k4kyv at hotmail.com
Thu Jun 24 12:06:37 EDT 2004
>Running tubes to hard will cause them to fail, excessive plate voltage,
>screen voltage, plate current.
>Cathode resistors going low, shorted/leaky cathode bypass caps?
>...Collins used to advertise they ran the 75s series at 150 volts to reduce
>extend the life of the tubes...
>Maybe run a test, and see just what the tubes are running at voltage and
I had the same problem with 6V6's in my receiver's outboard audio amp. It
is a modified 50's vintage 10w hi-fi amplifier that uses a pair in the
output stage. I would put in a new set of tubes that checked good, and
after a few months I would recheck them and they would test weak. A couple
of times the audio level actually dropped because the tubes had become so
weak. All tube voltages and currents measured well within the ratings
recommended in the RCA tube manual, but I tried reducing the screen voltage,
increasing cathode resistance, and put the whole amp on a variac and
reduced the a.c. voltage to 110v going into the amp. So far the latest pair
of 6V6's has held up.
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