Well, I built a tube amp and everything is absolutely perfect with it, except that there is a a buzzing sound when the volume pot is anywhere but max volume, at max volume there is no buzz (or hardly any at all) but as I move the volume lower the buzz steadily increases, and it is very lound at minimum volume. I'm gathering that htis is a grounding problem, I've got a star ground, and I'm not sure what else to try. The fact that it changes with the volume control (which is right at the input to the circuit) baffles me as to why this is happening, any help would GREATLY be apreciated.
I'd seperate the PSU ground and signal grounds, but whatever...
Assuming you use a normal voltage divider volume control, clearly there must be two things: 1. there is a signal (with respect to the first (input stage) cathode) at the pot GND; and 2. there is another source loading the pot, so that AC voltage drops to zero at the *open* end. If you have a signal connected, it could well be a sign of ground loop (try ungrounding some connected equipment). If it does this without anything connected, it could be a sign that the control is reversed and picking up stray noise in your case (is it near an HV line?).
The absolute best way to eliminate internal ground loop hum, BTW, is to 1. isolate PSU from amp circuits, and 2. connect input ground to 1st stage, and 2nd stage/output if possible, grounds. The quad 6L6 amp I made had nasty problems for a long time until I finally ran the 12AX7 cathodes to the input jack. (Everything was running through the chassis otherwise; damned if I know where the loops came from in the first place, but this nipped it in the bud. Bit of a coverup rather than fix-it-for-good situation but what are ya gonna do.)
Tim
Assuming you use a normal voltage divider volume control, clearly there must be two things: 1. there is a signal (with respect to the first (input stage) cathode) at the pot GND; and 2. there is another source loading the pot, so that AC voltage drops to zero at the *open* end. If you have a signal connected, it could well be a sign of ground loop (try ungrounding some connected equipment). If it does this without anything connected, it could be a sign that the control is reversed and picking up stray noise in your case (is it near an HV line?).
The absolute best way to eliminate internal ground loop hum, BTW, is to 1. isolate PSU from amp circuits, and 2. connect input ground to 1st stage, and 2nd stage/output if possible, grounds. The quad 6L6 amp I made had nasty problems for a long time until I finally ran the 12AX7 cathodes to the input jack. (Everything was running through the chassis otherwise; damned if I know where the loops came from in the first place, but this nipped it in the bud. Bit of a coverup rather than fix-it-for-good situation but what are ya gonna do.)
Tim
Oh, thanks Sch3mat1c, I didn't even think of seperating the signal and power grounds, I'll try that first. But what did you mean by "connect input ground to 1st stage, and 2nd stage/output if possible, grounds"? I think I just didn't understand the wording. And again, thank you alot, seperating psu and signal ground had never even crossed my mind!
An externally hosted image should be here but it was not working when we last tested it.
See the black wire that snakes around the left and top? That's the power switch wire. Well immediately below the switch (top center) is the potentiometer (it's an uh... 100k dual gang Alpha from Mouser). Above it is the input jack, which by its nature happens to be connected to the chassis. It's not easy to see on this small pic. Well anyway, the ground tab on this jack is connected to the pot's ground, and the first stage (triode; 6U8 on the right) is also grounded to this point. The other stages are grounded locally, including the 6L6s which are grounded at their sockets. They don't matter because the 50mV of hum pales in comparison to the 50V required drive; however, 50mV at the input of such a high gain amplifier will drive you NUTS.
Tim
ted the signal and power grounds and that helped, how it helped was decrease the buzz at lower volume levels. Now, at no volume, the buzzing is almost gone, but as Ibring it up the buzzing increases until it's at mid volume, then it decreases again and at max volume the buzzing is almost gone again. I'm using a linear pot so the loudest buzzing is when the resistance is equal going to ground and from the source, and it's loud. A max and min there's barely any, but in the middle there's alot.
Thanks for the tip, got some of it away that way.
Thanks for the tip, got some of it away that way.
Ah, so a high impedance signal fed into the grid would produce an even louder hum when no inputs are connected, right? Because I just tried that and indeed that's what happened. Although, I have no idea what could be causing that. Well, at least I know what to look for, thank you so much for the help and time btw.
I have same problem as Mr. Joebob too
there are more buzz only about 10 oclock till 1 oclock
0-9 and 2- max is no buzz.
May be I wiring Ground of RCA input to Potentiometer wrong
I wire 2 ground wires like this
1. GND from RCA input to GND of Potentiometer (stereo, I solder 2 GND pins together)
2. then I wire 1 wire from GND of Potentiometer to main Ground of AMP (not PSU of AMP but it is the same GND)
Should I change to ???
1. GND from RCA input to Potentiometer
2. Then GND from RCA input again to Ground of AMP??
Thank you
there are more buzz only about 10 oclock till 1 oclock
0-9 and 2- max is no buzz.
May be I wiring Ground of RCA input to Potentiometer wrong
I wire 2 ground wires like this
1. GND from RCA input to GND of Potentiometer (stereo, I solder 2 GND pins together)
2. then I wire 1 wire from GND of Potentiometer to main Ground of AMP (not PSU of AMP but it is the same GND)
Should I change to ???
1. GND from RCA input to Potentiometer
2. Then GND from RCA input again to Ground of AMP??
Thank you
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