845 Bias Problem - Advice Needed

fredt300b

Active Member
I bought replacement power tubes for my ASL 845 monoblocks and I'm having a bias problem with one of them. The recommended bias setting is 850ma. The old tubes which I am replacing were at 719ma and 623ma with the bias pot at its minimum setting. The new ones are at 719 (same as the old tube) and 1,020ma (versus 623 for the old one). These tubes came from the Tube Depot, and they do not sell matched pairs of 845's.

Would someone who is more familiar with SET amps than I am tell me whether I got a tube that is far out of spec and should be returned for a replacement, or should I consider changing a resistor value in the amp to move it into the recommended range? The ASL Model AQ-1006 schematic is posted at http://www.tubehifi.com/Circuit/C_106_845.jpg
 
Forgot to mention, I talked with someone at the Tube Depot and he suggested I run the tube a 1,000+ and see if it gets too hot. Somehow, this advice did not leave me with a good feeling. How hot is too hot for an 845, since you can toast marshmallows at normal bias? And what effect would this have on the tube life?
 
False Alarm! Looks like I should have waited a while. When I reinserted the new tubes and tried them again the bias was within normal range. Looks like some virgins are more reluctant than others. So I'll run them for a few hours and see what happens, but I believe they're ok.
 
Fred, glad you're getting it worked out. Just a heads up: the actual plate current must be 85 ma, not 850... perhaps you are giving the millivolt value measured to determine current. Even the sturdy 845 isn't going to be happy at 1000ma x 1000vdc = 1000 watts plate dissipation! :) Your operating point is something roughly on the order of 85 ma x 1000 vdc = 85 watts dissipation.

Have fun,
Tubino
 
Fred,

Just keep a close eye for the first 100 hours. New tubes really run wild until they get burned in. Tubino got you covered on the mA deal. when I read 850mA I said WHAT !!!!! It would take a power transformer the size of a Semi to power them babies at 1000V LOL !! Hearing the 845 gave me a glimmer of hope for the SET topology. Tubino did you get a chance to give a listen to the Almorro 6C33-b amplifier in Grumpy/Ray's room at Akfest? Interesting tube!

Enjoy Craig
 
NOSValves said:
Tubino did you get a chance to give a listen to the Almorro 6C33-b amplifier in Grumpy/Ray's room at Akfest? Interesting tube!
Nope, I regret that I didn't spend nearly enough time listening to that one and some others...

But yes, it's an interesting tube. Isn't it a low-mu high-current dual triode like the 6336, designed for use as a pass tube? I've heard several amps that use it, but mostly PP. I would guess that the OPT primary impedance doesn't need to be very high in any case.
 
NOSValves said:
Fred,

Just keep a close eye for the first 100 hours. New tubes really run wild until they get burned in. Tubino got you covered on the mA deal. when I read 850mA I said WHAT !!!!! It would take a power transformer the size of a Semi to power them babies at 1000V LOL !! Hearing the 845 gave me a glimmer of hope for the SET topology. Tubino did you get a chance to give a listen to the Almorro 6C33-b amplifier in Grumpy/Ray's room at Akfest? Interesting tube!

Enjoy Craig

What? Is that why the plates glow red before they self destruct? Actually, I meant 85mA, but the meter reads 850 when the bias is set correctly, so I was thinking 850 instead of 85. I did figure out what the problem was. The bias pot on one amp has a "catch" somewhere along its travel, and when I felt this detent I thought I had turned it down all the way. When I swapped tubes again I did, and the bias was right where I expected it to be.
 
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