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

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

  1. soma89

    soma89 Well-Known Member

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    Ok. But just to clarify, the goal is to have them all at or around 30ma ? Because all 4 of the 7591s are coming up as much less than that
     
  2. NOSValves

    NOSValves AK Subscriber Subscriber

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    One thing I wonder about is when changing those resistors from 470K down to 220K the cap value of the associated coupling cap should have been raise by at least double... The combination of both create the frequency cut and leaving at the original value could end up with the coupling cap saturating on heavy bass notes and limited bass response of the amp.

    I didn't read the entire thread so maybe I'm repeating someone...
     
  3. Dandy

    Dandy Super Member

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    This is one of the problems of these amps when stock, as Gordon explains in the thread.

    The other problems are that the boards are brittle and many of the resistors and all the caps need to be replaced (at least that was my experience). If you don't want to do the fixed bias mod, and even if you do, please check all the caps and resistors on the boards. A lot of mine were way out.

    At the same time, get your 7591s tested, or at least check that all are good in another amp.

    NOS: I don't think your point is raised in the thread. So, up those to 0.22?
     
  4. GordonW

    GordonW Speakerfixer Subscriber

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    When I did this the first time, I ran the numbers on the coupling caps- even with the change in bias resistors to 220K, the rolloff point of the transfer function of the original coupling caps and bias resistors is about 7 Hz. That's far below the power bandwidth limit of the output transformers (in fact, close to two octaves below).

    One could, of course, replace the coupling caps with .15uf or .22uf. In my tests, it didn't make any audible difference.

    Regards,
    Gordon.
     
  5. Dandy

    Dandy Super Member

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    Thanks for covering this Gordon. After all, this thread is 'the end-all FIX' for these amps. :)
     
  6. trudie

    trudie Ted

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    Heathkit AA-100 rebuild and Mods

    I'm a newbe. This site seems terrific and there seems to be a lot of knowledge here!!

    Some of this I get like putting in a 100 ohm 50w resister to drop the voltage to spec supplied in the 1960s.
    I get replacing the caps and testing the resisters.

    I'm reading a total of 4 12AX7 tubes 2 on the left channel and 2 on the right. Would you replace all 4 with 12AT7's or just 1 in each channel?
    if so witch one?
    I take it this swap doesn't require any other changes to the circuitry just popping in the tubes?

    The 470k resisters you are changing to 220k on the output tubes. Are they R85 R86 R91 and R92 on the schematic?
    Are these the same resisters you recommend cutting the leads on and then connecting in the to the 10K pots for individual biasing?

    Is there a newer schematic than the one I uploaded on the individual biasing from Gordon W's post? I thought I heard tell of an updated one but may have been confusing it with a different post.


    On to a couple tube questions.
    I understand that vintage tubes are better. I the spirit of trying to keep the cost down on the tubes knowing they are easy to replace, are Electro-Harmonix 7591A's an acceptable substation for the 7591's?

    With the 7199 tubes specked in this Amp look to be out of production. Has any one tried substituting 6GH8A Instead of 7199 Tube, with a plugin tube adaptor? Good results? bad results? Loss of sound quality?


    Sorry for all the beginner questions. I watched my dad built Heathkits before he passed in 1972. This looks like a fun thought challenging project made easer with the knowledgeable posts from this site. I have the manual, schematics and a AA-100 that needs some TLC.



     

    Attached Files:

  7. GordonW

    GordonW Speakerfixer Subscriber

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    Better coipy of Schematic

    A member asked me to put up a higher-resolution copy of the AA100 schematic, showing the bias mods that I did.

    The jpg file that I have is too big to post (it automatically gets truncated/shrunk down to 900 pixels wide) when posted as a .jpg file (the native format of the file). To try and address that, I'm posting it as a two-piece .gif file (.gif files apparently don't have the same size restrictions as .jpg files)... hopefully, this will allow reading of the file in the original size, for everyone...

    [​IMG]

    [​IMG]

    There's about 40 pixels of overlap between the two files (in the middle). If you are reasonably clever, you should be able to fit them back together as a single file in a paint program...

    NOTE: On the earlier diagram I posted of the replacement bias circuit, a few post above this, the resistor values are different than that on the full schematic in this post. EITHER set of resistor values should get you within the spec. I would recommend using the filter cap scheme (one 100u cap on the top side of the pots, to ground, and one 100u cap on the bottom side of the pots, also to ground), since it gives better hum filtering than the one cap in the diagram from the earlier post.

    Regards,
    Gordon.
     

    Attached Files:

    Last edited: Aug 5, 2015
  8. trudie

    trudie Ted

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    Gordon W, thank you very much for the larger schematic.
     
  9. djnagle

    djnagle Lunatic Member

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    Hey Gordon. I have an AA100 that I want to use the from to make a PP 807 amp with. Do you think the parts will work well in this configuration? I already made a sculpture chassis for it. What do you think?
     
  10. Shadowdog

    Shadowdog Super Member

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    Yes, Gord thanks very much also! Very nice and helpful to have!
     
  11. GordonW

    GordonW Speakerfixer Subscriber

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    I see no reason why not. I used the iron from a derelict AA100 (bought off of that auction place) to retrofit a dead Harman Kardon A500. I used 5881s as output tubes- it sounded great.

    As 807s are pretty much "parametrically equivalent" to the 5881/6L6 from all practical angles, I see no reason it won't work. They're about 7500 ohms primary impedance- a little high for 6L6s or 807s, but do-able (you just need more B+ and less screen voltage, a condition which suits the 807)...

    Regards,
    Gordon.
     
  12. stanko

    stanko Hoarder Subscriber

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    This is great reading material.
     
  13. djnagle

    djnagle Lunatic Member

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    Thanks Gordon. So when you say I may need more B+ does that mean the power supply the AA100 has now is not enough?
     
  14. trudie

    trudie Ted

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    I'm trying to find a top cover for the heathkit AA-100,I am also looking for what they refer to as wipers and rivets for the rotary mode selector switch sections. I need to remake 2 of these sections.
     
  15. GordonW

    GordonW Speakerfixer Subscriber

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    It would work, but you can probably get a little more dynamic range from the 807, using SS rectifiers to generate a slightly higher B+. Also, seemingly paradoxically, the higher B+ will take a little bit of the load off of the screens (as screen voltage stays constant, increasing plate voltage decreases screen current on average).

    Regards,
    Gordon.
     
  16. trudie

    trudie Ted

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    A thousand thanks To Gordon W. With the post and the revised schematic I was able to rebuild one of these. It now plays warm sweet tubular goodness!!!
    When I was rebuilding it I found a series of breaks in the Power amp board. I fixed them as I was going along. I did the recommended full recap, tested all the resisters many were far out of spec and I replaced them too.
    I built the bias circuit to replace the existing bias circuit, I put in the recommended preamp tubes, and replaced the out of spec 470 K ohm resisters on the power tubes.
    I did run into 2 issues when I powered it up. Half the heaters on the preamp tubes ( RT channel) were not working. It wasn't until I realized V12 didn't have heater power that I found the break just before the last power tube in the heater circuit.
    The other problem I ran into was V11 wouldn't bias. It was reading 80 V instead of .35V. Im a novis or I probably would have realized right off the ground was missing for this part of the bias circuit. As soon as I soldered in a ground every thing checked out. Again many thanks to Gordin W!
     
  17. djnagle

    djnagle Lunatic Member

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    I'm thinking about bringing this project off the shelf. I am only building the amp section without all the other stuff. Where in the schematic does the amp section start? Is the 7199 part of the amp or the "other stuff"?
     
  18. Pio1980

    Pio1980 AK Member Subscriber

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    The 7199 part.
     
  19. djnagle

    djnagle Lunatic Member

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    Not sure I understand Pio. Say more please.
     
  20. Pio1980

    Pio1980 AK Member Subscriber

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    The two 7199s combined with the 7591 output tubes form the power amp section of the amp, the power supply is of course common to all sections.
     

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