hi
it s just to share my hum test with ac heater
with stereo preamp
-for the 6.3 vac heater with 6sn7
1-pin 7 to ground and pin 8 to 6.3vac
=> zero hum to right but hum to left
2-pin 8 to ground and pin 7 to 6.3vac
=> zero hum to left but hum to right
3-my final solution is:
pin 7 to 33ohm to ground
pin 8 to 33ohm to ground
=> low hum to both channel but not zero
if my test is right maybe avoid dual triode
and always ground one side heater
but wait for guru confirmation
it s just to share my hum test with ac heater
with stereo preamp
-for the 6.3 vac heater with 6sn7
1-pin 7 to ground and pin 8 to 6.3vac
=> zero hum to right but hum to left
2-pin 8 to ground and pin 7 to 6.3vac
=> zero hum to left but hum to right
3-my final solution is:
pin 7 to 33ohm to ground
pin 8 to 33ohm to ground
=> low hum to both channel but not zero
if my test is right maybe avoid dual triode
and always ground one side heater
but wait for guru confirmation
jeapel said:hi
it s just to share my hum test with ac heater
with stereo preamp
-for the 6.3 vac heater with 6sn7
1-pin 7 to ground and pin 8 to 6.3vac
=> zero hum to right but hum to left
2-pin 8 to ground and pin 7 to 6.3vac
=> zero hum to left but hum to right
3-my final solution is:
pin 7 to 33ohm to ground
pin 8 to 33ohm to ground
=> low hum to both channel but not zero
if my test is right maybe avoid dual triode
and always ground one side heater
but wait for guru confirmation
jeapel said:
This isn't the real cause.. Almost all 6SN7 I have ever seen except for a few of the very last coin base types have parallel filaments, and I have never had a hum problem with this type with ac heating in pre-amps or power amps. Undoubtedly an issue of execution.
IMO Using two 6J5 where a single 6SN7 will do is both a waste of space and a socket. It confers no real advantage I can see. Last time I was aware the 6J5 wasn't being made any more..
jeapel said:
I guess that is an oddball design, I don't use them as my preferred Sylvania 6SN7 is readily available on eBay for no more money and to my ear sounds much better.
I've not had hum problems with the 6SN7EH either.
I have used 6SN7 in SRPP and conventional common cathode cathode follower and transformer coupled line stages as well as LTP and drivers in my power amplifiers and I have never had a problem with hum in any of my designs.
You may need to bias up the heater supply when using AC to something like 50Vdc to assure absolute silence.
Of the many people here I would say your experience with the 6SN7 is quite atypical and the problem points either to a defective tube (design) or something not quite right in your design. Do you have a scope, other test equipment - and how much analysis have you done?
Use DC on the 6SN7 heater, then that hum problem should go away.
Either rectify and filter the 6.3VAC heater supply, or if the output tubes draw enough current, you could use the 6SN7 heater as part of the output cathode resistor.
Another trick to consider is to disconnect the 6.3VAC supply from ground, and to bias it up about +40V. Making the heaters more positive than the cathodes tends to reduce hum leakage. .
Either rectify and filter the 6.3VAC heater supply, or if the output tubes draw enough current, you could use the 6SN7 heater as part of the output cathode resistor.
Another trick to consider is to disconnect the 6.3VAC supply from ground, and to bias it up about +40V. Making the heaters more positive than the cathodes tends to reduce hum leakage. .
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