Hum pot

[Demsy]

Hi,
I've used the Shuguang Black Treasure 300Bs for several months now and am very satisfied with the sound quality, it surely betters the already good stock EH tubes.

I have a slight hum out of one of the amps, while the hum-pot is already turned fully counter clockwise. I didn't measure the value, though, but when turned to the right I get more hum. On the other ampt it is free of hum with the pot turned about 3/4 counter clockwise, really no hum. I wonder if changing the pot with higher/lower value, would help taming the hum?

This hum is audible when using the 95db DTQWT horn speaker, gone if using the Martin Logan SL-3, which I believe is only 89db. So the hum is slight, but it would be nice to have it free of it.
There is no hum at all when using the stock EH 300B. Forgot to mention I use the TungSol 2C51.

Hope somebody can help, thanks.

[BNAL]

Doe the hum follow the tube if you swap them?

[Doc B.]

Before you replace the hum balance pot try swapping the 300Bs in the amps and see if you can get the tube to be quiet in the other amp. The circuit uses a 10 ohm pot with a 22 ohm resistor attached to each end. You could try adding a little more resistance to the 22 ohm resistor at the end away from the direction you are turning the pot wiper, and subtracting some at the resistor connected to the pot end that the pot wiper is being turned towards, to shift the pot into a point where it can balance the currents. For example you might try changing the 22 ohm at one end to maybe a 33 ohm resistor and the other end to a 10 ohm, so the pot wiper's adjustment range is "shifted over" closer to the 10 ohm resistor. Or you could replace the whole 22ohm/10ohmpot/22ohm setup with a 50 ohm wirewound pot. It won't have the fine tuning capability of the 10 ohm pot.

[vetmed]

From your description it is not surprising you get hum; turning the knob all the way to the left or right is guaranteed to get you maximum hum. The pot works best in approximately the middle of the range of rotation. Truly the surest way to minimize hum is to use your voltmeter set to AC, I believe the process is described in the manual. Unless, as the other poster suggested, your tube is on the way out

[Demsy]

Thanks guys for the reply. Lately I just use my ears to adjust the pot, when the hum is gone, it's good. But would try measuring it next time to see which values I get when the hum is gone.

The hum follows the tube, Dan. I'll try adjusting the resistor values as I think I only need a little bit more range and it will be good.

[Demsy]

Dan, do I need to do the changes to both amps or is that unnecessary?

[Doc B.]

If one tube is a problem, just do thst amp. and as I think more about it, a shift of only 5 ohms might be better than 10.

[Demsy]

Will try a 5ohms shift first, I think that'll be enough also. Thank you, Dan

[Paul Joppa]

Since it follows the tube, make the change in a way that can be reversed when the tube is replaced ... most tubes are symmetrical (in the filament) enough to not need that shift.

Another alternate is to substitute a 25 ohm pot for the 10 ohm. It will have more range but (naturally) less precision, and it will work with other tubes. Actually, Radio Shack sells a 25-ohm pot (called a "rheostat" the last time I looked).

[Demsy]

Thanks Paul, too bad we don't have Radio Shack here in Jakarta. Any specific type of pot I should use?

[Demsy]

Did the resistor change today, 5ohms shift as suggested. Hum completely inaudible, gone!

Thanks Dan for the suggestion

[Paul Joppa]

I prefer wirewound pots - and resistors - because they carry a lot of current and the resistance is unbypassed.

[Demsy]

I couldn't find the 25ohm wirewound here, that's why I go Doc's advise with the 15 and 25ohm 5 watt resistors.
But upon further inspection, I wil also do the same thing to the second amp, as the pot is also almost reaching the end of the travel counterclockwise. So these Black Shuguangs have all a-symmetrical filaments, obviously.

[Demsy]

Have done the 5ohm offset with the second amp, the wiper is now somewhere in between the extremes. Both are dead quite, but haven't measure the voltages, yet. So it is confirmed that both my shuguang black treasure filaments are not symmetrical, but real easy to cure. Thanks Dan and PJ