I200-I400 Power Supply transistor failure

[lazerbob]

I have an I 200 and an I 400 with what appears to be the same problem. The 2N6259 transistors fail at the rate of about 1 every 3 weeks and has to be replaced. The transistor shorts out and blows the 2 amp fuse. In operation, the passbank sumbuss voltage is 3.54 V. at 17 amps.

Does anyone know why these transistors fail after a very short duration in service? I normally operate the tube at 35 amps, getting 1 watt at 5.14 nm using a laser pure 60 closed loop with approx. 12 gals of building water.

Thanks for any help.

lazerbob

[mixedgas]

Passbank voltage excessive from tube pressure issues. Use of unmatched transistors. Unbalanced base drive, or unbalanced series string.