AB763 Single Channel - Helping a Buddy Fix His Build

[psb962]

I'm taking a look at a single channel AB763 a friend built years ago. It never worked well - noisy - and a tech recommended he replace some resistors and wiring. He did, and it stopped working. He put it in store for 9 years, and just gave it over to me to see if I can figure it out.

He appears to be using the Hoffman layouts and parts list.

It sounds like I'm looking for a bad joint or two somewhere on the board, so I have 2 questions before I start:

1) Until I get my hands on a 4 ohm load I plan to check voltages etc., without output tubes in. I assume that with no power tubes nothing is going to be overloaded. I expect voltages to the preamp tubes to be higher than normal. At least I should be able to check that power is getting everywhere it should, and that signal will get to the power tube sockets. Is that a good plan or have I overlooked something?

2) To keep it simple I'd like to pull the 12AT7s so reverb and tremolo will be dead. As far as I can tell from the layout, amp should still pass signal to power tubes. Is that correct or do I need reverb tank, tubes, etc., hooked up?

[sluckey]

2) To keep it simple I'd like to pull the 12AT7s so reverb and tremolo will be dead. As far as I can tell from the layout, amp should still pass signal to power tubes. Is that correct or do I need reverb tank, tubes, etc., hooked up?

The 12AT7s have nothing to do with the tremolo. One of them is the reverb driver so pulling that will kill the reverb. The other 12AT7 is the PI. If you pull that one NO signal will reach the power tubes.

I suggest you download the schematic/layout pdf from Hoffman and get familiar with the circuit.

[psb962]

Yes, apologies, I'm doing that and see what you mean. I'll pull the reverb driver only. Is it safe to power up with no power tubes in and no speaker connected?

I plan to use my current limiting bulb for first switch on.

[pdf64]

... Is it safe to power up with no power tubes in and no speaker connected?

Probably, but can't be sure, need to monitor how high the HT voltage rises, that it doesn't exceed the HT cap rating.

But whatever, an 8 ohm dummy load would be fine for your purposes.

If you've just want to kill the signal to the 6L6 output stage, link the pin 5 control grids together.
That way, HT voltages should be as normal.

[psb962]

I used an 8 ohm speaker as suggested. Tested with tubes in, found there are no plate volts on tubes 1-3, it's a missing wire on the turret board between the 4k7 and 1k resistors. Should be an easy fix.

Also the heater ground resistors weren't hooked up to the heater wires so that might explain the amp being noisy. Good progress.

Everything else looks ok - the soldering is all good and its all nicely laid out and labelled, so I will get the connections fixed and will retest it soon.

[sluckey]

found there are no plate volts on tubes 1-3, it's a missing wire on the turret board between the 4k7 and 1k resistors.

That would explain no plate voltage on V1 and V3, but not V2. V2 gets it's plate voltage through the reverb transformer red lead and a red underboard jumper that connects to Node B.

[psb962]

Yes, my bad, I now see that I noted 90V on one of the V2 plates.

With the missing connections made I did a current limited startup (bulb) and that went well. With the limiter removed I checked volts and found 450V DC on caps A, B, C and D, so the voltages aren't dropping downstream. I assume the preamp tubes aren't drawing any current. I then measured 444V on pins 1 and 6 of V1 (and a surprising 385V on the cathodes (3 and of V1 so shut it down. I get how the plates are seeing 444V but I'm puzzled by how the cathodes could read that high. Especially as the bypass caps are rated for 25V...

I think this amp still has a gross wiring issue I haven't found yet.

[sluckey]

I then measured 444V on pins 1 and 6 of V1 (and a surprising 385V on the cathodes (3 and of V1 so shut it down. I get how the plates are seeing 444V but I'm puzzled by how the cathodes could read that high. Especially as the bypass caps are rated for 25V...

That's a classic sign that the tube cathode has no resistance path to ground. Pin 3 and pin 8 of V1 should read 1.5K resistance to ground. Your meter provides a very high resistance path to ground and causes the high voltage reading on the cathode. Look for a missing ground wire or jumper on the preamp end of the board.

[psb962]

Oh wow you are correct. I'll fix that and retest.

I checked all the preamp tubes in another amp and they all still work so we are on a roll here.

[psb962]

Found and fixed the missing ground. Reran the voltage tests and everything now looks a heck of a lot better except the power tube bias - and the power tube screens which were 170V vs a plate of 450V. Didn't take long to see that the reason for that is that the screen resistors are 470k, not 470 ohm. That will do it...

I then went on a mission looking for any other erroneous resistor values and found the bias resistor was also 470k instead of 470, so the bias of the power tubes was down at -3V (instead of the -20 or so I would expect).

Finally I noticed that the blue and brown wires to pins 4 of the power tubes were reversed from the schematic so I assume that might be a' problem waiting to happen' as this amp has negative feedback and the reversal might cause a positive feedback squeal when the power tubes are properly adjusted.

My friend is coming over today so I can show him what I found and I expect he will want to take it away and rework it.

[sluckey]

I then went on a mission looking for any other erroneous resistor values and found the bias resistor was also 470k instead of 470, so the bias of the power tubes was down at -3V (instead of the -20 or so I would expect).

Bias needs to be about -35V for 6V6s or about -50V for 6L6s.

Finally I noticed that the blue and brown wires to pins 4 of the power tubes were reversed from the schematic so I assume that might be a' problem waiting to happen' as this amp has negative feedback and the reversal might cause a positive feedback squeal when the power tubes are properly adjusted.

Those blue and brown OT leads better be connected to pin 3 of the output tubes!

Do you have any experience working on tube amps?

[Stevem]

Wow!
Is all I can say!

I find it very hard to believe that your friend you got the wreck from ever got and audio out of it at all!

[didit]

Perhaps 3 & 4 had been bridged ..

[psb962]

Those blue and brown OT leads better be connected to pin 3 of the output tubes!

Do you have any experience working on tube amps?

They were on pins 3.

I built a 5e3 last winter with a selection of the Robinette mods. It works perfectly.

This is the first broken amp I've tried to debug. The main struggle I had with it was that the enclosure was not standard and the board obscured the sockets to a large degree.

Thank you for the help.