Help: Tube Amp Issue!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by bubingaisgod, Mar 3, 2012.

  1. bubingaisgod

    bubingaisgod Supporting Member

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    I don't find myself in a bind like this often, but I installed a quad of NOS Valvo(Hamburg, Germany, Mullard Made) EL84 power tubes in my Crate Vintage Club 50 amplifier(2x12 Version), and everything worked great for a few minutes, but I got greedy and turned the OD channel volume very high, and the instant I did that, the amp cut out completely on both channels. It's never done this before, through several tube changes.

    It powers up, both switches light up, and the fuse is intact,
    I've checked all internal connections for breaks or loose connections, and I've checked all the tubes, which seem to be fine and all light up normally. When you flip the standby switch, you don't get any sound, it's just dead.

    The only thing I haven't done is get out the multi-meter. I'd like some outside opinions on this before I start systematically checking every resistor and cap. My first thought was could it be a screen resistor? Of course I didn't bias the amp, because I've never had to before. I thought this was a cathode bias amp, as are most EL84s.

    Any ideas are appreciated. Thank you!
     
  2. Cyg_1963

    Cyg_1963 Member

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    Put your old EL84's back in?
     
  3. guiltless

    guiltless Member

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    Pull out the Multi-meter and make sure you didn't blow a speaker voice coil. If both speakers are fine, swap in the old power tubes.

    I say check the speakers first because you don't want that amp on too much with no load at the OT.
     
  4. bubingaisgod

    bubingaisgod Supporting Member

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    Just checked speakers, they are 100% fine. At first I thought it was the output transformer, because the amp really is dead silent. But really any combination of components failing could cause silence.

    Just remember the focal points. NOS tubes, turned level up high, amp dies. Amp powers on still, fuse didn't blow, both channels are out, etc.
     
  5. wall_of_sleep

    wall_of_sleep Member

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    Does your amplifier have an HT fuse?

    If the speakers make noise with a 9 volt battery and the amp lights up (tubes included), I would move onto checking that the standby switch is still functional and take the plate voltages of my tubes.
     
  6. bubingaisgod

    bubingaisgod Supporting Member

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    Thank you, I will take readings and post asap.
     
  7. Kurzman

    Kurzman Supporting Member

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  8. bubingaisgod

    bubingaisgod Supporting Member

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    Thats something I didn't know. This amp has a modular design, I think what I'm seeing are diagnostic tap points in certain areas. I'm going to need to pull the board in order to actually access the components.
     
  9. teleman55

    teleman55 Member

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    I had a Crate V32 do just what you described. Turned out the power tubes were red-plating (they're hard to see inside that amp). Replaced the power tubes and all was well.
     
  10. Mrmarshallhead

    Mrmarshallhead Member

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    Have you done what Cyg suggested yet? That would be the first step before pulling the guts out the amp.

    The process is known as swap with known good, and is always carried out right in the area that you last changed before it went bad.
     
  11. bubingaisgod

    bubingaisgod Supporting Member

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    Of course, that is the first thing I did. I understand where your coming from.
     
  12. Jerry Glass

    Jerry Glass Member

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    Your amp has a HT fuse; it's internal and located on the board. It's a 250mA slo blo fuse. Chances are that one of your NOS EL84s has a problem and blew this fuse.
     
  13. guitarcapo

    guitarcapo Senior Member

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    Don't some old EL84 tubes have an internal connection at pin 8 or something?
     
  14. bubingaisgod

    bubingaisgod Supporting Member

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    I've been told on this thread that many newer EL84 amps are pinned differently then the old production tubes. It looked like the HT fuse was intact, but on very close examination, it seems the fuse element has some erratic markings, like divets. I have to go purchase the 250MA ones, most of my amps use 2A slow-blo. If it's just the fuse, that explains why when I put the old tubes back nothing happened.
     
  15. Blue Strat

    Blue Strat Member

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    Check the fuse with your multimeter set for ohms. It does sound like it's shot though.

    Since you got sound from the amp for a while, it may be that the builder made the "mistaken" pin 1 connection on only one or two sockets. Or you have one or more shorted EL84s.

    When you say "NOS" do you mean that they're unused, or used old stock tubes? There are a lot of liberties taken with the designation "NOS".
     
  16. Zerksies

    Zerksies Member

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    Sounds like parasitic OSCULATION
     
  17. big mike

    big mike Administrator Staff Member

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    Worked fine with old tubes, new tubes (New OLD tubes whatever) amp dies.

    I think you have your answer.

    Board mount tubes, ran passed 100% I'd bet. Great amp, but not a candidate for spending NOS money on IMO.
     
  18. bubingaisgod

    bubingaisgod Supporting Member

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    No they weren't real NOS, they were very high rated "like new" tubes. I know what you mean though, it's not often anymore that you find genuine NOS stuff, some people just throw the best looking ones in their respective boxes and sell then as NOS. This person didn't exaggerate them or anything. I went to all over today, and I couldn't find a SINGLE store with 250MA fuses. It's ridiculous.


    Does anyone know of good vintage EL84s that will work in a newer amp?
     
  19. bubingaisgod

    bubingaisgod Supporting Member

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    Issue solved, ran to the ends of the earth to find a 250ma fuse, but the old tubes seem to work fine in there. I guess the old EL84s are pinned differently, or like guitarcapo said, the old amps probably have an internal connection for one of the pins.

    I sold the tubes on ebay in about 2 hours for $20 more then I paid with shipping from Germany too :)
     
  20. big mike

    big mike Administrator Staff Member

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    Hope you didn't just sell someone blown tubes
     

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