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In Reply to: RE: The MV75A had an Achilles' Heel posted by E-Stat on January 07, 2025 at 15:09:41
It eats 6550's like a moviegoer eats popcorn. Even GE's only lasted about a year.
Follow Ups:
The MV 75 had 590 vdc on the plates and if you used the LED bias, the idle current was 75 ma, which equals a dissapation of 44.25 watts of heat! The max rating for the 6550 is 35 watts for the plate. Is it any wonder tubes did not last or fail completely?
The solution was to change the cathode resistor from 20 ohms to 28 ohms and you can use the LED bias and not over dissapate the tube rating.
"Is it any wonder tubes did not last or fail completely?"
The company was formed by two economists and I suspect they didn't bring an actual engineer onboard til much later - but I'm just speculating.
Bill Thallman was employee number 3 and got the products going. Founded Music Technology after his stint with C-J. He was the go-to guy for updates on their early units.
While my VTLs run SED 6550Cs reasonably hard with 550V rails, I found a schematic for the MV75 and found it runs them at 590V!
My pair varied from 600V to 615V. Looked pretty cool when they red-plated!
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