Firing up new amp: Red Plate!!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by twinrider1, Jan 11, 2008.

  1. twinrider1

    twinrider1 Member

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    Alright, what's happening here.

    Short question: Will a tube red plating for maybe 3O seconds mean it's toast?

    All the details...
    New amp.
    Rockytop 45/50.
    EL34s
    London Power Power Scaling
    45/Plexi switch
    Tube/SS rectifier switch
    NFB switch 8/16

    Set switches to Plexi, SS, NFB to 8, and all knobs to noon.
    Connected speaker cable to left input of amp, and into the 2x12 8ohm cab ((2) 16 ohm speakers in parallel.
    Plugged instrument cable to upper left input of amp, then to tuner, and instrument cable from tuner to guitar.

    Turned amp power on, still in standby. Let it warm up for a good 5 minutes. Tried tuning up, but SS2 wasn't recognizing signal, just sat on C. Thought it might be an old battery. Switched it off, still in the path.

    Okay, I turn the standby to on...and strum a little....no sound.

    I go to the speaker cab and push in on the jack. Sound comes on. The jack was in all the way to begin with, but it can go in an extra 1/32". When you let it go it slide back out that 1/32." I did pull the input plate and the tip maintains contact in both conditions.

    Okay, play for 10-15 seconds and it sounds good.
    I turn the power scaling knob up to 3 o'clock, then back to about 8 o'clock, and then back to 3 o'clock, curious to see what difference it makes (really fizzy at lower power). Anyway now it's at 3 o'clock, I look up and the plate on the right tube is glowing bright. Okay, I'm a tube novice that has read a lot but very little real experience. I stare at the glow for probably 2-3 seconds until I realize, "OH, THAT must be what they mean by red plating!"
    I turn the PS knob back to noon and flip it back to standby.
    I think WTF, WTF, WTF. :jo What just happened.

    Okay, there it is. What do you think?
     
  2. Steve Dallas

    Steve Dallas Supporting Member

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    Hard to diagnose from just a description and no design information on the amp itself. Could be several things.

    Assuming you don't have a damaged tube...

    Tubes usually red plate when they are biased too hot. That can happen if the bias is set improperly or if the grid is shorted to ground on a fixed bias amp or if the bias voltage does not track properly in a power scaled amp.

    I would urge you to get it to an experienced tech to have the problem tracked down. Or if you want to do some safe experimentation, swap the power tubes with each other and see if the problem follows the tube or stays with the socket. If it follows the tube, you might try a fresh set. Maybe one of yours was damaged in shipping. If it stays with the socket, you either have a problem with the amp or the bias is set too hot for one tube if they are not a matched pair.
     
  3. twinrider1

    twinrider1 Member

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    Thanks. There were no funky smells, and no smoke. I'm up for some experimentation and can use a meter/read a schematic. I'll try switching tubes.
     
  4. teleamp

    teleamp Senior Member

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    If its new, why not let the builder take care of it?

    MikeY
     
  5. twinrider1

    twinrider1 Member

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    First, I should verify what red-plating looks like. Am I right thinking it's when the spiral filament in the center glows so brightly that it shows through the plate?

    I calmed down and double checked everything. Took the tuner out of the loop, and gave it another try.
    It seems fine, until I turn the 'power' knob up. I'm pretty certain that's the power scaling knob, but I'll confirm with the builder.
    I have a few other questions, but talk to Jesse first and go from there.

    Thanks.
     
    Last edited: Jul 6, 2008
  6. GearHeadFred

    GearHeadFred Member

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    No. The plate glows red from dissipating too much power. It would be completely separate from light from the filaments.
     
  7. twinrider1

    twinrider1 Member

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    MikeY, you're right. That's what I'll do. This is my first JTM-style amp, plus it has extra bells and whistes that I haven't used before. I've got an itch to try building a simple kit, learn the basics first hand.
     
  8. Blue Strat

    Blue Strat Member

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    One quick thing you could try is swapping the EL34s between the two sockets. Does the problem follow the tube or stay with the socket? If it moves, you probably have a bad tube. If not, there's something wrong internally in the amp.

    Since the plates aren't glowing you could have a tube screen problem which means a bad tube.

    One quick test is to check the bias setting with a bias probe if you have one. Short of this, I'd let the builder handle the situation.
     
  9. twinrider1

    twinrider1 Member

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    Swapped the tubes between sockets. The problem followed the tube.
     
  10. Blue Strat

    Blue Strat Member

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    Probably a bad tube, or it just happens to glow brighter than the other one.

    Checking the bias with a bias probe or having a tech check the amp out would be advisable.
     
  11. twinrider1

    twinrider1 Member

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    I'll pick up some tubes today and give them a try.
    I do want to check the bias to be certain of what's going on in there though. Couldn't I probe the appropriate pin with a volt meter to check the bias?
     
  12. Blue Strat

    Blue Strat Member

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    Not unless the builder provided 1 ohm resistors on pin 8. Otherwise, it's a lot more complicated than that. A bias probe is the easiest and safest solution.
     
  13. twinrider1

    twinrider1 Member

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    Neither appears to be a 1 ohm resistor.

    [​IMG]
     
  14. Blue Strat

    Blue Strat Member

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    Check with the builder. They could be located somewhere else, or not there at all.
     
  15. twinrider1

    twinrider1 Member

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    I'll be calling him tomorrow. He already replied to the email I sent yesterday (wasn't expecting that over the weekend). I'm sure it'll get ironed out.
     
  16. twinrider1

    twinrider1 Member

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    Spoke with Jesse and he is sending out a fresh set of tubes. He was very helpful and happy to go through the various features of the amp. My experience with Rockytop has been outstanding.


    EDIT: 07/06/08 He never sent the tubes.
     
    Last edited: Jul 6, 2008

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