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Thread: Hiwatt hi-gain 50 head

  1. #1
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    Hiwatt hi-gain 50 head

    just bought a hiwatt hi-gain 50 head equiped with 2 EHarmonics EL34 tubes wich have i stickers (A 35) and (A 35,5) think mA. i think hi-gain series have fixed bias and on hiwatt manual mention just to use matched pair of tubes when replace nothing more ... does anyone could help telling me if i just need a perfect much pair of EL34 tubes or i have to do something to keep the 35 and 35,5 mA at the same range as the manufacturers tubes??? i meen need rebiasing the amp? thanks michael

  2. #2
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    Hello there ... Well, EL 34's can be operated at any current between 30 and 100 mA (35 mA is typical idle current in Class B), and then again the difference between the two, in a guitar/bass amp can be up to 5%, perhaps 10% without any bad consequences. Just use a pair of tubes of the same make and age - that should do it.

    If you can measure idle currents with new tubes, and if they are not far away from 35 mA - just don't touch it. If they are much higher (over 40 mA) increase negative grid voltage (all amps have some means of doing it). In fact, operating your tubes at higher idle currents will to some extent shorten their lifetime.

    The other way around, if idle currents are below 25 mA, you might want increase them slightly - but if you're happy with how it sounds - why should you?

    A slight debalance in tube gains and idle currents might in fact "sweeten" the sound a little bit, as perfectly matched tubes almost perfectly cancel even harmonics, and umatched don't.

  3. #3
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    i think that if i would like to keep the amp in the builders tone, i mean factory tone if i like it i have just to keep with every new pair of matched tubes the same current builder had. means between 35 mA + - 5%
    is it right?

    thanks mate

  4. #4
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    if those numbers represent ma bias current then they're within 1.5% of each other- I'd called that a good match!

    Without getting into the more technical stuff about how to bias a given amp...does it sound good? If it does then I wouldn't worry about it!

    jamie

  5. #5
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    hi i bought a new perfect matches pair of el34 mullard reissue , so i examine my new hiwatt hi-gain 50 head before replacing tubes ,so on factory tubes stickers says 35,5 mA and the other 35 mA ,so i started i put the biasing tool and found that the head was biased at 24 mA !!!! i changed the tubes and placed the mullard i biased at 35 mA and really the mullard pair was perfect 35-35 perfect biasing..BUT when i put the jack to listen the head was very low output listening !!! i thought it was a big damage but ..when i replaced one by one the pream tubes the 3rd 12ax7 before the 4th 12at7 reverce phase tube was out of duty !!! when i replaced it with a new one the head plays perfectly very loud than before with the 24 mA biasing !!! god the factory says 35ma and bias at 24 poor service or cause of the head was DEMO and not new and due it was played after sometime need a new biasing ?? !!! but my question is why on the way to bias the head the 3rd tube destroyed ??? the tubes were all new !!! thanks mate i hope you have somethink on mind . Forgot to say but i had readen that is not good but i made the mistake to placed the jack to listen with probes ON during biasing !! maybe this destroy the tube?
    Last edited by carpediemgr; 03-25-2010 at 10:42 PM.

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