I finished up my little three tube amp today and fired it up on the variac. It works well but has a little hum until you get to almost full volume then it goes away. Tube lineup is 35Z5GT, 50L6GT and 12SQ7. Would there be any gains to adding a tone control to the setup? The schematic is based on the amplifier circuit from the venerable AA5. Will post some pics soon. Off now to start the AM transmitter!

I'm guessing you are using a volume pot of around a meg-ohm??? I've found higher resistance volume controls cause that issue, go to a 50-100K ohm, should eliminate the problem... Stems from using a low impedance input source, you can try hanging a 100K(maybe lower) resistor across the pot...

Tom

Agree with Tom.. Also be sure body of the control is grounded. There are plastic boxes for projects. Using one of these you will need to run a wire from body of the control to circuit ground.

500K volume pot. Wood chassis. Circuit ground is B return? Will see what a resistor across the pot will do for me. Thanks for the responses!

Using a wood chassis run a wire from volume control body to B-. Leaving it float can pick up hum.

Tried connecting the volume control body to B-. The hum got a little quieter but still there. I have a 100k pot around here somewhere. I will try swapping the 500K with the 100K and see what happens. Will have to set up a seperate power switch. The 500K pot has one integrated into it.

Having the power switch on the volume control can also cause a small amount of hum to be picked up. You never see that on high quality amplifier designs.

Also, reversing the AC power plug in the wall outlet can help minimize hum. Sometimes it will be quieter in one position vs. the other.

Oh I see how it is.. My design is'nt "high quality".. I did kind of figure that the culprit was that combined switch/volume pot. Gonna swap it out and see what happens.

Yep.. Works better with the detached power switch and 100K pot. Push pull 50L6 is next on the list..