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In the Forum: Audio Discussions
In the Thread: Initial thoughts about new/old Lamm ML2s

Post Subject: Bias current; rise during warm upPosted by jessie.dazzle on: 3/8/2010

After swapping all the tubes between the left and right amps, the cycling bias current issue (mentioned here in a previous post) seems to have settled.

I now have another question, which I hope other ML2 users might answer; first the context:

These amps came with two complete sets of tubes; one that had been in use, and one that the seller claimed were new, purchased from Lamm. The tubes did appear to be new (I have the equipment to test them, but have not yet moved it out of storage). For the past 6 months, I've been running the amps with these "new" tubes.

When the amps have been in use all day, I can set the bias current for .31 and everything is fine. The following day, I switch the amps on, and within 15 minutes, the current will have climbed to well above .31 (I just measured .385 for example), and returns to .31 only after the amps have been on for over 2 hours. The climb in current during warm up is quite a bit greater with one amp as compared to the other, but I suspect this to be due to the individual behavior of each V2 tube.

With the "old" tubes installed, I don't remember having to fuss this much with bias current, but it is possible that I've become a bit psychotic since then.

Can anyone using ML2s tell me if their amps behave similarly; that is, do they rise well above .31 during warm up, and if so, is the rise in one V2 as compared to the other approximately equal?

You might be saying to yourself, "why doesn't he answer his own question by simply reinstalling the old set of tubes?". Installation of the "new" set was prompted by a failed V2 tube. I assume that the other old V2 is pretty close to dead, and I don't have an alternate set on hand.

Thanks in advance,

jd*

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