KT88 and 6550

[Firestorm]

The KT88 (REAL NOS) can handle 800DC on the plates; the GE6550A specsheet says 600DC plate. Also, the KT88 is limited to LESS screen voltage than the 6550...compare the specsheets. The '88 also uses a little more filament current than the 6550. They are a close enough match to use the same OT. Hope this helps
tubetek

I think it's the other way 'round: 6550A takes max screen volts of 440; KT88 can run the screens at 600V. In my experience, 6550s WILL die from high screen voltage. Also, these tubes (both of the) "technically" need a separate bias supply for each tube. Also true, in my experience.

[Cygnus X1]

The ampeg V4 that I bought these for literally blew up on my bench.
I was scared enough that I completely parted out the project,
and held on to the 6550's that were intended for it.

What happened?
An entire series of power filter caps installed exactly backwards.
It was a milky, sticky, awful mess. Live and learn!

I power up slowly with a variac, now, and even then I would power up from the other side of my garage. Good thing!

Safety first, guys....