Re: Tube question...?
When changing out tube types there are some things to pay attention to, beyong biasing and pin configurations.
One important matter is the current consumed by the heaters. Some tubes consume more, and this can put a strain on the power transformer. For example a 6L6/5881 consumes 0.9 amps, but a KT66 consumes 1.3 amps. Nearly a third more per tube.
Some ratings:
6L6 - 0.9a
KT66 - 1.3a
KT88 - 1.6a
6550 - 1.6a
EL34/6CA7 - 1.4a
The EL34 and the KT66 are rather close, so going to KT66 from EL34, or from KT66 to EL34, shouldn't present to much of a problem. However, installing a KT66 into an American 6L6 amp may be problematic, despite the fact that the KT66 and 6L6 are similar tubes, and usually listed as a direct replacement. However, another consideration is the impedance matching with output transformer primary.
Despite what tube manuals may list as plate to plate resistance specs, it kind of depends on how the circut operates and the voltages. Generally, the EL34 types and the KT88/6550 types have similar impedance specs in the same circut. That's why we see Marshalls that run EL34's usually switched over to KT88/6550 instead of other types. On the other hand, the KT66 will want optimately, to be matched to a different output transformer than one designed for EL34, 6550, or 5881/6L6. Many people don't know this, but the Marshall plexi amps that ran KT66 tubes, prior to mid to late 1967, had KT66 specific transformers. After the the switch over to EL34, they used slightly different transformers optimized to the EL34.
The original JTM45 is another interesting case. While very similar to the Fender bassman, they didn't use anything like the Fender Bassman transformers. This is one reason why the JTM45 has a unique sound that is neither modern Marshall or Fendery. It close to vintage Marshall but a little rounder. The actual OPT used on the originals delivers quite an impedance mismatch with standard 5881 or EL34 tubes. It was probably all an accident, but that was probably one of the things that made the original JTM45 rather unique.
and i also think that no one here on my country can... that´s another bummer situation for me and my amp, i think that if i go to the store i bought it from they can´t either and that´s what you called a really bad luck. i rather learn myself the biased thing and change them myself, coz that´s what happend to my pups when i bought them, no one could change them and now i can 