Help! Blackfacing a Bassman/Bias Conversion Question...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Big Dan, Apr 17, 2005.


  1. Big Dan

    Big Dan Member

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    Hey guys, I have a few questions. I have a '68-ish Bassman (AB165 circuit) on my work bench and I'm in the middle of changing the bias set-up to blackface specs. I ran into a problem and need answers.

    Ok, I removed all the resistors going from the bias pot to the main circuit board. On the pot, I have one lug with a wire going to the bias board, one lug with a resistor soldered to the back of the pot, and one going to the main circuit board. I have read that all I need to do now is install the two 220K ohm resistors going from that junction to the top of each cap sitting there. Is that it? Note I have already removed the single original resistor on the right side of that junction.

    My real problem is, the AB165 uses two .022uf caps at that spot while the AA864 uses two .1uf caps. Can I leave the original caps there, or do I need to swap them out with the .1uf units??? This is a pic of where I stand now. I'm ready to shorten, bend, and install the two new resistors...

    [​IMG]


    While I'm at it, here is another question. The two 220K resistors that lead to pin 3 of each power tube...Keep them or remove them? The ones that are up on the little lug below the bias area I'm working on. AA864 does not have them, should they stay or go?

    The info I have been working from was on the Hoffman Amp Board. Here is that link.

    http://www.hoffmanamps.com/bias_conversions.htm

    Thanks,
    Dan
     
  2. Swarty

    Swarty Member

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    Just make sure the wiper is connected to the junction of the 2 220k resistors. As for the 2 resistors in the feedback network, I'd snip them.
     
  3. doctorx

    doctorx Member

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    Yes that is correct. Here is a layout you can refer to as well:

    http://www.ampwares.com/ffg/schem/bassman_aa864_layout.gif

    I would remove those 220ks. If you have those 2000pf caps on the power tubes snip them out too. Change the the lower resistors to 100k on the right and 82k on the left.

    .1 caps will give you a little more low end umph, I did this to my Bassmans and liked it.

    Install 1 ohm precision resistors between pin 8 and ground of the power tubes and you will be able to use your meter to bias the tubes. Or you could build a bias probe like doug has on his site, I have done that too.

    While yer at it, swap those turd brown caps out with Orange Drops or Mallory 150s. Those brown caps suck big time. Have you swapped out the electrolytics?

    Ok, I'm on a roll. Remove the power cord and install a three wire grounded one. Remove the .047 death cap on the ground switch.
     
  4. doctorx

    doctorx Member

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    By the way, that amp is older than a 68. it looks like it has cloth wiring and the tubular blue caps. i have a 68 which has crappy vinyl wiring and had all turd brown caps before I swapped them out for orange drops. dunno, maybe your amp is a transitional model. Read the codes on the pots and see.
     
  5. Swarty

    Swarty Member

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    Yes, that code # would indicate 38th week of '67.
     
  6. Big Dan

    Big Dan Member

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    Thanks Guys.

    Ok, lets see...Yes, the amp is a '67. It belongs to a friend and when I got a hold of it, the only things that were not 100% stock were the main a/c switch was an older replacement, and 1 resistor had been chaged. All else looked original.

    The plan was to BF it and give it a tune up. After he had it a while, we decided to hold off on the full BF treatment because he liked the stock tone.

    Anyway, I have already replaced the filter and power supply caps and resistors under the "doghouse". I also replaced the cathode caps on the board along with a "deep" switch that was faulty. I have a 3 prong cord ready to install as well. I just need to finish up the bias mod and I'm ready to turn it back on and give it a test (once I buy a bias meter that is).

    Once I get it running, then I will consider mods including cap/resistor value changes and messing with the feedback circuits. I just wanted to verify that I was doing the bias mod correctly.

    Most of the caps on the board are the blue Mallorys and that is why I'm not in any hurry to swap them out. The three right next to that picture are the brown turds, but the rest are Mallorys.

    Doctorx, what resistors are you talking about changing to 100k and 82K, and what will that do for me?

    Thanks,
    Dan
     
  7. doctorx

    doctorx Member

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    Hi Dan,

    That is a great amp. Even tho it is CBS, it still has touches from the house of Leo. If it is blackface, I have one just like it:

    http://bellsouthpwp.net/d/o/doctorx/pics/bassman bf.JPG

    Looks like you are doing all the right stuff. Make sure you pull that .047 on the ground switch when you install the new power cord.

    The 100k and 82k are part of the blackfacing process, look at that AA864 schematic. If you are modding it for adjustable bias, you should do that too. I am not 100% sure on the details, but IIRC it corrects an imbalance in the phase inverter.

    Keep all the blue mallorys, they are great caps. Dump the brown ones, replace them with Orange Drops or Yellow Mallorys.

    I usually run my power tubes hot, over 40 ma of current. The tubes don't last more than a year, but the amps scream when pushed hard.

    By the way, keep all the old parts.

    Here is the biasing process I use for all my amps:

    http://www.duncanamps.com/technical/lvbias.html
     
  8. Swarty

    Swarty Member

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