Is a KT-77 a plug in for a KT-88?

Not unless your amp is designed to accommodate them. Some amps can take both the smaller power tubes (KT77, EL34, 6L6GC, etc) and the larger ones (6550,KT88, KT90, KT120, and KT150), such as some made by Primaluna and Cary, most cannot. Check with the manufacturer of your amp or run down a copy of it's manual - that should solve your question.

The first check of course is the pinout, and they match.

In terms of operating points, the KT-77 is pretty robust (design max plate voltage = 800VDC, design max screen voltage = 600VDC).  It's doubtful that your KT-88 amp was designed such that it will kill this tube, but I'd check with your amp builder.

One overlooked point in tube substitution is heater current draw.  If you swap to a tube with significantly higher heater current draw, you my be outside the spec of your power transformer.

In the case of the KT-77 (1.4A draw) and KT-88 (1.6A draw), you'll be drawing less heater current with the KT-77 so you're fine in this regard.

So ... check with your amp builder.

Cheers,
Thom @ Galibier Design

Thank you for the reply.  Much appreciated