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Bought an old PA tube amp that...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Amit, Jun 22, 2006.

  1. Amit

    Amit Member

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    Israel
    makes me scratch my head about its circuit.
    I bought it in a flea market, no writing on it, looks like as a "one of" thing for stage work.
    Anyway, looking inside, it doesn't have a tube rectifier or diodes, the only thing that MIGHT be a "rectifier" is a big metal cube that has four posts for connections. at first glance I thought it was just a big old oil cap, but when I started going thru the amp to trace the rectifier/filtering and such, well...

    PT > 33uF(A) > cube > 33uF(B) > 1.8k > 125uF(C) >24k >25uF(D)

    (A) just goes to ground and the cube
    Between the cube and (B) there is a feed to the pin 3 on the two EL34 power tubes.
    (C) feeds a preamp tube that half of it is a split load phase AND the power tubes screens.
    (D) feeds two and a half preamp tubes, no cascaded stages.
    The cube, that has four posts, connects to (A) and (B) and the two remaining posts are tied to each other


    At first It of course sounded very bad, but replacing almost all caps (electrolytic and coupling) and some resistors that drifted like crazy (a 100K on the split load PI plate went to 800K!!!), it came back to life with a pair of Sylvania 6CA7 fat boys that replaced the pair of (maybe RFT) EL34 that were inside. on the other hand all four preamp tubes in it were AS NEW Siemens ECC83:D
    Didn't know that before I bought the amp as they were covered:)

    Any idea what is going on inside this old fela?:BEER
     
  2. tonezoneonline

    tonezoneonline Member

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    The "cube" is probably a bridge rectifier.You can check the Fender Reverb units schematics for similar rectifiers.
     
  3. Amit

    Amit Member

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    Ok here are some pictures.

    This is the amp. talk about rough life:)
    [​IMG]

    PT on the right, OPT on the left and the *cube* in the middle...
    [​IMG]

    This is the underside of the cube.
    Thin red/white wires go to filter caps, two thick red wires connect to OPT.
    [​IMG]


    Ok so this cube is probably the rectifier.
    Why cap (A) is between the PT and rectifier?
    Why have cap (B) feed the PI and power tube screens?
     
  4. Amit

    Amit Member

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    Hey Gary:)
    I did get it working after replacing almost all caps (all electrolytics and some others) and resistors that drifted like crazy but I still had too much noise, hiss and hum.

    First of all I saw that the heaters feed line wasn't twisted with a lot of care to say the least, and it ran all over the place including audio lines, so I rewired all the heaters and yea, it took away some of the hum, but I still had too much background noise.

    The power tubes seemed to heat up like crazy, plates didn't glow but it felt the tubes are being pushed hard. I never worked/had on a cathode bias amp so had some learning to do. without going into too much details, I found out that with the 130 ohm bias resistor that was there, my poor 6ca7's were trying to puch 68W!!!!
    Man, no wonder this amp felt like a pizza oven:)
    In the end I reached 940 ohm which should disspate about 16W and viola - most of the background noise was gone:)

    (Have no idea why I didn't think about that in the begining. I'm amazed my tubes could stand that situation for several hours...)

    No, don't have the RCA book.
    The secondary leads go to the cube with a 33uF cap between them going to ground.
     
  5. donnyjaguar

    donnyjaguar Member

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    I'll assume that all four EL34s use the same cathode resistor. If the plate voltage is about 360Vdc I'm guessing that 130R resistor is about right. 940R is profoundly too high.
     
  6. Amit

    Amit Member

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    It has two EL34 tubes that share the same resistor.
    Plate was about 410V, now it is about 435V
     
  7. donnyjaguar

    donnyjaguar Member

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    Hmmm...in that case I agree, 130R is too low. I'm guessing 330R would be a good choice. The idle current should be about 75-100mA for both tubes. What voltage are you measuring on the cathodes?
     
  8. Amit

    Amit Member

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    I'm away from home right now but from memory I think I had about 22V on the 130R resistor and now 38V on the 940R.
     
  9. Amit

    Amit Member

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    Ermmm let me get this straight, if the tubes share the same resistor this means I'm actually setting the bias for both of them combined and need to devied the dissipation result by two (in my case)?

    In that case you're right, 940R is way too cold:eek:
     

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