1. steve10358

    steve10358 Supporting Member

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    I think my bias caps have died. I popped a Main fuse on the 77 marshall I have, and the tubes got destroyed (glow red).
    How do I check this??

    S.
     
  2. Swarty

    Swarty Member

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    You need to monitor the grid voltage while monitoring the current draw. If the grid voltage starts to drop it is most likely the bias cap(s). If the current draw increases and grid votage does not drop then you are most likely dealing with bum tubes. You could just say the heck with troubleshooting and replace the caps and see if the problem is resolved.
     
  3. John Phillips

    John Phillips Member

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    If all the tubes glowed red it is much more likely to be a bias supply failure rather than a bad tube (which would tend to make only half of them glow, on the side with a shorted tube). The first thing to do before replacing the caps is to meter round the bias circuit for voltage with the power tubes out; it could also possibly be a failed supply diode or resistor. Find the cause before doing any work since you have to lift the board to change the parts, and that way you'll only have to do it once... and if it's a shorted diode - rare but it can happen - it will destroy the new caps you fit them without replacing the diode. But I agree with Swarty that it's worth changing the caps if you have to go under the board for anything, since they're over 25 years old now and will need doing sooner or later anyway.
     
  4. donnyjaguar

    donnyjaguar Member

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    If BOTH tubes glowed red then it implies that your bias circuit is defective. Or does it...

    What can happen (and this happened to me 2 nights ago, btw) is one of the output tubes fails and shorts out the bias circuit to ground (in my case via the cathode AND filament) and in-so doing drops the bias to the 2nd tube and causes a meltdown in it too. By the grace of the almighty electrons my fuse blew before the 2nd tube was cooked.

    I'd power up the amplifier sans output tubes and measure the bias voltage. If its okay, re-tube and rock on. I've found that some tubes that have glowed red will still work but the transconductance is often way down.

    DJ
     
  5. steve10358

    steve10358 Supporting Member

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    Well, the glowing happened with two sets of tubes. One brand new out of the box. At first, just the inner one was glowing. Then when I pulled the two inside, the two outter were bright red.

    1. Is bias voltage the same as plate voltage, or are you refering to at the tube. I use a probe to bias the tubes.

    2. When I did this, I was getting some wierd numbers. The amp is supposed to be at 30mA, and one tube was 53, one was 35 and so on. This was with a new HT fuse... before the main popped.

    3. How would I know if it was a failed component?? What do I do with my DMM?

    S.
     
  6. scottosan

    scottosan Supporting Member

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    If they are original, change them out. The same goes for any other electrolytics, if it has been a while.
     
  7. John Phillips

    John Phillips Member

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    This actually doesn't sound like a trivial problem... there's some odd stuff going on here. If it did it with two sets of tubes, it probably isn't a tube problem, but a general bias supply failure should not result in the tubes running away at different rates - especially not being much worse with two tubes than four. Bias voltage is not the same as plate voltage, or the voltage you read for setting current, it's the actual voltage the bias is supplied at - usually about -30 to -45V for EL34s. You can measure it directly on pin 5 of the tube sockets, or at the top end of the resistor that is connected to the bias trimmer. These will be slightly different due to the bias feed resistor value, and any difference in the voltage at the tube sockets may help to indicate where the problem is.

    If this sounds like Greek, you may be better taking it to a tech. It could possibly be something nasty like a short in a tube socket, which to accurately identify will mean metering directly inside the amp with the full voltages running. Hope that doesn't sound too pessimistic - whatever it is, it's fixable. Don't start changing parts without finding the problem first, if you can avoid it.
     
  8. steve10358

    steve10358 Supporting Member

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    Cool John. I do have mine set at 30mA. I guess my main thing was seeing if I had a bad solder or whatever, but I guess I'll just drop it off at the tech. :(
     

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