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

    Messages:
    3,889
    Location:
    North West, UK
    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.
     
  2. FireGuruMN

    FireGuruMN New Member

    Messages:
    19
    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

    Messages:
    2,005
    Location:
    Montréal, Canada
    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

    Messages:
    19
    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

    Messages:
    67
    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

    Messages:
    19
    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?
     
  7. FireGuruMN

    FireGuruMN New Member

    Messages:
    19
    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

    Messages:
    67
    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?
     

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