What should I check before powering up my amp?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Normster, Jul 31, 2005.


  1. Normster

    Normster Member

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    I'm about to connect the last few wires on my dumble clone and would appreciate any advice on safely powering up for the first time.

    From what I've read, the first thing is to plug it in and check plate voltages before installing tubes. I don't have a Variac so I'm going to use a light bulb in series to (hopefully) protect the trannys. Does the wattage matter? (100 watt?) How much will the bulb effect plate voltage?

    Also, are there any resistance checks I should make before applying power?

    Any chance of ruining the speaker if I use it instead of a dummy load? (If so, I have a "sacrificial" speaker I can use.)

    Any and all advice offered will be heeded. ;)

    Thanks,
    Norm
     
  2. Blue Strat

    Blue Strat Member

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    It's a little late to be mentioning this, but it's best to apply power after the power supply is complete rather than building the whole amp before applying power.

    EVERYONE (including seasoned builders) makes mistakes and it's much easier to track them down by testing the amp a section at a time.

    Don't install power tubes or connect your speaker until you've run a check on plate voltages. They'll be about 5 to 10% higher without power tubes installed and properly biased, but you'll know if there's anything seriously wrong.

    After you're confident that the plate voltages are in range, check to be sure that your grid voltage is getting to pin 5 on the power tube sockets and that your bias control varies this voltage over an appropriate range. Depending on which power tubes you're using, the grid voltage should be somewhere in the -36 (EL34s) to -50 (6L6s) volt range.

    Once those things are right, you can install your power tubes and bias them. It's best to use a dummy load at first. If you don't have one, it's ok to bias the tubes with no load. The OT doesn't care as long as there's no signal being passed to it.

    Make sure the power tubes bias properly.

    If you have a dummy load and scope, be sure that the amp isn't oscillating (at big possibility with a relatively high gain amp like a Dumble clone). If you don't have a scope, connect your expendible speaker and see what happens.

    Good luck!
     
  3. Normster

    Normster Member

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    Thanks for the run-down. I did check the power supply board and everything seems to be right, albeit a little low. (I kept the Pignose tranny and power board as-is.) Looks like I only have about -40 available bias voltage.

    On power-up I couldn't get the bias adjusted so I'm about to recheck all connections. The tubes got pretty hot right away so I'm sure I screwed something up.

    Thanks again for the tips!
     
  4. TheAmpNerd

    TheAmpNerd Member

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    1+ What mike said.


    -40, what is your plate voltage on them tubes?
    Actually it might not be that bad, you are close
    and it shouldn't fry good sturdy tubes.

    Keep in mind those pignose power trannies
    are just about the cheapest tihing I've seen
    in amps. You sould think about upgrading to
    one of those Canadian jobbies....
    (can't remember the name)..Hammond.

    I've replaced plenty of them in that small
    amp they make. I've had a few guys run those
    amp though 4 x 12 cabs.

    Have fun, keep us informed.
     
  5. Normster

    Normster Member

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    OK, it's working! :D

    Only problem I had was a bad solder joint to the preamp heater filaments, but it sure gave me a scare when it didn't make a sound.

    So here's the latest situation...I copied a dumble circuit as closely as I could and it came out sounding like a marshall! :confused:

    It's very fat and crunchy and will go into full-tilt distortion, but I haven't found out how to get that Ford tone yet. I'm so confused. I only had a few minutes to try it out so maybe tomorrow I'll have better luck tweaking knobs. I guess at this point I should be grateful it works at all. LOL
     
  6. Blue Strat

    Blue Strat Member

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    What schematic did you use?

    The only real Dumble I've played through was a "skyline tonestack" model. Out of about 100 different tones you could get, there were 99 mediocre or bad ones and one great one. Took a long time to dial it in.

    I've played through Two Rock and Fuchs clones which had about 70% good tones versus 30% crappy ones. These were much easier to dial in than the real Dumble mentioned above.
     

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