Metropoulos Forum

Are these tube really so temperamental?
I was playing pretty much on 10 and i think I kicked my my wah and poof
it blew a fuse and hummed really bad
I checked it out and put a new fuse in and some 6L6's
and off to the races again,
runs fine even if it doesn't quite as sweet as the KT's
may my set was just bad?
any thoughts ?

I think KT66 draw more current than 6L6, so assuming your Power Transformer can handle KT66 current needs, I think a re-bias is all that is needed.

Mike

New production tubes in general are far less rugged than old stock tubes. New out of the box is no guarantee of a tube lasting more than a few hours anymore, or even working right off the bat.........

The flip side is that you can buy ten to twenty pairs of new KT66's for the price of one good pair of old KT66's.

I guess that's true of most things these days...
I'll get a new pair and hope I get lucky

watch for redplating, esp. if you're running high voltages

Roe wrote:watch for redplating, esp. if you're running high voltages

I have a just finished JTM45 metro kit
I don't think I'm running the amp in any unusual way
it just got a spike probably due to the antique house wiring I have

Janglin_Jack wrote:I think KT66 draw more current than 6L6, so assuming your Power Transformer can handle KT66 current needs, I think a re-bias is all that is needed.

Mike

AIR, the plate characteristics of the 6L6 are closer to that of EL34's than KT66's, and so the impedance selector needs to be set one 'click' higher than that used with KT66's. That is, with an 8-ohm speaker load the impedance selector would be at 8 ohms with KT66's, but set to 16 ohms with 6L6's or EL34's. Maybe somone else can validate this, but that is also what my OT spec shows referencing the impedance value of each secondary winding taps.

no need to mismathch the impedance unless your running very high voltages.

I was reading that if you go by the tube spec sheet, that a kt66 matches up with a standard marshall OT, no mismatch impedances or anything.

Now I'm no EE, but I have KT66's in my 1987 clone. Biased to about 37ma, I was noticing they would get real hot after playing for a few hours, with a mismatched impedance (8 ohm cab, set amp at 4 ohm). My headbox would get very warm.
I set the amp to the correct impedance, and it runs much cooler.