Forums - Bias on Tubes Seems Low

Blueone302

Seasoned Member

Posts: 136

Bias on Tubes Seems Low
12/29/22 at 16:27:18

Good Morning. Hope you've all had a great Holiday season so far. I've got a question on tube bias in the ZMA. Got my amp exactly a year ago. It came with the KT66's for the output tubes. I've noticed when I check the bias now and then that it hovers right at 50ma consistently. However, when I try to increase the bias a tad, I'm unable to go any further. Ive read many posts about folks running their stock tubes at about 60ma, or a bit higher. Just wanted to play with it a bit. I've also tried an old quad of TAD EL34's I had in my office amp. Which by the way sound really really nice. Not better so much as the Tung Sol tubes, but very rich. Great sound! Anyway, I am only able to bias the EL34's at just above 40ma. Neither set of tubes have more than 200 hours on them. Any thoughts on why I can't get the tubes to bias a bit higher? Your experience/input appreciated.

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GroovySauce

Seasoned Member

Posts: 864

Re: Bias on Tubes Seems Low
Reply #1 - 12/30/22 at 21:55:30

I don't have a ZMA. I do recall reading somewhere that Steve can set the bias window higher or lower on the ZMA. I have a little experience with the ZMA. Had two different quads different of EL34s one Chinese and one Russian. The lowest the would bias was 5-10mA different. Didn't check for max.

I found I liked the sound more when they were biased a little lower.

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Blueone302

Seasoned Member

Posts: 136

Re: Bias on Tubes Seems Low
Reply #2 - 12/30/22 at 22:11:43

Thanks GS. I’m very satisfied with the Tung Sol stock tubes. But I just wanted to see if there was any discernible difference with a higher bias. Like you mentioned, I wondered about the meters. This was the first time I’ve even thought about tube rolling with the ZMA. Thought While I had a few days, I would experiment. I’ve got some time tomorrow. Might pop in a quad of the Tube Depot’s Preferred Series KT88’s just to see what I can hear and how the bias goes on those.

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Carlsbad

Seasoned Member

Posts: 202

Re: Bias on Tubes Seems Low
Reply #3 - 01/07/23 at 18:54:10

Bluesone,
I bias my KT66s at 50mA. I've tried them a bit higher and didn't find anything better there. How do we get 50mA as the right bias? The KT66 has a 25Watt max plate dissipation. The ZMA has about 390V plate voltage. 70% of max is a good place to run your tubes. so 25/390 x .7= 44.8mA. 100% of max is 64mA. Clearly the tube will work at up to, and probably over 64 mA, but for long term stability I use 50.

I'll attach a photo of my ZMA. I've put brass extensions on my main bias knobs which are tucked behind the regulator. to run at 50, these knobs are right in the middle of their span. See the indicator pointing about 12o'clock? I just turned off the amp and confirmed it can be turned all the way counterclockwise to 6 o'clock. turn counterclockwise to increase the bias current . are you saying that you have your bias adjustment knobs turned all the way counterclockwise and get only 50 mA current out of your KT66's?

You'll notice in the photo that my bias is set on 40mA. That is because I'm running a set of 6P3S tubes which I bias at 40mA because they have lower plate dissipation. Do yourself a favor and try these tubes. They are cheap on ebay but you'll have to wait 60 days for them to arrive from eastern Europe. See the ZMA tube rolling thread for all the tubes I've tried and ended up with these. However, my second favorite is the tungsol KT66 that you have.

Jerry
https://photos.app.goo.gl/2j4tzU8SXVYT9UaS7

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