biasing an IOD?

Discussion in 'Amps and Cabs [BG]' started by jasonbraatz, Dec 10, 2001.

  1. i really want to learn how to do this myself. i had to replace the tubes in my IOD right away, and i didn't have them rebiased because i don't have the $$$ and the place that does warranty work is a few hours away. do i do it? what tools do i need?

  2. mthoople


    Nov 1, 2001
    Denver, CO

    I replaced the tubes in mine as soon as I got it as well. I biased it using the crossover distortion method. This requires a scope ,a voltage meter, and a constant signal source (I used my Yamaha DX-11). It took about 30 minutes after the tubes were allowed to warm up completely. If you want specific instructions, let me know.

    What tubes did you put in?

  3. Tapp


    Aug 29, 2001
    USA, Mississippi
    Jason, I bias my tube amps (including the ISO) using the transformer shunt method. I will preface by saying that if you don't know what you're doing you can hurt yourself doing this so knowing the exact place to put your test leads and using clip leads for your meter are ESSENTIAL.

    There's a lot of debate about various biasing methods but I find (and many other knowledgeable techs I know) that the tranny-shunt method is a more accurate way of biasing output tubes. The crossover method is fine yet it tells you nothing about the static current flowing thru the plate and this is what you need to know to calculate static plate dissipation. One drawback is if you have a meter that has high shunt resistance you can bias your tubes "hotter" than what is reading on the meter. If you bias at around 50-60% dissipation then this is really not a big deal.

    Another very safe and accurate method is installing 1ohm 1% resistors from cathode to ground of each EL84 and measuring voltage across them which corresponds to the plate current. This does not take into account the screen current yet it's not much of an issue with this preamp IMO.

    If you've got a good meter that reads current and have clip leads I can step you thru.

  4. ok, i'm going to my parents house tonight to borrow my dad's meter - what does it have to be able to read? volts?

    i know i've got clip leads where do i put 'em? i haven't popped the thing open yet, so i don't really know where things are right now.

    i put in groove tubes :( ONLY because they the ei's they shipped with were crackly and microphonic. after i figure out how to bias, i'll get a set of Ei EL84's and a 12ax7 from lord valve so i know they'll be good.

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