Can an ARC VM220 be upgraded to KT 150 Tubes?

Stick with 6550’s/KT88’s. Both are excellent tube types. The newer KT types draw more heater current, thereby risking damaging the power transformer’s heater windings. Don’t make the assumption that the newer KT types are somehow sonically "better". I don’t think so! Don’t second-guess the original designers’ choices!

Don’t assume the power supply is over-speced! You could end up needing a new - and costly - power transformer! More current drawn = more heat - never a good thing for an amp's power supply!

I think it unwise to substitute any tube without first checking with the manufacturer or its authorized dealer.

I agree with all of the above!

 one area you can take a look at is the 6N1P

 and experimenting with some of the older versions.

  Best JohnnyR

ARC dealer NJ

+1 all of above comments.