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Bias Settings for Air Tight ATM-2 with TS 6550s

I have an Air Tight ATM-2 power amp which I bought used about six weeks ago on A'goN and want to roll some old stock Tung Sol 6550s (greyplates/no hole) into it.

About a month ago, I tried a quad of used 50s black plate 6550s when I first got the amp, but I didn't know then about slowly settling the tubes in--gradually increasing the bias over 4-5 hours. I lost a tube and a resistor in the amp, which I got replaced. I ran it with new re-issue TS 6550s for a month, to good effect, then tried some SED KT88s this past week, which were much warmer and mellower, though not as clear or focused or extended in the trebles. Now, I want to try some 60s TS 6550s which I bought used (about 100 hours) but they test very strong.

For about two hours now, I've been running the old stock greyplate TS 6550s at 1/5th the recommended bias (which is the low end of a "bubble" on the bias meter on the face of the Air Tight). At 1/5th the bias, four line increments below the bubble, the amp sounds terrific--big, mighty, mellow, fast, and with a warm string tone to orchestral music and good treble extension for choral music.

My question is this: why increase the bias to the lower end of the bubble (the recommended setting as I was told by The Analog Room) if the amp sounds good at the low setting about 1/5th of the way to the bubble? Am I risking any damage to the amp's transformers if I keep it at this low setting?

The only reason I consider this, by the way, is to shy away from the high bias settings which might stress these old tubes. The Air Tight is a high current amp and I've already had one old stock 6550 tube run away on me and blow. I don't want a recurrence.

Does anybody have any experience with this sort of thing? Please advise.
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Showing 2 responses by bblilikoi

bblilikoi's avatar
bblilikoi OP
99 posts
 
Mahalo Roesch,

I had the bias settings to the recommended left end of the bubble (as suggested by Scott at the Analog Room) too. But I found that old stock Tung Sol 6550s, both greyplate and blackplate (no holes), sounded better biased lower.

New issue 6550s and KT88s I've only tried at the recommended bias settings, but I will try a bit lower setting to see if that changes the sound for the better. Thank you for the suggestion and even more for the report of improved results.

I too have experienced that the Air Tight ATM-2 can sound "lean and pitched-up tonally," as you describe it, though this varies from CD to CD. I've consequently gone away from silver ICs (Audioquest) and back to copper (Verbatim and Cardas).

Where do you set the bias? I've just tried lowering it by two markers and noticed resolution in the treble range is improved, but only slightly. I'm using SED KT88s now.
bblilikoi's avatar
bblilikoi OP
99 posts
 
Aloha Roesch,

Following up, I can report that lowering the bias settings by two marks has improved resolution in the massed trebles of choirs--the music I listen to and which was the main reason I got the Air Tight in the first place. It's still not perfect, but it's better.

I don't know what electronics are involved--do you? With the lower bias settings, I mean. Less current? I thought that would result in poorer performance?

You said lowering bias won't hurt the amp, but what does it control? Current or plate voltage or both? Do you or does your friend know? I'm sure this is very basic to cognoscenti, but I'm a novice.
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