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I bias my amps with a mA bias tool socket that plugs into my mA (volt/ohm) meter. If I begin biasing around the EL-34 36mA range, then the range of the bias pot slowly starts shifting upwards, even though I can't get it lower than 52mA, what is happening? I'm not visibly frying my tubes, but after 15 minutes of messing with it, the bias range seems to read 48mA - 70mA. Is there an accumulated charge on the plates, or should I let them cool, then try again?


Originally Posted by GearjoneserI bias my amps with a mA bias tool socket that plugs into my mA (volt/ohm) meter. If I begin biasing around the EL-34 36mA range, then the range of the bias pot slowly starts shifting upwards, even though I can't get it lower than 52mA, what is happening? I'm not visibly frying my tubes, but after 15 minutes of messing with it, the bias range seems to read 48mA - 70mA. Is there an accumulated charge on the plates, or should I let them cool, then try again?

Everytime you do an adjustment, leave it so it settles down. Dont turn the amp off, just turn your meter off and give it couple of minutes. Then do another adjustment, etc..

that happenes when I bias my tube poweramp. it sorta jumps in values even if i am not turning the trimpot inside. i just turn my meter off and leave it for couple of minutes and then turn the meter on for another reading.hope that helps!

I think what's happening is that the tube is getting too hot, and distorting the reading, even when you bias it colder. I turned the amp off for half an hour, then turned it back on, and it was back within the 34mA range. I put it at 35mA, which is what the Bogner manual suggests, and left it.

Owning a $15 bias tool and a voltmeter makes biasing fairly easy, as long as you know how to handle a live and exposed chassis. I definitely use a mini-screwdriver with a plastic handle.

I'd try another pair of tubes. I found a old pair of RCA short bottle 6L6's that I had for ever. I put em' in my HRD. The amp sounded good, No visable sign of the tubes baking? But when I took a Bias reading I was drawing 90 Mv with the trim pot all the way off?? WTF ! I put another pair of 6L6's in... They when right into range. Can't offer you a technical reason why? But maybe some tubes go bad like that?
Edit : I just remember. My bias probe dosen't work on my Laney for some reason? I get screwy readings all over the place. I soldered a 2watt 1ohm resistor from pin 8 to ground and I use aligator clip's on the resistor to my meter set on the DCV side. And it's right on the money. Google Lord Valve amp; bias he has a lot of info.

Thanks for the info KMC. I actually haven't changed the tubes in a year on my Ecstasy head, but had the feeling they were still perfectly good. I just opened it to rebias it. I probably answered my own question, by shutting off the amp for awhile, then starting again. I guess when you tweak the bias too much while it's running, you may heat up the tubes, cause them to run hot regardless of where the bias is set. If you turn of the amp and start again when it's cold, that seems to be the best time to get an accurate reading. I may do that from now on.......leaving it cold, and checking one last time, before reassembling the amp/headbox.

Much difference between using a bias tool and a meter?

I just biased my JCM2000 with the multimeter and it couldn't have been easier.

That Marshall has the bias points on the back, which I wish all amps did.
On mine, i've got to use the little adapter socket, which runs into my mA meter.
I've got to tilt the amp up on one end, read the meter, strum the guitar, and watch the numbers as the amp sounds it's best. I like doing a lot of it by ear, so the amp sounds as good as it can.

from : localhost/bias calculator page here....