal connected to the amp. Oh, even with no input signal, guitars etc, the stage should be oscillating, right..? I'll get some measurements on that pin, with the stage turned on and report back. Ahh... That's the reason I got voltages close to plane. The stage is turned off, hence not conducting any current, pushing the voltage up to plate voltages. I guess I'll be reporting a min and max voltage then.

I'm pretty sure I pulled v6, along with all other tubes. (One tube at the time to see which stage is rendering the noise.) The power section seemed fine, voltages close to schematics and it sounds good, pin 1 and 5 was somewhat low, - 64V. Channel 1 perfect, like most Fender amps. :-)

Goodness gracious, I'm back to checking components then, or? Mhmm...

[MWJB]

V6 feeds the power tubes, so with v6 pulled only the power tubes can be making any noise, as they are disconnected from the preamp...but like I said, the noise would be on both channels if this were the case.

-64 vat pin 5 might not be bad, depends on power tube plate voltage & brand of power tubes, how much curent they draw. You need to check plate current, you can't bias by negative plat voltage alone.

You menttioned the preamp tube brands but didn't say whether you had bought some low noise tubes specifically?

[überfuzz]

Well, I noted that the preamp had JJ tubes labelled ECC83S. I don't really know if they're good or not, they sound good enough, talking about the tone. Data sheet - JJ ECC83S (ECC83S is the European version of 12AX7.)

[MWJB]

This is the first thing that you need to establish, if you don't know if the tubes in V2 & V4 are good or not (by good, I mean sound good & provide quiet operation) thn you wouldn't be the first person to start rewiring an amp, only to find that you only needed to find a good tube! :-)

When troubleshooting, always try the simple plug-in/plug-out stuff first, it helps to have "known good" tubes on hand to try. I am very familiar with the JJECC83S...great tube, even so, for V2 in a AB763 I would use tubes that have been pre-screened by a trustworthy vendor, to ensure that the don't have microphonic issues.

Your problem may well be more complex than this, I'm certainly not ruling it out...but do try a screened/7025 spec tube first.

Also, be sure to realistically evaluate the hiss, the vibrato channel will hiss more than the normal channel, if only beacause the signal is amplified another x35+. If the hiss is only an issue with the treble at 10 and volume above "7" for instance, it might not be tht much of a problem.

[überfuzz]

Well, there's a whole thread about me picking and poking trying to find out if there's a simple solution, like bad tubes etc... Right here.

Seriously

Thanks for helping! The hiss isn't a big problem, but it's out of the range of ok hisses. Starts as soon as the volume is turned from 0, keeps getting higher throughout the range of the volume pot.
I'll see if I can try other tubes in v2 and v4. Any suggestions? Preferably European dealers.

[MWJB]

watfordvalves.com

[martin manning]

The normal channel is quiet, so V1 is not a noisy valve. Exchange V1 and V4. Does the noise persist in the Vib channel? If so, it's not the valve formerly in V4 making the noise, it's something in the circuitry around it. The most likely source of this kind of noise is the plate resistors, so I would replace those and see what happens.

[überfuzz]

m&m - The changing the plate resistors are in the pipeline. Just need to get time to do it. I don't have free access, it's standing in a rehearsal studio down town.

Update - Last night I was able to test it, at first when it fired up and then later, a couple of hours later. The noise was more evidently when the amp was hot. To the extent that I'm now prepared to say it IS a big problem, maybe not so loud but the 2:d channel isn't really usable like it is now. Is heat likely to affect the noise level in the plate resistors?

[martin manning]

Is heat likely to affect the noise level in the plate resistors?

Yes, that is likely.