JTM 5881

[woolenmammoth]

I just took out a pair of kt66's from a JTM I built, perfectly quiet. Put in a pair of JAN 5881's as this is usually my preferred tube, Im getting a low level hum that is acceptable, but wasnt there at all with the 66's. Put in a set of 6l6 STR's and it was quiet again. Back to the 5881 and the noise returns. Is there something internal to the 5881 that makes it more delicate to hum reception? Id be happy recording this amp as is with the 5881, its not high level 60 hz hum or anything, but it is present where it wasnt with a different tube design. Anyone ever experience anything like this?

[EFK]

How well matched are they for bias current? There is nothing inherent to a 5881 that would promote greater hum, as far as I know. I've certainly never experienced anything like that with either new or old-stock tubes. However, if they are spread a few mA apart or more, noise that would typically be canceled in the output section can begin to creep back in. If your KT66 and 6L6 are better matched, perhaps this is what you're hearing.

[woolenmammoth]

I thought this as well, the two pairs I have been putting in there match tighter than the other 66 and 6l6. odd, huh?

[EFK]

Well then, that is definitely very odd. Never heard of this previously with 5881, anywhere!