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

Hi everybody, 
looking for some advice here. I would like to try out before mentioned...would you:
just plug&play, while monitoring the bias on one/two ?
dial back the bias, run for 30mins and adjust coming from the lower values?

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..


Thanks a million, will report back if I felt there was a sonic difference,

Audio Research VS115

yes, turn down the bias to about 50-60% of what it was driving the kt120’s, and then insert 6550/kt88, then turn on, wait 5-10 min, and bias up from there to the target 65 mv across the test resistor


a good reference post for you, op  ^^^^
Interesting outcome. It didn't change the overall neutral character going from KT120 (TungSol) to 6550 (Shunguang), still very happy with the entire setup. Immediate impression was...it's a bit on the lean side, bloodless and thin...but that changed from song to song, on others I indulged into the more precise, refined and thorough presentation. As time went on...especially around voices there was more detail. It eventually came down to two different flavors, right now I like the 6550 better and can't wait to find a detail here and there that I hadn't picked up through the KT120.
The setup is USB/streaming through a Raspberry Pi, then Digi +v1.0 into dbx venu 360 loudspeaker management system, splitting the signal at 50Hz to open baffle Beyma 15 B 100/R (Alesis RA150 driven) and Quad 63s (ARC VS115 driven).
Thanks again for the help,