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So I just got done building an IT-coupled 6B4G PP stereo amp per the Ralph Power's design (http://www.digitalakuten.com/circuits/ralph_amp/ralph-amp.htm). It's got Lundahl ITs, Western Electrin 417As, Tamura OPTs, etc, etc, etc., and I will post pictures of the amp soon.

Anyway, right after I finished wiring the amp and fired it up for the first time, I noticed that there was a loud hum coming from one of the channels. It sounded like an AC-heater hum, but I went through a systematic troubleshooting steps and found no smoking guns. Then my thoughts went back to what sounded like the AC-heater hum (unplugging the 417A did not rid of the hum, so I knew that it was coming from the 6B4Gs alone). Sure enough, I did not wire the 6B4G heaters on the humming channel in parallel, i.e. pin 2 to pin 2 and pin 7 to pin 7. The hum was completely gone after addressing this.

Now, I had no idea that this would make such a HUGE difference! I guess it does matter for DHT tubes like this in a PP circuit. I usually twist the heater wires than randomly connect heater pins in PP circuits that use non-DHT tubes and never encountered an issue such as this.

Min

By having the heaters , out of phase and the output tubes out of phase the 60 HZ from the filaments added , normilly with the filament in phase and the output tube out of phase the 60 HZ cancels . I have a PP 6B4G Amp with the input transformer doing the phase shift and two 6J5GT's . I see you are using a 470 ohm resistor for bias , mine uses a 850 ohm resistor and is way into class A , and only puts out 8 watts , have you measured the the power out .