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THE end-all FIX for the Heathkit AA-100 integrated amp.

Discussion in 'Tube Audio' started by GordonW, Jan 16, 2009.

  1. elbreapoly

    elbreapoly New Member

    MPLS, MN
    Gordon has done a great service here, if it hasn't been mentioned tons of times before with eight pages of replies. I just have done my work on one Heathkit AA-100. Indeed those anode resistors were at 880K and 950K, replaced and done, as well as the four 220K replacements on the 7591's. Recapped the entirety, including filter cans (why are they insulated from chassis ground?). With a ton of dust, got this one working. I was really unsatisfied that on another AA-50/AA-100 that I couldn't repair the selector switch as the solder just wouldn't take. It confused me at first, but now working at an electronics shop it was suggested that it must have been made using plumber's solder or something. Grabbed a new unit, followed Gordon's procedure and check process, and am listening to an excellent amp as we speak (luckily I could solder this one because the correct solder was used). This advice is a goldmine. Thank you for the advice Gordon! The 12AX7 swap for 12AT7 I'm still playing with--not sure which I like better at this point but will post once I do. The input tubes V-5 and V-6 really react to the character of the amp much more than I've ever seen with tube-rolling. Sound best right night with Bugle Boy 12AX7s, but Tele's 12AT7's are close as well.
    Cheers and Thank you Gordon for your advice. And yes, I'll get better output tubes later than JJ's. IMG_1951.jpg IMG_1950.jpg
    dlucy likes this.


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  2. Glassweights

    Glassweights Compulsive about tubes Subscriber

    Hey Gordon
    Been playing with tube amps lately because not feeling the guitar building and my cohort is a EE who learned when tubes were the thing. So we got this aa-100 on Epay and after fiddling with it I realized it had your 220k and 12AT7 mods, but nothing else done to it! Kinda fun. I formed the big caps and replaced the paper caps, checked the resistors you recommended, then while chasing a few gremlins checked every resistor and replaced a few more.

    Here is where I run into trouble with the changes from 470k to 220k. According to the EE and you if I understand correctly, those along with the .01uf are only used as a high pass filter due to the crazy high impedance of the control grid. So every time I calculate the -3db frequency cutoff with the formula Fc=1/2pieRC I get about 34hz using .01uf and 470k and 70hz using .01uf and 220k.

    Given the undisputed fact that you are a crossover network guy, all I can think is you are approaching this math/frequency stuff from a different perspective. Can you educate me on this?

    Also, on the whole 12AT7 tube likes a different resistance. I have looked at the tube charts for x7 and t7 and really other tubes, and I cannot find a place where resistance from outside the tube is mentioned. Where do I see this data?

    Thanks much in advance!
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    Last edited: Oct 13, 2018 at 7:14 PM

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