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I recently finished a 6sl7 //, dc 2a3 amp, it has been operating very stable at 250V, 60 ma, +/- < 2%. The output tubes are RCA blackplate, used, different production date code, measured over 85% though. After about 30 hrs , there is some color change in both tubes, the very top of the envelope, used to be clear, now it looks tinted, it is kind of like charcoal, greyish in color. Both tubes so far measured the same as before. I had used both tubes in my Sun Audio RC circuit for few hundred hrs before the switch.
Any idea why?
Thank you
Wal
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Q. " I am running my new car continuously at 100 MPH on the freeway, and the bright red color on the cam cover has changed to dull red, .....what should I do " ??A. Slow down to 62 to 70 MPH. and enjoy the scenery some.
You are running the tubes at 15 watts plate dissipation, which is their maximum dissipation. This means you will NOT get great tube life from them, and that they are now continuously degrading. Additionally, they are now operating thermally stressed, and they SOUND that way.
The tubes will last tens of thousands of hours, and, (if your internal wiring and layout is sound), the amp will be more relaxed, less stressed sounding, if you REDUCE the plate dissipation from the 15 Watts to about 10 to 10.5 Watts.
To reduce 2A3 dissipation and current draw requires (1) a proper DC voltage on the 2A3's grid, and usually simultaneously, (2) a higher value self-bias (Rk) resistor.
Jeff Medwin
Edits: 05/21/12 05/21/12
What is your heater voltage?
I borrow my friend's tester, both tubes tested borderline. I think I need to work on my power supply circuit again. find a way to slow ram up the B+.
wal
I am with Chris. Me thinks your filmant voltage is about 3.5 volts.
.
You posted 5 volts in other thread. The glow of the filament should have been evident. Ouch.
.
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