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joshuahhhhhhh 26th October 2007 08:50 AM

heathkit aa-81 mono blocks
Hello all I am new to this forum a person I had contact with on ebay told me this is a great place for help. I am some what of a newbie the most experience I have is with tube gear that has come out of old stereo and mono consoles like magnavox. Any ways I have 4 heathkit aa-81 mono blocks they use 2 el34 or 6l6 1 6an8 and 1 5ar4 I have been collecting these amps to drive 2 pairs of quad esl57 electrostats 1 amp per speaker.
3 of these amps work fine after cleaning the pots and a slow power up on a variac one amp had a bias problem that turned out to be a resistor that was shorting out on the chassis. The 4th amp I am having problems with
it makes a hum but its not the trademark 60cycle hum like the filter caps in the power supply are bad. Its a kind of static hum that gets worse as I turn the gain up. At first I thought it was a leaky coupling capacitor so I pulled one side off all the coupling and used some orange drops of the same value to test to see if one was leaky I still get the same hum. I have also disconnected all the old power supply cans and mocked up some in there of the same value and again I get the static hum I have studied all 4 amps and compared b+ voltages and I cant seem to find where this static hum is coming from . I am also having a hard time finding the manual to these amps. My uncle who used to be into this stuff told me it could be the 50v cans in the bias supply I get -40 to -50 on these 50v 100uf cans
and from what I have read on other sites this is normal for fixed bias amps but I am not for sure these are fixed bias as they have a voltage and bias pot. I am sort of stabbing in the dark here and as you can tell I have limited knowledge of this stuff. Any help would be great!!! :gnasher:

SHiFTY 27th October 2007 12:37 AM

Usually the term fixed bias actually means adjustable (you have to set the bias manually), whereas cathode (autobias) is the other configuration.

If you are getting dull red glowing anodes on the el34s (the outer grey part of the tube) shut the amp down immediately as it has lost bias or needs the capacitors replaced.

To ensure long term reliability you should get the bias circuits overhauled on all the amps, it will cost very little and reduce the risk of overheating tubes and catastrophic failure. They use selenium rectifiers which can easily be replaced by a couple of UF4007 diodes. The bias capacitors should be replaced with new ones of the same value. In fact you would probably be well advised to replace all the coupling and cathode capacitors in the amplifiers for better sound and better reliability; it's a cheap and easy repair, and well worth it on such nice amps.

Regarding the noise, is it just in one channel? Could be a noisy 6AN8 preamp tube, try swapping the preamp tubes around and seeing if the noise follows the tubes.

Also try gently pinging each preamp tube with a pencil, see if they have gone microphonic.

Otherwise, it could be a bad solder joint, or a noisy plate resistor.

joshuahhhhhhh 27th October 2007 08:23 AM

2 Attachment(s)
Shifty these amps have no selenium stacks in them I don't have the schematic for them either. there are test points on top of the amp and I have been biasing them at 1.56v they seem to hold well the amp Im having problems with was not holding bias till I went in there and cleaned up some cold solder joints and now it seems to hold bias fine but Im still getting that hum. Heres a pic of the guts and a link to alil history about them you figure they would have a manual online so I can figure out how to bias these properly for now I have just been biasing them like a dynaco
stereo 70.

yel17 27th October 2007 09:56 PM

heathkit aa-81 mono blocks
I think your AA-81 amp is the mono version of the AA-40 and AA-121, except your amps have a tube rectifier, where as the AA-40/AA-121 have solid state rectifiers. I have an AA-40 amplifier, and a AA-121 manual with schematic (electrically identical to the AA-40), let me know if you want me to email it to you. It's a 7.2 mb pdf file. I spotted the bias rectifier in your photo, it's the white round component (with "Model 10" on it), it's between that 1 watt 22K (red-red-orange) resistor and the output transformer.
If the hum/noise gets louder as you turn up the gain, that makes me think the problem is before or at the gain control itself, which is at the very input of the circuit. So I would look at the pot and attached wires/components, and the input jack.
I hope this helps.


joshuahhhhhhh 28th October 2007 06:00 PM

I had another try at that amp last night I cleaned the gain pot again and blew it out with compressed air and dryed it out real well it seems to have done the trick I think I might have soaked that area around the pot a little to much with the contact cleaner. I had them up and running at low volume all night on the quads and didnt have any problems they sound great!!! I think the amp just needed to put under load for awhile to work the bugs out but im still going to replace all the caps in them and Im still a bit confused as to what my meter should be reading
when Im setting the bias on the amps . Thanks for the help!!

yel17 29th October 2007 09:55 AM

Heathkit AA-81
From the picture you posted, the EL34 cathode circuitry looks to be the same as my AA-40. To verify this, measure the resistance of the black resistors connected to each EL34 cathode (pin 8), and the brown resistor between their junction and ground. The black resistors should measure 6 ohms each, and the brown resistor should measure 12 ohms. My manual says set the bias voltage to 1.5 volts. Keep in mind that you should not set the bias voltage too high as the excessive current that results will shorten the life of the EL34s, and could result in other problems. Heath, Dynaco, Harmon Kardon and others set the output tube bias so that the tubes ran very hot (near full class A operation). When you couple that with today's higher AC line voltages, the result can be tubes that are running far too hot.
After you set the bias voltage to 1.5 volts and balance the tubes bias, measure the plate voltage (be careful in there). Now multiply the plate voltage by .05 (that's the cathode current when you set the bias voltage to 1.5 vdc), the result will be the tube dissipation in watts. I would try to keep the tube dissipation under 20 watts for the best overall reliability. You will probably need to reduce the bias voltage to around 1.3 volts to get the tube dissipation down under 20 watts.
I would also recommend replacing the bias diode and electrolytic caps with new components.

joshuahhhhhhh 30th October 2007 12:34 AM

I'm going to get some new matched pairs before I start in on them I have some of the coupling on the way I just want to get every thing together before I start tinkering with them. If you could email me the manual cablefreak1 at that would be super Its going to be a long journey but I would like to get these amps up to par the other night I had them running they seem to do pretty well but I could tell
some of the old el34's I was using were not very well matched one of the amps was vibrating to the touch with some old mullard el34's I had in it and I shut it down and put a matched set of EH and the vibration stopped
that kinda freaked me out and then I remembered that the hickok cardmatic I have only matches using mutual conductance another thing that freaked me out is that those mullards or at least the ones I was using in that amp were glowing green on the inside I never seen that before usually you see a nice dark blue on the el34's. Another thing I was wanting some input on is the 2 cans in series in the power supply should I keep the series and replace them with the same values or I can get my hands on some that are 600wvdc that are 20uf and just parallel them up Im going to keep the old paper ones in there to keep the stock look. yel you have been great help most the time I spend my time tinkering with old magnavox amps but when I got a pair of these and seen the output transformers were bigger than the power trannie
I knew these amps have some real potential the 1st pair I bought was off my uncle and there cherry not a scratch on them and look like they have never been used its like they were assembled yesterday and seem to work perfectly but have all the old parts in them . I have been biasing them at 1.56 and iv never had a tube in them run off my uncle tells me not to turn my back to them HAHA last weekend I had them on for almost 2 days running the quads there is a fan over the amps to keep the air moving well enough to keep them cool . They drive the quad esl57 panels better than any amp I have and I have alot of them I was using some old magnavox amp 150's that are pushpull parallel using 6 6v6's per channel and they work great but sound lethargic compared to the heathkits. Its going to be awhile but Ill get there Ill put up some pics of the setup soon as I can scale them down.

yel17 30th October 2007 11:37 PM

Heathkit AA-81
The manual will be in your email after I finish responding here. The Hickoks are good for checking transconductance (I have a 539C) and many other parameters. But, there is no Hickok, B & K, or other old tube tester that can match tube current because they do not test at the higher voltages found in typical amplifiers.
I've never seen a green glow either, blue yes but not green, amazing....
I would keep the two cans in series setup and replace them with new caps of the same value, you might also want to replace those big old 100K carbon balance resistors with a couple new metal oxides of the same value. I suspect any 600 volt electrolytic cap you buy will likely have 2 caps in series in a single container.
I am not surprised that the amps drive the Quads so well because the output trannies are probably good quality Chicago Stancor iron, like my AA-40 has. Let me know if you have any additional questions...

joshuahhhhhhh 1st November 2007 09:08 AM

got the manual and printed it all out. Sorry you went through all that stuff with the email and scaner. Ill study the schematic this weekend and see If I can figure out what rateing of diode to use in the bias supply

DigitalJunkie 1st November 2007 10:42 AM

(just spotted this)


My uncle who used to be into this stuff told me it could be the 50v cans in the bias supply I get -40 to -50 on these 50v 100uf cans
I'd replace them with caps with a higher voltage rating. 80-100V would be good.. (63V caps might cut it,just.) ;)

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