Help on repairing messed up AMC CVT3030 - diyAudio
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Old 6th September 2014, 11:23 AM   #1
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Default Help on repairing messed up AMC CVT3030

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 do not match my cabling to 100%, but i figured out how the routing is in mine)

Thanks a lot for your help
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Old 6th September 2014, 02:43 PM   #2
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Location: Bulgaria
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.
Projects: OTL-OCL 6AS7 Gen. Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, E84L Siemens, E236L Telefunken, headphone ampl. OTL Loftin White 6AS7 RCA,... PPs 6005 Gen. Electric, EL34 Mullard, 6L6....
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