F6 Build Questions

Are you sure the low hum is from the speaker? Or it's from the F6 chassis (power transformer mechanical hum)?
Offset 2mv DC and drifting a bit is normal, as Zen Mod said, +/-100mV range is ok, I have seen some 200mV ones also no hum from speaker. What's the AC voltage then (where you measured the offset DC)? Without input to F6, this AC should be zero mv.
Can you post some your F6 photos?
 
Increasing bias from the recommended may have some beneficial effect on the measured distortion, but in practice I’ve found that there is no real sonic change.

Hum at speakers could be a number of things, wire managment is a possibility. As noted, offset has noting to do with it. Short the inputs, does it still hum?
 
Hum at speakers could be a number of things, wire management is a possibility. As noted, offset has noting to do with it. Short the inputs, does it still hum?

Important question to answer.

Increasing bias from the recommended may have some beneficial effect on the measured distortion, but in practice I’ve found that there is no real sonic change.

Moreover, 6L6, somewhat illustrated this experiment with his Aleph J build guide, years ago. He measured the changes in overall distortion as bias current was increased. It hits a zenith and then a plateau.

Scroll to post 3 and digest 😀:
https://www.diyaudio.com/forums/pass-labs/241729-aleph-illustrated-build-guide.html#post3615479

In conclusion, without a detailed spectrum THD analysis (unless you have the golden ear), it can be difficult to ascertain which bias point is best. More might be just a space heater, if that's what you want :clown:

Best,
Anand.
 
I certainly never have claimed to have golden ears, (not that you implied that) but I have heard most of these amps.

A conclusion I can share from experience is this - the recommended bias by Nelson is almost always at or very close to the “sweet spot” for the amplifier.

Play with it all you want, but I bet you’ll settle on to something close to the “normal” bias. 😀
 
Thanks. Have you ever run into an explanation of why those places on the PCB aren't marked? I did search through the build guide without finding anything helpful but it is so large I might have missed it.

When in doubt about a PCB and it's markings, always compare with actual circuit diagram. F6 has an extremely simple circuit so it is very easy to do.