heathkit aa-81 mono blocks

[yel17]

The rating of the bias diode isn't critical as long as it is rated higher than the original. I use 1N4007 (1 Amp, 1000 Volts) when replacing bias diodes. They're cheap and available just about everywhere, I bought my last bunch at MCM, but I think Radio Shack might even have them.

[joshuahhhhhhh]

http://members.cox.net/cablefreak1/IMAG0088.JPG theres the setup sorry took me so long had some computer problems that picture is from like a month ago Im still waiting on some of the caps for the bias supply I did play around with one of them tonight and i put in one of those new diodes in one of the amps I was having bias probs with and now the voltage almost zero's out when I turn the voltage pot all the way down. It
was at like 1.3 volts even with the voltage pot turned all the way down I thought it was just bad tubes and the noise out of that amp is still there like a high pitched spiting sound but I think once I replace all the caps in the amp and do some heavy resistor checking it will go away. Cant wait to get the rest of the parts but for now Ill haft to listen to some vac gear... http://members.cox.net/cablefreak1/IMAG0080.JPG or another juicy peice of pushpull parallel http://members.cox.net/cablefreak1/IMAG0062.JPG

[mark02131]

Originally posted by yel17
Hi,
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.

tube.guy

I am getting a Heathkit AA-40 and would really like the manual if you would like to email it to me.

Thanks
Mark

[yel17]

Hi Mark,
The file is a 7.3 MB pdf and my ISP will not let me send files that large. I will send you the same files that I scanned and sent to Josh, the schematic and bias setting instructions. I recommend setting the bias voltage to 1.2 volts which equals 40 mA per tube. The manual says set it to 1.5 volts, but that's too high, your EL-34s will probably red-plate and die an early death at 1.5 volts bias setting. Here's a link to a thread with more info on this subject.
http://www.audioasylum.com/cgi/etv.m...=202788#202788

Best regards,

tube.guy

[Mush]

Hi:
Would someone be willing to email me a copy of the manual or schematic? I have a pair of aa-81 amps that are pretty beat up and I need to rebuild. The manual would be very helpful.
Thanks and have a great new year -- Minto