hylaphone 7/10/2018 1:14 PM
12A_7 @ 12.6V
A quick question about running 12A_7 tubes at 12.6V --
If I connect the 12.6V to pins 4 and 5, can I use pine 9 as the center tap, either grounded or elevated?
 
Enzo 7/10/2018 1:29 PM
You would normally leave the pin 9 unconnected. You COULD use it as a center tap, but then you make the amp depend upon the tube being there. For the couple of cents a pair of 100 ohm resistors cost, I would recommend staying with the common Fender style virtual center tap elsewhere.
 
hylaphone 7/12/2018 9:22 AM
Thanks Enzo, it's more a theoretical question if the actual filament center would behave the same as the artificial CT, elevated or grounded.
What if this were a single bottle preamp?
 
printer2 7/14/2018 8:16 AM
I think the whole idea of the centertap on the heater is that you have 6.3V coming in from both ends with the same phase and the hum cancels out as it does with a P-P transformer. If you have 12.6V in on one end and 0V at the other I can't see the center of the heater being grounded or elevated canceling out the hum. Now elevating the 12.6V would get the heater current farther away from small signal voltages but you can do that to the heater supply without using pin 9.