• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

GU-50, GM70 and KT88


A google search for GU50 triode will reveal good results! I read somewhere that triode connected, the GU50 can take up to 400V (due to the limitation on G2).

I am also curious how it compares soundwise to a KT88... The GM70 will need some more volts on its plate, Brett (member) said it starts singing from 600V onwards.

GU50 is a Russian version of Telefunken LS50 whose data sheet
can be down-loaded below.


I purchased a Chinese amp using FU50 Triode Connection
Parallel single ended. FU50 is Chinese version of GU50.
My amp Manufacturer's Web page is below.


The schematics is indicated at the bottom of the Web page.

My amp is introduced in my blog(in Japanese) as below.



2004-06-21 8:04 am
big smoke
There's a thread somewhere here concerning G2 limits of the GU50. The rather contentious consensus was G2 is more power than voltage limited at lower B+. Use a screen resistor, monitor G2 current and voltage, watch the product of the two. I've run it up to 600 VDC and heavy A2 on the bench without issue for about 12 watts SE into a 10k load. FWIW, I very much prefered the harmonic signature of the GU50 into that load over a KT100.