power tube/bias/mismatched tube question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by johnmfer, Mar 3, 2005.


  1. johnmfer

    johnmfer Supporting Member

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    The situation: I have an Egnater TOL50. It's my baby. Two weekends ago I did a show at a video game store, no stage, PA only for vocals, and ended up having to turn the amp all the way up to get the volume out of it I needed (the place was so full, the crowd was bumping into me as I played). I've never turned it up that loud before. The next day I had practice with my other band and after two songs I blew the mains fuse. I replaced the mains fuse and checked out the power tubes. One of them was not glowing blue like normal and had burn marks on the insides of the glass. I replaced it with a spare EL34 I had laying around. I don't own a multimeter to bias the amp, nor do I much like poking around in amps since electrocuting myself inside one a couple years ago.

    The problem: One band is starting to record a demo today and will be recording the next four days. I have at least 8 or 9 shows in the next two months including a SXSW gig and cannot leave the amp at the shop for two weeks to get it biased and checked out. I have a backup (a 50w JCM800) but it doesn't sound anywhere near as good.

    The question: Will it be okay until I can get a break in my schedule to take it into my tech? Obviously the tubes are no longer a matched set, and I don't want to cause more damage.

    EDIT: Sorry for the long sob story!
     
  2. drbob1

    drbob1 Silver Supporting Member

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    Biasing an amp like that should be a 5 minute job. I'd get a decent set of new tubes sent out second day from Lord Valve, then take the amp to your tech and ask him to bias it while you wait (if you're really kind remove the chassis so he doesn't have to do anything but hook up his meter). Or, since bias isn't too hard, get the tubes and a Bias Rite from Weber (should take about 10 days to get there) and bias it yourself. If the tube plates aren't glowing red, you probably don't have to worry about the mismatched tubes hurting anything...
     
  3. johnmfer

    johnmfer Supporting Member

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    Thank you! Nothing is glowing red at this point. Everything looks normal with the "new" tube. I'll pick up a new set and see if my tech might be free sometime soon.
     
  4. Blue Strat

    Blue Strat Member

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    Note that power tubes don't tend to glow red until you hit close to 200% of maximum power dissipation (100% is considered a design limit).

    The "red plate bias method" has to be a top 10 hack guitar tech tidbit.
     
  5. bruce egnater

    bruce egnater Member

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    You should replace the pair of power tubes. Tell your tech there are one ohm resistors in the cathodes of the output tubes so a bias proble (or similar device) should not be used. Simply locate the resistors (not too difficult for any decent tech) and measure the DC voltage across each resistor.
    Adjust the bias trim pot for ~~35mV across each resistor.

    Note: Often when a tube blows in those amps, the one ohm resistor will open. This will also be obvious to your tech.

    Egnater
     
  6. johnmfer

    johnmfer Supporting Member

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    Thanks to everyone for replying in this thread. This weekend I was running errands, and happened to find myself driving past Maple Ave where Antique Electronic Supply is located, and it just happened to be the second saturday of the month, the only Saturday they're open for counter sales. I bought some new power tubes and swung by the electronics store where I bought an analog multimeter for $12. The cheapest digital one they had was $30 or so, and I'm trying to keep to a budget.

    I once watched my tech bias the amp so I knew where the 1 ohm resistors were. He complimented your design, Bruce, said it was very intelligent. I set the multimeter to the "0.5 DC mV" range (the other options were 10 mV and 250 mV) and adjusted the bias to 3.5 on the scale, it was at 2 or so when I took the initial reading. Does this sound right? I had to guess a little, I've never used a multimeter and it being a cheapy the manual didn't really explain anything.

    EDIT: I went with the 0.5 setting because at 10 the initial reading it was so low that I would have had to add quite a bit to it to bring it up to 3.5. I also figured at the 0.5 setting I'd still be biasing it cold if I was setting it wrong and wouldn't risk blowing anything up.
     
  7. bruce egnater

    bruce egnater Member

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    Not sure if you did this correctly or not. Please reread your post and correct as needed. Be sure about what scale you used, etc etc etc. and be accurate with the results. "0.5 DC mV" does nto make sense.

    Egnater
     
  8. johnmfer

    johnmfer Supporting Member

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    Hahaha, sorry! Whoops! Read mA where you see mV. I'm new to all this and made a layman's error.

    http://www.extechproducts.com/products/extech/38073.pdf

    This link is an Adobe PDF file with a closeup of the multimeter I bought. I had it set to the 0.5 DC mA setting on the dial, then using the bottom row of black numbers (0, 2, 4 etc.) I set the bias to 3.5. Like I said it's a cheapy multimeter and I only plan to use it for biasing the amp, and I'm guessing on how to use it.

    Thanks again for your help!
     

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