I have a bit of a mystery with my "Mystery Amp" that maybe another ZMA owner can shed light on.

I just put a quad of Gold Lion KT66s back in my Anniversary modded ZMA after maybe 5 years. These were lightly used. The last time I had them in was around the time I first got my ZMA. At that time I was biasing, as per Steve's recommendation, at 50 mA. Last night, these same tubes wouldn't go below 64 to 68 mA (left vs right). I ended up biasing at 70mA.

Can anyone account for this? Since they were last in, the amp has new meters and the full complement of anniversary mods.

My meters go to zero with the power off. I use a ZLC and even at the higher bias I don't think my transformers get too hot -- I can keep my hands on them for a long time.

It's possible my memory is faulty but back then we weren't biasing high.  Jeff sort of started that trend with his fore into KT88s and other tubes.

I am wondering if the Anniversary mods might have changed something.

But since your GL tubes bias high, I could be wrong.  Stone-of-Tone used the GLs as well.  Unfortunately, he hasn't posted here in a couple years.

I did some searching of old posts and I found this in one of mine from December 2014:

"One of my stock Tung-Sol KT66 tubes was getting funky so I bought a matched quad of Gold Lion KT66s from Cryoset.  I don't have the aural memory or the language to describe what I hear but I'd say they are an improvement over the Tung-Sols.  One thing is that I can't bias them below 58+ ma."

So it seems my memory isn't what I thought it was and that the GLs always biased high.  It is interesting though that the lower limit of bias is considerably higher post Anniversary mods.

And I pulled my GL KT66s out this morning!  I prefer the Tung-Sol 7581As that I have in as well as the 7027As.  I figured I'd give the KT66s another shot since it'd been so long since I'd heard them.  The 7581As have a lower bias window than the GL KT66s by a lot.  I tend to bias them on the high side at 64mA.  

Thanks Steve.  Can you account for the high minimum bias of the Gold Lion KT66 as compared to, say, the Tung-Sol KT66?  I'm sure the ones you originally sent with the amp biased below 50 mA.  I was under the impression that the bias window of a tube had something to do with it's power output.

When setting bias, as far as amp health goes, is there any other consideration besides transformer heat?

Following up on the GL KT66's I haven't been able to bias them below 60ma for a long time now. I'm going to run them a little hotter now and see how they fair. They are starting to drift a little so they might be coming to the end of their lifespan.