John P, Trisonic, anybody: follow-up: correcting factoy bias on sf Vibro Champ

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by drano, Dec 11, 2005.

  1. drano

    drano Member

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    I finally have my Champ back to fix the crappy sound:
    John Phillips wrote this a few months back:

    The amp is biased wrong.

    That may sound surprising, since it's a cathode-bias amp and from what you're saying is probably totally stock, but it is true. Fender built them like that.

    (What probably happened was that in about 1965 they raised the internal voltages to get more power - 6W not 4 - but for some reason forgot to alter the value of the cathode resistor. The result is that the 6V6 runs far too hot, which not only shortens its life but makes it clip far too soon at the forward end of the waveform, resulting in low clean headroom and a mushy, farty distortion. I don't know why no-one noticed, but they built them like that for over 15 years...)

    In order to make it run correctly, you need to replace the 470-ohm cathode resistor with a 1K (preferably 5W wirewound instead of the old 2W carbon), and replace the cathode cap with a 50V-rated type since the bias voltage goes up to around 30V; possibly 50uF not 25 if you want the same bass response. In fact, if they haven't already been done, it may be a good idea to replace all the electrolytic caps in the amp, since they're well over 20 years old now - if it has ply sides it was made no later than the middle 70s (you can date it accurately from the pot and transformer codes usually). It's also a very good idea to fit a screen-resistor, which they left out too - 1K 2W wirewound is ideal.


    Here's the amp. Please tell me which resistor to change. I can drain the caps if I have to. I would appreciate some help on the date codes too. What is the circuit number of this one? Would the circuit number be found on the amp anywhere? Thanks much.
    [​IMG]

    If my interpretation of color code is right this is the only 470 Ohm resistor I can find. Change it to a 1K?
    [​IMG]

    Sorry but I had to split the close-ups but I think you can see what you need to see. Let me know if you can't.
    [​IMG]
    [​IMG]

    Here's the rest.
    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG] This pot says 1377348. Is that the 48th week of 1973?
    [​IMG]
    [​IMG]
    Thank you. Thank you. Thank you.
     
  2. RL in Fla

    RL in Fla Member

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    Yup , that's him , and drain the caps .

    "40692" calls it a '74 , so the late '73 pot is about right too .

    AA764 schematic


    ;)
     
  3. VacuumVoodoo

    VacuumVoodoo Member

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    You correctly identified the resistor in question. When changing the electrolytic cap parallel to the resistor make sure you connect it with (+) to the cathode. But I'm sure you knew this.

    edit: beaten by 2 minutes.....
     
  4. drano

    drano Member

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    I forgot to ask, but I presume that cap next to the resistor in question is "the cathode cap" which sould also be changed out?

    Thank you for that reminder on the polarity. Actually, I'm such a novice that it didn't occurr to me. But I have a good friend with an EE degree and a beautiful shop who disdains tube technology but is kind enough to help me with the details like that if I can keep it simple. He will show me how to drain the caps properly and his soldering is masterful.

    Thank you for identifying the circuit number? How do you do that? Is it on there somewhere?
     
  5. VintageJon

    VintageJon Member

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    OK, go to minimum 1K 5W resistor and High Temp 22uF @ 100V cap.
    (Email me if you can't find them...)

    Ohm's Law tells us that E=IR, thus if you measure the cathode resistor and the voltage avcross it then Ik= Ie/Kr. (Cathode Voltage / Cathode Resistance)

    Now the MAX Plate Disiaptation for a 6V6 is 14W so-
    Ik X Vp = IPD. (Vp = Plate Voltage.)

    As this is a cathode-biased - single-ended amp you can run very near the Max Plate dissipation, (MPD- as I might as well coin a new term having coined IPD.)

    The 470 Ohm Cathode Resistor from the factory worked with the 100VAC that we had then but doesn't work with the 117VAC to 13VAC we get from the wall now.

    -Jon
     
  6. trisonic

    trisonic Member

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    John P. Trisonic, Eh?

    John gave me the data for me to give to an amp tech. As it happens I met Jimmy Somma who knows all about BF Fenders too - I got lucky both ways.

    Me, I just play 'em.

    Best, Pete.
     
  7. drano

    drano Member

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    It was, decidedly, part of the solution, however. Thank you.

    Apparently there's some disagreement here about the right cap to use. VintageJon's math up here looks persuasive.
     

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