Volume pot noise

[rhaigh]

Something i noticed recently with my Northland tube amp...

When the volume 0, there is no noise. As I increase it, a buzzing noise grows and is loudest center pot. From center, to turning all the way up, the noise gradually goes away.

I disconnected point 'B' on the shcematic and the noise goes away.

I reconnected point 'B', and disconnected and grounded point 'A' and the pot gets noisy again in the same way. No noise from around 0-2 and 8-10.

Any ideas? My guess is a grounding issue as this amp is not star grounded.

I tried another brand new pot but it doesn't help.

[Helmholtz]

First of all, disconnecting the grid (point B) is no good idea, as it leaves the tube without bias/ground reference.

Is this a newly developed problem?

Did you try a different tube?
What happens if you disconnect the pot from the grid at point B and replace it with a 1M resistor between grid and ground?
Are the filter caps good?

Do you have a scope?
It would help to know if the fundamental of the buzz is 60Hz (50Hz) or 120Hz (100Hz).

I assume the amp works fine otherwise?

[catalin gramada]

Check this...

[nevetslab]

So, this is a 1 Meg Linear Taper Pot, if the 'B 1M' label on the schematic is correct. Though if you have the pot at 50% rotation in the photo, and the black lead of your meter is at the Ground connection (can't see that contact), then it look more like a Log Taper, that reading being 10% of the pot's resistance. At any rate, as far as noise and stray field pickup on pots, it's worse around 50% rotation. At that setting, you have a lot more resistance added into the circuit, so the source impedance is highest. At CCW setting, you've grounded the grid of the following tube, and at CW, you're at the source impedance of the tone circuit. With the pot disconnected, and grounding Point A (top of pot), and pot is rotated to around 50%, you again have high source resistance, so there is field pickup from your amp's circuit. We'd guess this is AC Mains stray field from the power transformer, or perhaps wiring that is carrying AC field. Can you provide photos of the inside of the amp, showing the heater wiring, tube circuit wiring, control wiring, power transformer location?

[g1]

Though if you have the pot at 50% rotation in the photo, and the black lead of your meter is at the Ground connection (can't see that contact), then it look more like a Log Taper, that reading being 10% of the pot's resistance.

The photo is an example for a different user, not the originator of the thread.

[Chuck H]

I'm thinking filter caps. I think Helmholtz is too. The halfway point on the volume pot would produce the most noise to signal ratio if there's hash on the power supply. That's if it's a linear pot though. "B" doesn't always mean linear. IIRC I think there are some old Euro pot designations where B actually means log.?. In either case you should still have lowest noise at zero or full on the volume knob if that noise is on the power supply.

[J M Fahey]

1) Poor/no shielding between pot wiper and next tube grid.

2) cut a piece of shielded wire long enough to go from pot wiper to next tube grid plus, say, 1" extra for slack.

3) peel and tin both hot ends and both screen ends.

4) connect hot from wiper to grid.

5) this is important: "ground" is not "ground" anywhere.
Ground "here" is not the same ground "there" , 1 inch away and onn the same conductor or piece of chassis
Ground is DIRTY.
Ground "seen" by tht pot is NOT the same Ground "seen" by that tube.
SO:

6) lift/disconnect pot ground lug from wherever itīs grounded now and leave it floating.
Solder that screen end to lug.
Solder other screen end to where next tube 1k+30uF are grounded.
Both (cap and resistor) lugs should be grounded together to exact same spot; if not move one as needed.

PS:curious about that Piezo reverb and *how* piezo crystals are attached to springs.
And how spring is actually mounted.
Could you take a couple pictures?
Specially a closeup showing spring end, the piezo transducer and spring mounting.
I viewed Uncle Dougīs video and reverb is quite reasonable, I was surprised.
Thanks.