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

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

  1. Tube Radio

    Tube Radio Super Member

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    I may try that with my Scott LK-72A amp. Even though the power transformer is fused a little more protection right at the power tubes cannot hurt anything. I may try and add a neon lamp across each resistor so I will have some indication if a resistor burns up. I used the JJ 76591 once and one of the four tubes developed a short to the grid after only using them a week. If only the Groove Tubes 7591 were a little smaller in diameter and not quite as tall.
     
  2. Mike Stehr

    Mike Stehr Poverty Audio

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    Hi Gordon,

    What's the purpose of the 2200 ohm resistor from the bottom side of the 10K pots going to ground?

    And do the 10 ohm cathode current sensing resistors necessarily have to tie in at the same ground point as indicated in the diagram?

    This is what I wanted to do with my Altec 345A amplifier, having the capability to bias each output tube.
    (The idea was brought up by a local retired AE.)
    The amplifier's original negative bias circuit had one 5K pot to adjust bias for all four tubes, with the cathodes all tied to chassis ground.

    I went and put 10 ohm 1/2 watt resistors to ground for each EL34 cathode with test points, for measuring bias current above chassis.
    And I then was going to source four 5K bourns trimpots for each grid of EL34. Mouser catalog indicated a panel mount clip-on bracket for the little 3/8" trimpots.

    They would have mounted nicely right in a oval chassis punchout or two.
    I ask Mouser for the panel mount feature to the Bourns trimpots. They don't know nothing about them.
    They contact Bourns and ask about them, then they contact me.......Bourns says no......lame....

    I got tired of figuring out what I was going to do, or how to custom fabricate four trimpots mounted on a piece of perfboard. Then figure out how to mount the assembly to the chassis and still be able to bias the tubes from the top of the amplifier. I'm still gonna figure something out....

    I went ahead and just went with two 5K bourns 2 watt precision wirewound pots, mostly because it was a drop-in mounting in a larger oval punchout on the chassis.
    Of course that will only bias each pair.
    When you measure across each 10 ohm resistor to a pair of tubes that you can only bias as a pair, you can really see where individual bias becomes a nice option.

    After reading your writeup, this makes me want to follow through with the individual bias idea.
    I'm pretty much set-up for it........

    What were the size of those Radio Shack 10K pots that you used?

    Thanks,
    Mike




    [​IMG]
     
  3. GordonW

    GordonW Speakerfixer Subscriber

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    The 2.2K resistor (and the 6.8K on the top end) is to limit the range of adjustment... to prevent the "ham-handed" from accidentally turning the bias voltage ALL the way down to zero (or up to 60 volts). You only need 5-6 volts of range... from, say 16v to 22v. This does that, nicely.

    The 10 ohm test-point resistors can just be put in series with the cathode leads... the cathode grounds can go where they originally went, if desired. It'll be fine that way. In this amp, the original circuit board had the cathodes wired back to the power supply ground lead... so I duplicated that.

    The RS 10K 15-turn pots are about 5/16" square by about one inch long each, IIRC... maybe a little smaller than that. They're also pretty bright blue. :D

    Mind you, for other amps, you will need to find what resistors will go in place of the 6.8K and 2.2K, for the amount of voltage you need. This is ONLY for these 7591-based amps such as Heath, Scott and such, which have a somewhere near -60v bias supply, and need around -20v of bias voltage...

    Regards,
    Gordon.
     
  4. Mike Stehr

    Mike Stehr Poverty Audio

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    Thanks Gordon.

    To limit the range of adjustment, I like that idea.

    I'll have to check the Radio Shack that actually has parts, and see what they have.....
     
  5. soundmotor

    soundmotor super modified Subscriber

    Doesn't matter if they do as you can order in the store and they'll ship to you at no charge.

    :thmbsp:
     
  6. Chrome

    Chrome That there be funny......

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    That's what I just did.

    Well written too. :thmbsp:
     
  7. Mike Stehr

    Mike Stehr Poverty Audio

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    Cool.......There is a RS a block away in the Ben Franklin.......I may just have to blunder over there and do that......
     
  8. GordonW

    GordonW Speakerfixer Subscriber

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    And now a moment for the (not so) dearly departed...

    :D

    The dead pool. ALL of these caps came from this one amp. And I think I may have dropped a couple under the bench, so there may have been a few more... :yikes:

    [​IMG]

    There were some 10-15 resistors as well... but they don't photograph very well. :D

    Off to that great dumpster in the sky!

    Regards,
    Gordon.
     

    Attached Files:

  9. soundmotor

    soundmotor super modified Subscriber

    Before you do, measure them and post how far off listed values are.
     
  10. GordonW

    GordonW Speakerfixer Subscriber

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    OOPS. :D

    This reminds me of a conversation in Return of the Jedi:

    "INTENSIFY FORWARD FIREPOWER!!"

    "TOO LATE!!!"

    BOOOM!! (star destroyer blows up).

    They're already in the trash... and the bag got carried out to the dumpster. I don't really feel like dumpster diving in 20 degree weather either... :D
     
  11. drgoodfellow

    drgoodfellow New Member

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    Thanks for the very thorough info! I recently acquired a AA-100, but I haven't even powered it up yet. Now i definitely will (after the mods that is) :yes:

    I have a question though. What about cathode bias this amp? Any big downsides? I kinda like the sound (I own a Eico ST-40 & some other cathode biased amps). I realize it'll be harder to balance the amp, find matched pairs of 7591's and whatnot - but that aside?

    Cheers!
     
  12. barebones

    barebones Well-Known Member

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    I'll print this too!!! Unfortunate you had a battle with it. I grabbed mine plugged in in and voila ...low watt goodness from the get-go. Great info to have in case it tanks though!!!:thmbsp:
     
  13. GordonW

    GordonW Speakerfixer Subscriber

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    You'll lose a lot of power. 7591s (at least modern production) can't take much idle bias current, compared to big mamas like 6L6GCs and such. Once you get the bias down to no more than 70ma idle, you won't have much drive voltage left, in cathode bias. Might be able to get back to 25w/ch of the stock unit, but I doubt it. Probably no more than 15-18 watts that way.

    This is why you rarely see 7868s or 7591s in cathode bias... they just don't make much power that way...

    Regards,
    Gordon.
     
  14. BrassTeacher

    BrassTeacher Uncurable Tweaker

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    Gordon, I think it was the awesome paint job on the scorched louvers that made the biggest improvement in the sound...:D

    For those of you who are skeptics of the AA-100, Gordon or myself will gladly take them off your hands!

    If you have one lying around, do yourself a favor and follow Gordon's rebuild advice, especially the individual bias trimpots, the 100-Ohm resistor between the rectifier and first stage of the filter cap (doesn't exactly have to be 50Watts, but it does look pretty cool under the amp with its big, gold anodized heatsink/body), a good set of 7199 tubes, and subbing the 12AT7s in for the 12AX7s.

    And besides, the amp just looks cool!
     
  15. Pio1980

    Pio1980 AK Member Subscriber

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    Very interesting thread.
    I 'think' I have one of these AA-100 things in a very safe place in my house somewhere. They do have a certain rep as being crap-sounding iron donors at best and the captive board circuitry is a total PITA, I have regardless considered eventually attempting to find and rebuild it into something usable. I also have an AA-151 I've decided to keep 'for the duration' with much better tech-friendly excecution, otherwise the observations apply for both. I do hope they both have the "good iron" in them.
    I cannot see why a decent smaller-scale integrated cannot sound as good as the sections separated at the same level of build. I am not interested in "tube sound" per se, preferring accuracy re openness and transparency without a signature sonic imprint.

    I'd like to ditch the 7199s for 12AX7s (I loathe small-signal pentodes in audio) and otherwise reduce the *TMS factor* during the upgrading with tone section clean-up and bypassing option and a serious rebuild of the phono section into something worthy of a modest but competent audiophile system.
    BTW, did you get to the phono stage yet?


    TMS factor= Too Much Stuff in the signal path chain of custody. I can't understand how adding stuff to the signal path unnecessary for proper basic operation improves sonic transparency of the system. Competent simple approaches tend to sound cleaner, more 'direct' and 'open', IE more "transparent" to the source signal to me.
     
  16. tube.guy

    tube.guy New Member

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    4
    Heathkit AA-100 iron

    These amps do have decent iron in them. The output trannys are stamped with an EIA code prefix of 138 (mine are stamped 1386208), which indicates that they were made for Heath by Chicago Stancor during the 8th week of 1962. Here is a link to the EIA codes for various manufacturers of electronic components http://www.triodeel.com/eiacode.htm
    I've had this AA-100 sitting around for ages, just never got around to doing anything serious with it. After reading Gordons posts, I've decided to upgrade mine with individual bias pots, and the other upgrades.
    Regards,
    tube.guy
     
  17. GordonW

    GordonW Speakerfixer Subscriber

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    I'm upgrading another one, right now. Will try to take pictures once its done. A lot of clean-up was required on this one, due to it having been taken apart and put back together again, before it got here. It's getting the total re-cap, bias pots, and such.

    I tried to simplify the procedure, as much as possible, on this one. Less having to run wires around all over the place, as the first one I did. Put the 1/4w 10 ohm resistors on the backside of the PC board... that saved a lot of wire- that sort of thing...

    BTW: On the subject of pentodes- to quote Stuart Hegeman, arguably the best single amp designer in the heyday that these amps were being made: "Why would anyone want to use a triode front-end when they could use a pentode?"

    Regards,
    Gordon.
     
    Last edited: Jul 12, 2011
  18. Pio1980

    Pio1980 AK Member Subscriber

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    Stu may be the only person I've heard of who might use them because he wanted to rather than because he had to.
    I still think I have an AA-100 somewhere around here but have no idea where it's hidden itself. I know where the AA-151 is and cutting out the 6AU6 tone stage effected a vast sonic improvement. It's still a 'someday' project.
     
    Last edited: Jul 13, 2011
  19. tube.guy

    tube.guy New Member

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    Gordon, I went to the Shack and bought the pots and a copper-clad perfboard to mount the pots but have not yet decided where to mount the board. I intend to mount the 10 ohm resistors on the main board like you are doing. But, the physical layout of the AA-100 isn't very mod-friendly. Where did you mount your trim-pots?
    One more thing, the packaging for the trim-pots rates them @ .75 watts at 25 C. That's only 77 F, room temperature. I hope that's a misprint because trim-pots wattage ratings are generally rated @ 70 to 85 C (a much higher temperature than 25 C). Did you ever have any problems with those Radio Shack pots?
    Thanks
    tube.guy
    Thanks
     
  20. GordonW

    GordonW Speakerfixer Subscriber

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    No, I haven't had any problem with the Radio Shack 10K 15 turn pots. AFAIK, those are made by Bourns.

    I usually get a 7-terminal, center-ground terminal strip, and mount the Radio Shack pots at 45 degree angles, facing up. There's two groups of three terminals, on each side of the center ground/mounting terminal... the center of the three is common to the two pots (I solder the bottom/back terminal of each pot to it), and the ends of the three are what I solder the middle terminals to (splayed, one on each side, facing 90 degrees out from each other). After doing both sides (all four pots) like that, I then run a "bus bar" line across the four top/front terminals. Bottom set of terminals goes to the 3.3K resistor that was connected to the 8.2K (I also put a 22uf 50v cap in parallel with the 3.3K ohm resistor, positive side to ground, negative to the pot strip). Middle set of terminals is output to the grids. Top set of terminals go to the output of the 8.2K resistor. I also put a 22uf 50v cap from that junction (again, positive side to ground, negative to the junction), to the same ground as the cap across the 3.3K...

    As for mounting location- I tend to make a small "bridge bracket" out of a piece of 18g sheet metal... a "z-shape" piece, that goes under the power transformer mounting screw nearest the can caps, on one end, and mounts to the terminal strip mounting point on the other, to space the terminal strip rearward (if you mounted it to the transformer mounting screw,it'd run into the wires to the can caps and the heater pots) enough to clear everything. Usually winds up about 3/4" behind the transformer mounting screw.

    BTW: Update on the one I'm working on now: Man, I think that this is becoming an exercise in "what can possibly go wrong with one of these preamp circuit boards, and how can I fix all of the things that have gone wrong". Whole bunch of broken traces. Not only on the board itself, but on the little "card" that mounts the stereo/mono mode switch- I had to jumper the connecting traces from that card, to the connecting points on the main board. Now, I actually have BOTH channels playing.

    However, now I have one channel louder than the other, and the weak channel distorts. It's not the outputs (I've jumpered the preamp supply to the amp board, channel-to-channel). Somewhere, there's some resistors out of spec, I'm pretty sure. Now, just to find the little f***ers...

    Regards,
    Gordon.
     

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