AB763 help needed..

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tubejunky
Posts: 5
(4/2/04 9:11 pm)
 AB763 help needed..
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 OK, so I 'had' my SR clone up and running seemingly fine, and so I decided to upgrade all of the pots for better tolerance, and add retainer claws to the power tube sockets. The problem is that with the new pots, the amp has sort of a screeching (not piercing) distortion to it at high volumes, which can be tamed intermittently by rotating the treble pot. With the treble pot all the way up the amp screeches, and the decay is harsh. I've replaced all of the tubes, and basically it seems as if the treble pot is microphonic?

I'd really like to get this solved. Any ideas?

[Edit]
The treble pot alone seems to be driving the amp over the edge, when I back it off, the amp sounds ok.

Edited by: tubejunky at: 4/2/04 10:19 pm
 
bnwitt
Senior tube assistant
Posts: 312
(4/2/04 10:49 pm)
 Re: AB763 help needed..
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 do me a favor and disconnect the negative feedback from the speaker jack and see if it goes away or only happens it full treble. If it does, tell me about your grounding scheme.
 
tubejunky
Posts: 6
(4/2/04 11:59 pm)
 Re: AB763 help needed..
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 Disconnecting the negative feedback does nothing, and the OT is wired correctly. All I did was replace the pots so I don't understand what the problem could be, and I just tried a new Alpha 250k-a pot. I ommited the trem circuit, and normal channel in this amp btw.

I'm at a loss at this point. Could this be the result of a bad bypass cap?
 
bnwitt
Senior tube assistant
Posts: 313
(4/3/04 12:07 am)
 Re: AB763 help needed..
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 Well you are experiencing a high frequency oscillation. I have had this problem with a grounding scheme that did not follow Doug's suggestions and it was exacerbated by nfb. The AB763 has nfb without a pot control so I thought that might help. Tell me about your grounding scheme. Is it according to the hoffman suggestions?
 
tubejunky
Posts: 7
(4/3/04 12:25 am)
 Re: AB763 help needed..
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 That's interesting because I previously had the bypass caps grounded to a bus wire that ran across the back of the pots, which I have since moved to the bottom of the brass grounding plate (like the original) for easier access to the pots. What would you suggest?
 
bnwitt
Senior tube assistant
Posts: 314
(4/3/04 12:49 am)
 Re: AB763 help needed..
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 I would suggest you follow Doug's grounding scheme to the letter. It works when all else fails.
 
LooseChange
Senior tube assistant
Posts: 234
(4/3/04 12:03 pm)
 Re: AB763 help needed..
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 I have had strange results with different pots. I don't know why either. Try putting a grid stop resistor between the Volume pot and the tube (on the tube socket). Try 33k or 68k. Just a thought. It has helped me with little problems like that and it can't hurt if it does not work.
 
tubejunky
Posts: 8
(4/3/04 3:30 pm)
 AB763 help needed..
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 I don't know what's wrong with it. I don't think it's the grounding. I was swapping out reverb driver tubes, and now the reverb doesn't work. However, the initial problem does not occur with the reverb driver tube pulled.

  
 
tubejunky
Posts: 9
(4/3/04 7:34 pm)
 AB763 help needed..
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 I can't figure out why there's 300+ volts on the plates of the reverb recovery tube? All the other voltages are reasonably close. I've got a ~440v B+ from this PT.

This amp sounds GREAT with the reverb driver tube pulled, extremely close to a real SR. I'm just ready to put it trough its paces!

Edited by: tubejunky at: 4/3/04 7:39 pm
 
bnwitt
Senior tube assistant
Posts: 315
(4/3/04 10:10 pm)
 Re: AB763 help needed..
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 Are you sure that is a 100k ohm resistor on the board between the B+ rail and pin 1 of V4? Chech all four of those 100k resistors on V1,V2,V3,&V4 and make sure they are the same. Other than that, maybe the V4 tube isn't conducting. Bad Heater pin connection or other pin connection.
 
tubejunky
Posts: 10
(4/4/04 12:09 am)
 Re: AB763 help needed..
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 Yeah I double checked those, and V1 is +270v on the money. I tried tensioning the socket in question, and I did find a tube out of about 5 that brought the voltage down to below 300v. I can't imagine what the problem could be.

Thanks again, and keep the ideas rolling!
 
Lucid Alice
Senior tube assistant
Posts: 188
(4/4/04 6:49 pm)
 Re: AB763 help needed..
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 "That's interesting because I previously had the bypass caps grounded to a bus wire that ran across the back of the pots, which I have since moved to the bottom of the brass grounding plate (like the original) for easier access to the pots. What would you suggest? "

I believe Doug suggests throwing that brass grounding plate away. It can lose good grounding to the chassis as the amp gets older due to oxidation, etc. I would move those grounds back to the bus wire on the back of the pots. A poor ground could be causing your oscillation. From what you've said this is one of the few changes you made as you replaced the pots, and it apparently didn't have that problem before the change.


 
tubejunky
Posts: 11
(4/4/04 6:53 pm)
 Re: AB763 help needed..
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 I found out that I had a 1.5k resistor on the cathode of V4 instead of an 820 ohm, causing the high voltage.

Now I'm going back to the grounding issue. This amp hums like crazy, even though I have it grounded in accordance with Doug's scheme. The reverb pot is more like a hum boost control..
 
bnwitt
Senior tube assistant
Posts: 316
(4/4/04 6:55 pm)
 Re: AB763 help needed..
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 Ok,
I guess I didn't read that previous post close enough. Yes I agree with Lucid. Throw that plate away and follow Doug's grounding scheme to the letter.
 
tubejunky
Posts: 12
(4/4/04 8:47 pm)
 Re: AB763 help needed..
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 Could it be a bad reverb transformer? The grounding is good, and like I say I can pull the reverb driver tube, and the buzz at high volumes goes away. Also, the reverb is not as strong as it should be. It's possible I shorted it when checking voltages once.

 
 
tubejunky
Posts: 13
(4/4/04 9:58 pm)
  Reverb Transformer
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 I just pulled the reverb tranny from my reverb unit, and installed it in the SR clone. The amp worked great for about a minute, strong reverb, everything was fine! Then aparently this reverb tranny blew (same symptoms), and I'm back to where I started!

WTF??


 
tubejunky
Posts: 14
(4/5/04 5:46 pm)
  Reverb Transformer
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 Well, the amp is working now. I've got a resistor and cap in series across pin 1, and 2 of the reverb driver for negative feedback. I don't know why this is required when everything else is wired according to the AB763 schematic with the exception of the 1st channel and vibrato circuit. The reverb tranny is from Mojotone, and Andy recommended I have this cap/resistor in place.

Can anybody explain why this is needed in my amp, but not in a real SR for example?
 
 
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