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.

    {Edit: I figured out my math mistake on the frequency calculations - doh - I understand the frequency cutoff points now for changing the coupling caps and associated resistors.}

    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 24, 2018
  3. Glassweights

    Glassweights Compulsive about tubes Subscriber

    Slept on it and decided to replace the can capacitors, stuffed a bunch of individual caps underneath and left the cans as is up top for looks . . . Unit is functioning fine but I think it needs some fine tuning as it has a small issue with a motorboating sound when the high gain phono is selected and cranked. (I do not actually have any phonos set up, but I want to chase that down.) Still hoping someone comments on the RC circuit question above. I suppose this may have been covered in the comments that were removed earlier? Not my intent to stir anything up, I am really only interested in learning facts about tube circuits that can be measured and calculated if possible on this topic.
  4. Patrice B

    Patrice B AK Subscriber Subscriber

    Montréal, Canada
    Mine does the motor boating thing also if phono gain is cranked sometimes.
  5. Glassweights

    Glassweights Compulsive about tubes Subscriber

    You know, we have at least three of these aa-100 in the bench area. I'll listen to another one and see if it does it. I only noticed this motor boating when I was tracking down a hum. The hum is pretty much gone, and the motorboating is not as consistent. Just got a scott lk72 to play with, but I am waiting on parts, so a little investigation will be fun!

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