EBERGER@B.PSC.EDU (04/12/90)
My friend got a NYAL Moscode 150 power amp in to fix this week. The owner blew both speaker fuses... (sometimes things are this silly) The moscode amp uses a 6DJ8 (ussr) input tube and a 6CG7 (westinghouse) driver tube with 1 complementary pair of Hitachi power mosfets for the output stage, for each channel. The circuit board & heatsinks have spaces provided for another pair of output mosfets, which would probably be useful after tube changes. We weren't very impressed with the sound... Since we had the time & tubes, we tried a few different ones. using the ussr 6DJ8 with GE 6CG7 (snarfed from an audio research amp) - The sound was a definite improvement, forwardness was decreased and the amplifier sounded more cohesive and warm like the old audio research. using a GE 6DJ8 with the westinghouse 6CG7 - This was really strange! from 2000Hz down it sounded like a tube amp, overly warm and thick sounding, above that It sounded like a cheap solid state amp (hard, thin & glaring). It was almost like biamping an amp... The wrong way! using a GE 6DJ8 with a GE 6CG7 - This was overly thick sounding, possibly unstable as it had what sounded like mistracking distortions. And for the surpise performer.... using a westinghouse (made in japan) 6DJ8 with a GE 6CG7 - This was the clearest sounding combination. It was cohesive sounding, and I could begin to hear the sound of the preamplifier circuits upstream. using a westinghouse (made in japan) 6DJ8 with a westinghouse 6CG7 - Back to glare & hardness without much tube warmth to tame it down. Summary: - Dump the Westinghouse 6CG7 and replace with GE 6CG7 If you feel you've lost detail doing this try the westinghouse (japan) 6DJ8. If you value warmth you may prefer to keep the ussr 6DJ8. - If I were trying new caps in it I would use the westinghouse (japan) tubes, to clearly hear effects, then possibly swap back the ussr 6DJ8s to get more warmth & tuby sound. - I'm really surprised at how good the westinghouse (japan) 6DJ8 is. I really expected it to be awful, just like their 6CG7 was, and it sounded comparable to my favorite 12AX7s. I'll keep my eyes open for more. -Ed Berger eberger@b.psc.edu [My favorite 6DJ8 is the JAN-6922. If you can't get that, buy Amperex 6922. I've found that MIL/IND numbered tubes generally sound better. -tjk]