Hum only at volume 0 5F2A

[basso]

5F2A build, hums at zero but as soon as you inch the volume knob up it goes dead quiet and the amp works perfectly. Voltages look good, measuring the 2nd half of V1 (pin 6 or 8) eliminates the hum completely with the DMM on the pin, measuring all B+ voltages quiets the hum and gets quieter the farther down the line you go. Touching the shielded cable from the volume knob quiets it some which leads me to grounding but it goes away when you move the volume knob so that doesn't make sense.
So far I've replaced the 12AX7, volume knob, disconnected the tone control, replaced the shielded cable from pin2 on the volume to pin 7 on V1, of course I've made sure the grounds are good, hoping someone has an idea

 

[King Fan]

For sure I'd suspect the pot; they often have a dead spot at 0, and in some cases are noisy at that spot. Somebody smart may be able to correct me. Or maybe explain how that dead spot (when the wiper makes poor or no contact with the track) could introduce hum. Are those CTS pots, or ??? Not that CTS are immune to the problem, they're definitely not. Are Alphas better? Dunno.

This fault is usually from the factory, but as a side note, experts advise against soldering the bus to the pots, even though big makers and many builders have done it. Their main issue is small ground loops, but mine is (slight) potential heat damage to the pot, but even more, the extra work of installation and triple extra work of pot replacement. If you decide to replace that pot, a star washer does well to maintain case grounding and prevent rotation; then you can just run a bus or wire off the back of the pots. No harm no foul, and I'll say again *if* this is a pot fault, it's probably not from the bus solder.

 

[basso]

For sure I'd suspect the pot; they often have a dead spot at 0, and in some cases are noisy at that spot. Somebody smart may be able to correct me. Or maybe explain how that dead spot (when the wiper makes poor or no contact with the track) could introduce hum. Are those CTS pots, or ??? Not that CTS are immune to the problem, they're definitely not. Are Alphas better? Dunno.

This fault is usually from the factory, but as a side note, experts advise against soldering the bus to the pots, even though big makers and many builders have done it. Their main issue is small ground loops, but mine is (slight) potential heat damage to the pot, but even more, the extra work of installation and triple extra work of pot replacement. If you decide to replace that pot, a star washer does well to maintain case grounding and prevent rotation; then you can just run a bus or wire off the back of the pots. No harm no foul, and I'll say again *if* this is a pot fault, it's probably not from the bus solder.

They are Alpha pots, I use star washers and dremmel the area to ensure good contact, I also dremmel the back of the pot which limits time to solder the bus, I've already replaced the pot and it's exactly the same so I don't think it's the pot but I suppose it could be

 

[AntonyB]

You haven't mentioned if this is with or without any instrument+cable connected?
And is your base schematic EXACTLY a 5F2A?

 

[King Fan]

Helps to know you tried replacing the pot. Not that it couldn't happen in more than one pot, but less likely. And I won't push you on the bus if it's your thing.

Good question re ± cable inserted? Input jacks can also come with short/switch problems. You can inspect the switch prong for contact and separation, but here's a better test:

 

[basso]

You haven't mentioned if this is with or without any instrument+cable connected?
And is your base schematic EXACTLY a 5F2A?

Plugging an instrument in makes no difference, touching the input jacks has no effect, V1a is good, the noise starts at V1b. Yes it's true to a 5F2A other than filter caps are 20/20/10/10 vs 16/16/8/8

 

[basso]

Helps to know you tried replacing the pot. Not that it couldn't happen in more than one pot, but less likely. And I won't push you on the bus if it's your thing.

Good question re ± cable inserted? Input jacks can also come with short/switch problems. You can inspect the switch prong for contact and separation, but here's a better test:

View attachment 1080315

Yup that's all good, the issue isn't the input, it's V1B or after.

 

[Lowerleftcoast]

Ground is not necessarily quiescent. When the vol pot is all the way down there is a direct path (no impedance) from the chassis to pin7 of V1. I suspect a ground loop or ripple as the trouble.

Follow a better ground scheme.

Or you may be able to Rube a resistor on the ground of the vol pot to have some impedance to the ground plane.

 

[King Fan]

Heh, maybe this is a case where the ground loop of bus-on-pot is audible. But you gotta wonder where else ground loops pop up in any 5F2a, and what ground scheme details exist in this one.

 

[schmee]

I've had this issue before, and may have it now on one amp... can't remember. Hums at 0 on volume, goes away and silent at about mid volume pot. More noisy again with volume cranked. Not sure I ever solved it.

This question pops up from people on TDPRI now and then. I wish we knew what it really is.

 

[Lowerleftcoast]

Cut the Green wire and report what happens. (Red arrow.)

EDIT: I see now the terminal the green wire is connected to is not attached to ground... nevermind.

 

[basso]

Ground is not necessarily quiescent. When the vol pot is all the way down there is a direct path (no impedance) from the chassis to pin7 of V1. I suspect a ground loop or ripple as the trouble.

Follow a better ground scheme.

Or you may be able to Rube a resistor on the ground of the vol pot to have some impedance to the ground plane.

I'll give a different ground scheme a shot

I've had this issue before, and may have it now on one amp... can't remember. Hums at 0 on volume, goes away and silent at about mid volume pot. More noisy again with volume cranked. Not sure I ever solved it.

This question pops up from people on TDPRI now and then. I wish we knew what it really is.

Cut the Green wire and report what happens. (Red arrow.)

View attachment 1080353

The green wire that you are referring to is the preamp ground, it is on the non ground side of the terminal so it is solely grounded at the input jack

 

[peteb]

5F2A build, hums at zero but as soon as you inch the volume knob up it goes dead quiet and the amp works perfectly

it sounds to me like the amp works perfectly overall.

how often do you play or idle the amp at volume zero?

 

[basso]

Thanks for all the inputs, so I rebuilt the volume/tone stack and pulled the ground off the pots, new pots etc. The preamp section is single point ground to the input jack. This did not solve the issue.

 

[Lowerleftcoast]

Try a 1M, or so, resistor from the vol pot wiper to the ground side of the vol pot, to try to rule out a weird connection of the resistive element of the pot to the terminal.

Then try a control grid stopper on V1 pin7. Maybe 22k, or so.

Single point ground scheme?

 

[basso]

Try a 1M, or so, resistor from the vol pot wiper to the ground side of the vol pot, to rule out a weird connection of the resistive element of the pot to the terminal.

Then try a control grid stopper on V1 pin7. Maybe 22k, or so.

Single point ground scheme?

Ok good idea, Ok I guess you'd call it a Star ground, all preamp grounds to single point and all power supply grounds to another

 

[basso]

1Meg on the wiper didn't help, 22K grid stopper did the trick!! Works perfect, thank you all for your help!!!