What does this trimmer do ?

[Rustynuts]

Hi, I'm a newbie here and this is my first post.  If been gradually leaning about tube amps and have a few.  I have 4x el34  stereo amp.  This came from a church that had an old organ running into it.  ( No, it wasn't a B3 ) it hadn't been used for decades.  I replaced the valves and caps and added some 1/4" sockets.   It works great.  Tons of power.   I noticed this trimmer inside, not sure what it does.  Can someone enlighten me ? It's connected to the two sides of the heater filament circuit.   The orange wire goes to ground.   If I move it the heater voltage does not change. 

[DummyLoad]

tapped resistor for filament hum reduction. adjust for minimum hum when idling with a speaker load, or with an oscilloscope, or AC voltmeter.


--pete

[vampwizzard]

"Hum balance". Depending on the actual circuit there were a couple of ways they did this back in the day.

[nandrewjackson]

+1 for hum balance pot.




I like that amp ! 4 - EL34s !