Help on repairing messed up AMC CVT3030

[Motobias]

Hi everyone

First i got to say that i am an absolute beginner in tube technology.
My never ending hunt for more "exotic" gear recently brought me an AMC CVT 3030, which was running when i picked it up but distorted quite a lot.

I took the whole thing apart and checked for bad components.

First thing i found were the blown G2 resistors on one tube of each side.
I know that these are important but not necessary for the amp to run.
I'm gonna replace these later, when i can get spares.

What worries me the most, is that as soon as i connect to ground/bias/b+ cable from the input board to the tube board, one of the tubes per side goes completely nuts and starts glowing red hot, while the other one stays in (what i would say) normal range.

I verified the b+ voltage, which is around 450V and tried to get a measurement on the bias voltage, but it seems to be very low - i get about 2V at best with ground reference on the connector.
The transformer outputs about 33V AC on the bias outputs, before it gets rectified and filtered.

I changed the caps to better ones already but i don't have a rectifier on hand to try swapping it.


Can somebody tell me why one of the tubes is getting way too hot, with the b+/bias/ground cable connected and everything is perfectly normal with the cable unplugged ?

(Somehow the schematics on http://www.amchome.com/sites/amchome.com/files/Service Manual-CVT3030_k_0.pdf do not match my cabling to 100%, but i figured out how the routing is in mine)

Thanks a lot for your help

 

[azazello]

Check fuss F 402, Diode Gretz D405...on caps after D405 You must have about minus 35-40 V DC. On R521 and R522 You must have about minus 30 V. On the plates of both EL34 You must have about 350-400 v. On R 525, R 526 You must nave +U little bit more /5-10v/ than on the plates.