hey.. i need help with my vibro champ

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by gbisn1960, Jan 26, 2004.

  1. gbisn1960

    gbisn1960 Member

    Messages:
    816
    Joined:
    Dec 10, 2002
    Location:
    Baton Rouge, Louisiana
    the damn thing keeps eating tubes!!! i dont know whats wrong with it.. if i run it on 10 for too long with a guitar, or even a bullet mic for microphone it blows the tube... how do i test things to see and what should i test?? i have the chassis out of the cabinet right now and im looking at it.. im guessing the HUGE resistor is the screen resistor?? the previous owner put a weber alnico 8" speaker in it.. could it be a mismatch in impedance?? this is pissing me off cuz i cant use it.. and i kinda want to just sell it off, but i dont want to invest 65 bucks on getting the thing fixed... it could be a very very simple thing.. especially if i can only get 200 for it when its fixed! please help!!! thanks
    Jon
     
  2. John Phillips

    John Phillips Member

    Messages:
    13,080
    Joined:
    Mar 17, 2002
    Location:
    Scotland
    If it has the stock 470-ohm cathode resistor in, that's why.

    Believe it or not, Fender made these amps with completely the wrong biasing (WAY hot) from 1965 to 1982. I don't know why no-one spotted it. Typically, the 6V6 runs at well over 40mA (some as high as 50), which is a dissipation of 16 to 20W on a tube rated for a maximum of 14, even for a 6V6GTA. It also actually reduces the clean headroom and makes the distortion farty and rough, because the tube clips the 'forward' end of the waveform far too soon.

    The big resistor on the board is the cathode resistor. The 'correct' value varies a little (and you really need an oscilloscope to get it dead on), but in nearly every Champ I've worked on, 1K (ie double the stock value) is very close, if not exact. Use a 5W wirewound type for maximum long-term reliability.

    If you do this, you MUST replace the cathode bypass cap, which is in parallel with it, with a 50V-rated type, since the cathode voltage will rise to about 35V (the stock cap is 25V), and to maintain the same bass response you really need to go up from 25uF to 50uF too, since the resistor value has doubled.

    It's also an extremely good idea to fit a screen resistor - another big reason for tube failure is that the plate voltage can easily dip below the screen at high power, since the Champ OT has quite a large primary resistance, causing a big voltage drop if the tube is drawing too much current due to the very hot bias - if that happens, the screen pulls a heavy current which it isn't designed for. I'm guessing that this is probably the case with your amp, since it blows the tube when cranked up. Move the wire from pin 4 of the power tube socket to pin 6, and fit another 1K (preferably at least 2W, wirewound) resistor between pins 4 & 6 - the typical 470-ohm used in other Fender amps is not really enough here, due to the high primary resistance.

    If you do those things, you should have the slightly cleaner-sounding, better-distorting and much more reliable amp that Fender should have made in the first place... you might even want to keep it! :)

    Also, check that the speaker is 3.2-ohm or 4-ohm, not 8-ohm (Weber do in fact make specific 'Champ' speakers, so it may well be right). There's some debate about whether there ever were true '3.2 ohm' speakers (not just a DC measurement of a 4-ohm speaker), but either is fine.
     
  3. gbisn1960

    gbisn1960 Member

    Messages:
    816
    Joined:
    Dec 10, 2002
    Location:
    Baton Rouge, Louisiana
    well.. time to go to work! ill have to see if i can find some parts in my room.. and ill have to print this out... if this doesnt work.. ive thought about sending it to voodoo amps to get it modified into a mini plexi kinda amp.. thanks man
    Jon
     
  4. gbisn1960

    gbisn1960 Member

    Messages:
    816
    Joined:
    Dec 10, 2002
    Location:
    Baton Rouge, Louisiana
    ok.. just one question.. im going to call later and get those parts.. i have 1 of them already (the 2w resistor) anyways.. what i was wondering.. there are two wires going into pin 4 on the power tube socket.. one coming from a set of resistors on the board, and the other coming from a pin on the preamp tube.. which do i move??? hopefully this will work..
    thanks
    Jon
     
  5. John Phillips

    John Phillips Member

    Messages:
    13,080
    Joined:
    Mar 17, 2002
    Location:
    Scotland
    Both of them. When finished, the only connection to pin 4 should be the resistor - pin 6 is just used as a convenient wire-tag, since it's not connected to anything in the tube.

    On some Vibro-Champs, the two wires run from the resistors on the board, one to each tube socket; on others, it goes to the power tube socket then on to the vibrato tube. It doesn't matter from an electrical point of view. (Champs just have one wire.)


    BTW, the reason for all this is, I think, that in about 1965 CBS/Fender decided to increase the power of the BF Champ and Vibro-Champ from 4 to 6W, by using a Princeton PT. This substantially increases the internal voltages in the amp. But they forgot to change the cathode resistor (which is correct for the older, low-voltage circuit) to compensate. There are actually schematics of the time which show self-contradictory voltages in the power supplies of these amps, so obviously they just weren't paying attention.
     
  6. gbisn1960

    gbisn1960 Member

    Messages:
    816
    Joined:
    Dec 10, 2002
    Location:
    Baton Rouge, Louisiana
    awesome.. when i can get the parts ill let you know how it comes out! good info to have! thanks
    Jon
     

Share This Page