So...is anyone using the KT120's yet?

[kg]

the simple and elegant solution is to build a good quality driver/bias power supply, and then drive the grids with either followers or a transformer.

[kg]

ie:


lowzbiassupply by k.a. gilbert, on Flickr

[jmaf]

ie:


lowzbiassupply by k.a. gilbert, on Flickr

Heh! I'm staring at a 3055 on a heat sink I scavenged off a car inverter, right in front of me! Guess who'll be victim soon
Thanks for that kg

[jmaf]

I'll have to wind the power trafo or order one, how many milliamps do you estimate I need for the 46VAC bias secondary for that 2n3055 emitter follower circuit?

[es345]

Maybe es345 can share with us how he drove that kt120 - Something like black_labb's 2 triode dc-coupled driver?

Well here is the schematic of the powerstage: driver, LTP PI, followed by a mosfet follower and direct coupling to the G1 of the KT120 . Important: the negative supply voltage of the follower must be much more than the peak to peak G1 signal driving the KT120.

[kg]

I'll have to wind the power trafo or order one, how many milliamps do you estimate I need for the 46VAC bias secondary for that 2n3055 emitter follower circuit?

depends on how hot the 3055 is run... which depends on how negative you need your bias supply. the current through the follower will be set by the voltage that appears across the emitter resistor and it's value.

[jmaf]

Thanks. Then I'll calculate for full -60 VDC and then double the current for the sake of goold ol' Texas size excess. Sound good?

[kg]

you may need to go more negative with the entire rail. for comparison i use about -75v to keep kt90s cool with 620vg2.

[jmaf]

Will do. Haven't had time to experiment at all, but hopefully I'll build this experimental circuit soon.

[imaradiostar]

Interesting thread. I still wonder how they compare to a GU50.

jamie

[es345]

you may need to go more negative with the entire rail. for comparison i use about -75v to keep kt90s cool with 620vg2.

FYI: Measurement at my powerstage with KT120: Ug1 = -86V, Ik= 47mA

[Steve Conner]

I think they would smash a GU50 right in its novelty coffee pot head. What is the transconductance of the KT120?

[jmaf]

It seems like they pasted a whole bunch of old images onto a PDF to make it look like an old tube....

http://www.jogis-roehrenbude.de/Roeh...120TungSol.pdf

With Ua = 400 VDC Ug2 = 225 VDC and -14 VDC bias the transconductance listed is 12.5 mA/V

[es345]

...from Datasheet

Ua= 400V
Ug2=225V
Ug1=-14V

S=12,5 mA/V

[Steve Conner]

The GU50 has a transconductance of 4mA/V, so I think the KT120 wins.