Switching from KT120 to 6550, just plug&play or dial back bias?

What amp?

Audio Research VS115

OP if it has an adjustable bias, you ALWAYS back it down, no matter what you do. All the way Counter clock wise, it will still play right?

NEVER plug and play when there is a bias setting..

I’ve worked and biased a LOT of amps.. Certain thing you just DO..

Cary is wide open meaning the settings go from ZERO to off the charts, they can max out and actually pop a valve on startup because the bias is to high. I turn the bias up slowly until I hear the speakers on a Cary THEN I pay close attention to a DMM.

Stick gages, are NEVER close enough and once a year they are suppose to be calibrated. That is a US/UK standard for analog gages.

Most valve amps (NOW) have a range within a range, so you really can’t pop a valve but you could burn one up pretty quick.. The old VTL was that way..

Machine OFF, turn the pots CCW, install your valves, get your bias tools (screwdrivers, etc). Turn on your DMM, let it calibrate, and hook up.

THEN turn on the machine keeping an eye out for red plating on new valves or just transported valves. After 10 -15 adjust the initial settings.

Re check at 30 minutes and an 60.. If there is drift.. BE LEERY. The bias should not be changing only slightly as the valves wear..

I bias low, NEVER high (unless it’s very low volume listening)..

I can get a much better wattage before distortion or clipping, sometime 50 to 100% more.. When adjusted with a scope.. Tuned bias method..

Enjoy

Regards

+1@ oldhvymec
Excellent!