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Heathkit AA-121 amp - under chassis picture

Posted by PeterI (A) on December 13, 2007 at 13:55:17

Since there had been discussion when I posted a pic of this pristine AA-121 amp, and since EL34s are being mentioned, here is a picture of the underneath of the Heathkit amp. The pots at the top are the balance and bias voltage adjusters on the front of this amp.

Besides the 4 filter caps above (150uf/300v and 200uf/300V) there are a number of electrolytics underneath. I plan to find good quality replacements.
But I really need a replacement resistor - 12ohm, 1% (or thereabouts), and 1/2 watt. No one seems to have these. Need to replace the one on cathode that popped when I had a tube go red plate. I guess I could put 2 in series to get that value?

I think the surgistor (lower center left) has to go - it doesn't appear to be working (an arm that is supposed to bend and make contact when it heats up). But I do get 499V at the OP tubes (schematic says 495V).

Pete

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