6SL7 Hum Problem
Greasehorse
A few years ago I started a thread about an amp I was building that
had severe hum problems. The amp used 6SL7 tubes in the pre amp. I
tried everything under the sun, but was unable to reduce the hum until
I finally installed a DC heater supply for the 6SL7s. That eliminated
the hum completely.
Now I'm bread boarding a bass amp that's loosely based on the Ampeg
B-15-N "Fliptop". Again I'm using 6SL7s. And again the amp has a lot
of hum, which a DC heater supply eliminates. I was resigned to
building a DC heater supply into the amp to get rid of the hum, even
though I don't really have room for it in the chassis.
But I just had a major breakthrough. I noticed that in the B-15-N,
Ampeg referenced the (AC) heaters *not* to a positive voltage, but to
a negative voltage -- namely, the fixed bias supply for the power
tubes (around -50V). I decided to give it a try. I hooked up a
variable negative power supply to the heater center tap, and started
cranking up its voltage. When the voltage reached minus a few volts,
the hum dropped *dramatically*. With the heaters referenced to the
amp's fixed bias supply, the hum is almost nonexistent, even though
this is just a breadboard setup with the circuit spread out over most
of my workbench.
For most tube types, a positive heater reference seems to work the
best. But for the 6SL7, a negative reference is dramatically better.