The 6SC7 is used as a "floating paraphase" phase splitter. At least
in the fixed bias version it is very sensitive to hum in the bias
supply. Maybe not in the self bias version as used here. However, I am
very suspicious of this tube. Easy to prove it by changing tubes.
There is some information about the floating paraphase in the
Radiotron 4th edition. Very widely used circuit with several variations.
The illustration in the RDH is almost identical to what is used in the
SX-28.
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On 2/16/2025 5:37 PM, Mike Langner via groups.io wrote:
6SC7 tubes are renown for developing heater-cathode leakage and for
developing inter-element shorts and leakage as well.
Often, if you shake one near your ear, you can hear loose whatever
rattling around inside the tube.
In addition, thumping a 6SC7 while it¡¯s operating in a circuit can often
vary the hum and noise produced by the stage.
I spent over 60 years in broadcast engineering and facility
maintenance.? 6SC7 tubes in phono preamps (remember LP¡¯s?) was a
continuing headache.
May or may not be relevant in this case.
Mike/
K5MGR
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
