ARC VT100 with KT-120 tubes ?

Howdy!
Some slight technical corrections:

Bifwynne, you wrote:
"the KT-120's ability to handle plate current of 700V dc (versus actual bias being set at 65mV dc) is impressive."

700VDC is plate voltage (Volts DC), not current.

Karma16, you wrote:
"Until you prove me wrong, I stand by Ohm's Law and state that simply installing KT120's will not increase an amps power output."

Ohm's law does not apply to tubes, because tubes have plate current/voltage CURVES, not straight lines.
Ohm's law is a first order equation, therefore it represents a linear relationship between voltage, current and resistance. its graphical representation is always a straight line.

Also, a tube with a different set of voltage / current curves may interact differently with the same output transformer at the same bias point, because the OPT does not follow Ohm's law either. An OPT's main electrical spec is impedance, a non-linear measurement, it varies with current AND frequency.

So I guess we are all laymen and should wait for the manufacturer's findings.

In the meanwhile, do not pop KT-120s into your 6550 tube amps, KT-120s draw more heater current and may burn the power transformer beyond repair.
Enjoy the music!

Bifwyinne:
Thanks for being a good sport!

More plate voltage is not very important in the context of most tube amps, because the plate voltage is fixed by design. However, the ability to take more plate current is an advantage, for the following reasons:

The Svetlana 6550C has the following limiting values (only ONE limit can be exceeded at a time, and only momentarily, for example, during power up):
-680V plate voltage
-plate dissipation: 35 Watts as triode, 42 Watts as tetrode

For best sound / least distortion, ARC tends to run their tubes at about 450 Volts B+ and 65 ma current, which is aproximately 30 Watts, pretty close to the tube's limiting value.

The KT120 has the following limiting values:
-650 plate voltage connected as a triode, 850V as a tetrode
-Plate dissipation: 60 Watts

Running a 60W tube at 30W dissipation is much easier on the tube that running a 35-42W tube at 30W dissipation.

So, instead of plate voltage, power dissipation is the main KT-120 advantage.
A 60W tube running at 30 W dissipation will be loafing along and will probably last much longer than any 6550 tube, provided that the power transformer can take the extra heater current and that ARC makes the (depending on amplifier model) necessary bias supply changes.

I agree with you, this tube has great potential but is not a simple drop-in replacement.
Enjoy the music!