found some info on KT120's at the TC tubes website
"KT120 substitution notes
The KT120 tube is one of the more exciting developments in new production vacuum tubes. Generally speaking it is a drop in replacement for 6550, KT88 and KT90 tubes, as long as the larger bottle fits in your chassis. This tube also draws more filament current (100-300 mA more per tube) than 6550/KT88/KT90 tubes do, so you need to be sure your power supply can handle the increased demand.
Many people report improved bass response and increased output with KT120 tubes. The reason for this stronger response is obvious when you look at its test results. New production 6550 and KT88 tubes we test range from 90-140% of the expected value for mA and Gm. Under the exact same operating conditions (500v plate, 300v screen, -30.5v bias) the KT120 rarely tests lower than 125% and can test as high as 185%. (We reject KT120 tubes that test higher than 165%--at some point you have to say enough is enough!)
Most fixed bias amps have enough range to dial KT120 tubes in to the recommended current draw for that amplifier, though some do not. Even if you are unable to bias the tube all the way down to the recommended current draw you can probably still use KT120 tubes. You'll just have to do a little math to be sure the tubes are running safely. Simply take the plate voltage and multiply it by the current measured in mA. For example, if your plate voltage is 500v and you are measuring 70 mA across a single tube then the math goes like this 500 x 0.070 = 35 Watts. Since this is a 60 watt dissipation tube and the rule of thumb is to run tubes at 65% of their maximum dissipation (60 x .65 = 39 Watts), 70 mA across a single tube in this case would be just fine even though the manual is likely to recommend 50 mA for a 6550 tube.
If you have an autobias amplifier you will want tubes at the lower end of the spectrum of KT120 tubes."
