Volume Control Hum

[BRSHiFi]

I just added in the second channel of a preamp I am constructing. It uses an alps blue velvet pot.

The unit is quiet on the left channel but a hum emerges when the volume is not at either end of the pot's travel on the right channel only. The same issue occurs with other pots as well.

It probably is a ground loop of some kind. The issue is probably simple, I just could use some help in tracking it down.

Thanks,
kingneb

 

[gingertube]

It may be a ground loop BUT....

Is there a grid stop resistor between the wiper of the volume control and the grid of the following tuibe grid.?
If not, then add one.

When looking away from the tube grid, back into the volume control, then maximum impedance back to AC ground (0V) is with the volume control at mid settings and minimum impedance to AC ground is with the volume control at either end.

Any tendency for the following tube to break into ultrasonic oscillation (which you often notice only as hum as the power supply is stressed) will occur at the mid settings.

Similarly maximum pick up of noise from heater circuits occurs at these (maximum impedance) mid volume control settings. Check the right channel heater wiring to make sure you have either a real or pseudo centre tap. A heater circuit DC elevation may also help.

Cheers,
Ian

 

[BRSHiFi]

I'll add grid stoppers as I forgot to do so. The heaters are powered by a 7812.

 

[BRSHiFi]

I Added grid stoppers.

They did not help. Even if there were an oscillation going on, the left channel would hum too as the power supply would be loaded down. The hum only happens in the right channel.

There is something else going on.

 

[deicide67]

Dumb question, but did you swap the tubes from channel to channel. Is all of you wire routing away from the PS? Sometimes coupling caps crossing B+ can have an effect like this.

Do you have an up skirt shot of it?

Blair

 

[BRSHiFi]

Up skirt, lol.

Anyway, I think I killed the hum loop by breaking the ground for each channel into two paths. I didn't think I would need to do this but I think it stopped it. I will report back as soon as I make the wiring change permanent.

 

[deicide67]

Dual power supplies? I've had to break the ground on preamps like that, but not usually on a single supply preamp. At any rate, glad you found the culprit!

 

[BRSHiFi]

The hum was actually coming from the stupid selector switch's unconnected positions. I soldered them to the RCA's and that cured it.

The only problem left is hiss (static or snow) at a fairly low level (audible at 6 inches to 1 foot from the speaker). Is that coming from the tubes? I am using all metal film resistors and the alps pot. Changing to the alps from a carbon pot lowered the hiss some. The hiss level changes slightly with the pot adjustment with no input connected. Shorting the input changes the hiss little.