Re: 6550's vs 6V6 vs 6L6.....
Usually, you can bias 6550's to work in a 6L6 spec'd amp, even though their more akin to a KT88. The plate voltage of most 6L6 amps is much higher than the 6V6 spec, though you can run new, hearty tubes like the EHx 6V6 (which I'm told isn't really a 6V6) and the brand new JJ 6V6S (supposedly a great tube, btw) at up to 500v.