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

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

  1. Dandy

    Dandy Super Member

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    Your experience shows up the problem with those old PCBs. A new preamp board would probably fix the problem(s) there too, ... if it was available.
     

     

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  2. FireGuruMN

    FireGuruMN New Member

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    Here is a bit of an update. I found one original 10k resistor hiding on the preamp board. I replaced it. I also reflowed all the solder connections from my previous work. I methodically shuffled the tubes on the preamp section to see if there are any problems there. I found a configuration that sounded better than the others so I think there are some weak tubes in the preamp that need replacing some day. However somewhere in that process both sides started making music. The left side sounded "dull" but I eventually determined that it was just quieter than the right side so I adjusted the volume pots and they are both sound good now. There is a bit more buzz than I would have hoped for but the bottom plate is not on yet so I could be picking up some noise that way. I bought a new set of 7591A tubes from Antique Electronics. Russian. Electro-Harmonix. $90.00. They make everything sound better than the original tubes that were testing "marginal" on the tube tester. But even with the bias pots turned all the way down I can only get the bias voltage down to 45mv. That's pretty hot from what I read. But I ran them for an hour and had no red-plating. Any advice on how to get that voltage down to the recommended 33mv? If anyone cares these tubes have a beautiful purple/blue glow near the tops of the plates which is worth the price of admission alone! Oh and the new tubes are about 1/4" too tall for the cover. So I guess I have to leave the cover off for everyone to see my handy work. And the glowing tubes. I might make a vented wood case someday but the original textured metal was sooo 1960's. . .
     
  3. Patrice B

    Patrice B AK Subscriber Subscriber

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    I've built a circuit with four pots for mine also with EH's 7591. Yours has already the pots on the PCB I assume?

    And yes, 45mv is too hot. You'll have to play with resistors or pots values in the bias circuit methink.
     
  4. FireGuruMN

    FireGuruMN New Member

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    The board I installed replaces the entire amplifier board. It has 25 turn pots to adjust the bias. It worked fine with the original tubes but it cannot pull down the voltage on the 7591A's. maybe the "A"'s are different? I don't know enough about the circuit design to start plugging in new resistors. Suggestions always welcome.
     
  5. volvogv

    volvogv Member

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    FireGuru, You need a little more negative bias voltage. I recommend that you replace the R105 resistor in the power circuit with a lower value like 6.8k or 5.6k or 4.7k. Alternatively you can change R110/R112/R113/R114 with higher values.
     
  6. FireGuruMN

    FireGuruMN New Member

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    I put in a 5.6k for R105 and the bias dialed right in at 33ma. Now I have quite a bit of buzzing in the right channel. The "buzz" pots do not seem to change it. I am getting a lot of clipping noise in the speaker on that channel with louder bass notes. I notice that when I tap on the 7199 for that channel I can hear the tapping in the speaker. But not with the other channel. So is it normal to hear the tapping or not?
     

     

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  7. FireGuruMN

    FireGuruMN New Member

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    I forgot "Trouble Shooting 101". I swapped the 7199's between the two channels and now I can go to full volume without any clipping. I'm lost. I might just have to go "if it ain't broke, don't fix it".

    I noticed the level adjustment pots in the back of the amp are super crusty sounding when you turn them. Is it worth replacing them or just tweak them in and leave it alone? It would be nice to leave something original on the amp. . .
     
  8. volvogv

    volvogv Member

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    Good "shooting" there! Sometimes they just need to be turned back and forth to clear them. If not, there's no shame in replacing crusty pots if they don't shape up. Intermittent problems might be found in the pots, stereo mode switch, or the phase reverse switch. That's why I bypassed all of mine.
    May I suggest that instead of hijacking Gordon's thread that you start a new one?
     
  9. moediggz

    moediggz AK Subscriber Subscriber

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    It's mind blowing the amount of misinformation going on here regarding the restoration of the Heathkit aa-100. Most of these were build using good all caps which are survivors that don't need to be replaced. Everyone cutting good vintage caps out of their hifi please send them all to me!

    It's a shame people like to carve this stuff up because they lack the real world experience to do a period correct original restoration. All of the mods and changes suggested are absurd. My Heathkit aa100 is 100% period correct parts and 95% original. It's plug and play with no issues and sounds fantastic. I have racks and racks of vintage hifi all with period correct and original parts. Every piece I own can be taken off the shelf plugged in and enjoyed. Its mind boggling how people dont realize the engineers who designed this stuff knew what they were doing and take a random forum post as gospel.
     

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  10. volvogv

    volvogv Member

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    Moediggz: I see you've resurrected Gordon's old thread. Congratulations on a successful restoration! Once in a while somebody gets lucky and finds an amp that's in great condition and I'm glad it turned out well for you.

    Most of the rest of us ended up with amps that are/were in horrible condition. The common problems with this amp are well documented (poor circuit boards, carbon comps gone bad, Broken wafer switches, leaky caps, etc). You are correct that the HK Engineers knew what they were doing, but they lacked the technology that we have today. If they had poly caps, metal resistors, and 2 layer fiberglass boards available they certainly would have used them. I agree that it's un-necessary to go to the extremes that Gordon went to in order to make the amp sound good. Merely replacing the above mentioned components with fresh modern ones, and adding bias controls makes a big difference in the sound, but to keep 50 year old components in the audio path for the sake of authenticity is not making the amp any better. If it was an antique radio I could see it, but we're after a sweet, pure, flat frequency response and aged components just won't get us there. I'm not here to argue the finer points of vintage vs fresh components but I will say that the majority of people I've known who own these have a great need to clean up the audio path. I love historical things as much as the next guy (maybe more so), but not in this case. I've been a professional audio engineer for 34 years and I stand by my decision to replace ALL electronic components.
     
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  11. trinhsman

    trinhsman AK Subscriber Subscriber

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    I know Gordon personally. I have seen some of his work, and he has done work for me. The man knows his stuff. One thing I do know is that if something can be kept original, he will try to do that. While changing caps in a crossover is not the same as restoring an amp, Gordon re-did my crossovers in my Snell E II speakers. He was very reluctant to do it, because of the uniqueness of Peter Snell's crossover designs. After much prodding he relented. Now what he did was way above and beyond what most people would have done. He sourced a better quality cap, but got every single cap to the exact specs of the originals. He also "bundled" the caps, like Peter Snell did in the original crossover. When he got done it looked exactly like the original crossover, with the exception of the color of the caps. So......How did it sound? I'll let my wife's comment tell you: she said: "What did you do? That sounds so much clearer and better than before." If I had gobs of money, I would trust Gordon to build me a high powered tube amp to play with. IMHO
     

     

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  12. Dandy

    Dandy Super Member

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    Hi Moediggz, welcome to the tube forum. Take a while to read up on the excellent posts by Gordon before criticising this thread. This and others by him are certainly not 'random forum posts'. This thread helped me to get my AA50 from sounding terrible to sounding very good. It certainly was not carved up in the process, and the new power board by volvogv is a great additional resource. It also happens to be very respectful of the original board's layout. It's good to see that you've replaced some of the resistors on your preamp board, as those carbon comp resistors do drift.

    Having said that, there is a lot of love for keeping vintage hifi as original as possible here and I'm sure we'd enjoy some future threads from you documenting some of your vintage restorations.
     
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  13. gadget73

    gadget73 junk junkie Subscriber

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    You may be surprised how little leakage it takes to screw things up. I recently worked on a Pilot SP-210 that had assorted behavior problems that I traced to bad caps. My ancient cap tester indicated very low leakage, a blink every couple of seconds on the light, but it was enough to cause problems. I had some 60v on the volume control, which made it very noisy, and random pops and crackles that also went away when the caps were replaced. These were the same Pyramid caps that Dynaco used that I have occasionally been told hold up fine, and a pair of Ceracaps that are also supposed to be durable. They were NFG in the Pilot, modern basic yellow caps made it behave just fine.

    So yes, consider me sold on replacing "good" old caps for reliability reasons unless they can be proven good with full voltage leakage testing. I don't feel that replacing these original parts with modern says anything bad about the engineers that designed it, they were working to a price point and with the components available at the time.

    Also, the circuit mods presented here are not substantial at all. Change one tube type and install a better bias control system is really the sum total of it, beyond replacing degraded components. I would also not consider this to be a poor reflection on the engineer's original design. At a time when output tubes were more consistent in terms of performance and behavior maybe the extra cost of a bias adjustment system wasn't justified but we don't live in 1960 anymore.
     
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  14. GordonW

    GordonW Speakerfixer Subscriber

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    1) The individual bias mod is a necessity, if you plan to use any of the modern 7591 tubes. Even "matched quads" of current production 7591 tubes aren't nearly as matched as anything that you could get in 1965.The sound is measurably and audibly better, when the bias current is matched- and it's easier on the output transformers as well. Plus, the tubes last longer (which is a BIG plus when you're trying to milk all the lifespan out of expensive NOS 7591s), because the bias can be turned down to be within spec of the tube (most of these amps, unless the rectifier is changed out or a lot of resistance is added to the power supply, over-bias the output tubes- this was not accidental, but a result of the "horsepower wars" going on in tube amps in the mid 1960s- a somewhat lower tube life was considered worth it, to get a few more watts on the label).

    2) It's well documented that the screen resistors on the 7199s on these frequently fail (much higher resistance than spec). Those, and sometimes the phase inverter resistors (for the same reason) frequently need to be replaced. When doing so, it's good practice to replace them with resistors with better thermal stability and freedom from drift, compared to the original types. Fixing the same problem again, later, isn't my idea of a good time...

    3) The power supply caps- and any other electrolytic caps made in the 1960s- are junk now. Period. In fact, mine was popping (shorted) internally when I got the amp. It's a wonder someone didn't blow up the GZ34 or the transformer, it was so bad. Running one of these with original power supply caps is careless at best, and reckless at worst (there's this thing called FIRE)...

    4) The circuit boards on these, were far from "state of the art construction". Bypassing failed connections at components, as needed is a necessary part of making many of these work at all, much less reliably.

    5) It's well documented, as well, that power line voltages ARE higher than they were when these amps were built. Slightly reducing the power supply B+ voltage is a very prudent thing, to have the amp running within it's design specs.

    6) Many of these amps were assembled with the green Sangamo caps. Those are known unreliable caps, compared to most of that era. Mine had those, and at least a third of them showed leakage- and a few of them were even cracked open and PHYSICALLY leaking. Hence, a wholesale replacement was warranted. I'm not sending out a time bomb. This, along with the higher-than-original power line voltage combined with the already high stock bias points, is the reason why you see many of these amps with burn marks on the covers (I had to carefully paint-match and refinsh mine, in fact- the paint was actually burned away over a couple output tubes), due to run-away output tubes at some time during the life of the amp...

    7) When I rebuild one of these- unless requested otherwise, I'm using mylar film caps (not poly film). Those are a very similar sonic signature as the caps of the original era, if not functionally identical in sound- but they don't leak.

    These amps, like almost any other consumer product, were all "price engineered" to an extent. We have an opportunity to correct the shortcuts that were made then, in the interest of bottom line and sales volume. They were built to be sold at a price, and to do so, some things that could have been done better were done "good enough", for the time- but that's all you get from them, in original form...

    Regards,
    Gordon.
     
    Last edited: Feb 14, 2018
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  15. dlucy

    dlucy dlucy67 (Doug) Subscriber

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    Excellent points, packed with useful information, and a level-headed, careful, polite rebuttal.

    Well said, Gordon. Thank you!
     
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  16. Patrice B

    Patrice B AK Subscriber Subscriber

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    Gordon is not the first one to suggest mods in the AA-100 circuit and tube replacing (12AT7's in place of AX7's) and he's spot on his rebuild thread... Who are you to criticize like this? Please take a deep breath.
     
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  17. Celt

    Celt Super Maude Staff Member Super Mod

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    Let's back off the attitude and mis (bad) information, folks...
     
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  18. pustelniakr

    pustelniakr Silver Miner at Large Staff Member Super Mod Subscriber

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    Moderation Note:

    This thread has been cleaned up of recent thread-crapping, insults, disrespect and its afterglow. Let's get back on topic, and stay there.

    Rich P
     
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  19. Dandy

    Dandy Super Member

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    I was fortunate to find some affordable NOS Sylvania 7591 tubes recently and have added them to volvogv's power board, plus all Gordon's other advice here. The result is very good indeed. With these fixes the AA-100 must be judged a sleeper.
     
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  20. volvogv

    volvogv Member

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    There are now 28 output board kits scattered around the world, including USA, Canada, UK, and Germany. Many thanks to those who have supported this!
    IMG_20171226_141458.jpg
     
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