Biasrite issues

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mmorse, Sep 6, 2008.

  1. mmorse

    mmorse Member

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    The other day I put a pair of KT88s into my 2204. I used a Biasrite to bias them. One side read fine but the other gave a weird negative reading. The tubes are matched fairly well so I went with the one side reading. Played the amp for a couple hours without incident.

    When I went to play the amp the next day, I popped an HT fuse. Sure enough, one of the 88s must have died. So I pulled them and put in a pair of 34s.

    Now my Biasrite is acting weird. I tried to bias the 34s and I get this really bad squeal through the speakers if I turn the bias pot past a certain point. With the Biasrite removed, there is no squeal no matter where the pot is set. Amp plays fine. I just don't know where the bias is on the tubes now. It should be OK as the squeal starts at about 30ma (if I can trust the Biasrite) and 70% is 46ma for 34s with my amp's plate voltage which is on the low side.

    I'm thinking one of the probes may be bad. I don't know. I'm wondering if anyone else has had a similar problem with a Biasrite.
     
  2. wixedmords

    wixedmords Member

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    Easiest way to that I can think to check the validity of the BiasRite is to get the plate voltage manually with the multimeter and then check it against what the BiasRite reads for plate voltage. Using a known good multimeter would also eliminate that from the whole process, a basic troubleshooting method, the elimination of each part in the chain individually.

    To get the plate voltage manually use a multimeter set to be able to read hundreds of volts and get a reading from pin 3 of a tube socket inside of the chassis.

    There have been some issues with the BiasRite, last year if memory serves me correctly. I would test the plate voltage manually, against the BiasRite and then contact Weber if needed. You may also need to clean the contacts inside the testers socket adapters and/or retention the socket adapters.

    Wixed
     
  3. Groovey Records

    Groovey Records Member

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    46 for EL34's is way high I'd say 37 is the upper end depending on plate voltage

    Do you have the Bias rite that reads plate voltage?

    And are you sure your amp is equipped to go from EL 34 to KT 88's or vice versa?

    Lets see what others have to say
     
  4. Blue Strat

    Blue Strat Member

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    To prove your theory about one probe being bad, try one at a time.
     
  5. mmorse

    mmorse Member

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    Well, according to Weber's bias calculator, 70% dissipation for 34s in an AB amp with a PV of 375 is 46.6ma. My amp has a lower PV than 375. More like 360-365. I thought that was too low but found out that is typical for late JMP and early JCM800 2204s.

    I also had an '87 2550 with PVs around 425.

    As far as 34 to 88, I had a tech change the bias resistor so I could use 88s, 34s, and even 6v6s.
     
  6. mmorse

    mmorse Member

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    I think I may have found the problem. No need to further prolong this thread. Thanks to all that responded.
     

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