Princeton Volume Pot Hiss

[Ed_Chambley]

Modded Princeton with Paul C and Stokes.  Volume pot hisses when all the way down.  Gets quite from about 200k ohms to 700k ohms and after begins to hiss again and gets louder all the way to 1 meg.  Treble pot will add some hiss when dimed.
Replaced pot and tubes, no change.  I am thinking I have a bad resistor, maybe?  The #1 input is very sensitive.  Posting to find out is anyone has had this experience and where this could be coming from.  Pulled the reverb tube, still hisses.  I am assuming it may be prior to the tone stack.
Some input would be appreciated before I begin pulling and checking resistors as to what area the origin may be.
Other mode were changes in the first 2 bypass caps from 25uf to 2.2uf on both.

[plexi50]

Did it hiss before doing the Paul C or Stoke mods?

[Ed_Chambley]

Did it hiss before doing the Paul C or Stoke mods?

No, it did not.  I can understand the higher volume hiss, but the low volume is what is got me confused.  Also in the middle it is very quiet.
It also did not hiss when I completed the job.  It started later.

[plexi50]

Did you try moving the wires around with a stick? Sounds like an osscilation some where near the power supply end. Do you have any pics?

I love my Princeton stock so much. I put a Jensen C10Q in mine for a tad more headroom. The clean and breakup is awesome

[Ed_Chambley]

Did you try moving the wires around with a stick? Sounds like an oscillation some where near the power supply end. Do you have any pics?

I love my Princeton stock so much. I put a Jensen C10Q in mine for a tad more headroom. The clean and breakup is awesome

I did move the wires around with no change.  If I touch the wiper of the volume pot with my meter, it stops.  I did not have time to check much else as it is not loud enough to keep the guy from playing the amp, but it still needs repair IMO.  No photos as it is not my amp and is gone with the owner.  It will return on Friday.  I would say a plate resistor on v1, but I have never had one to cause a hiss at "0" on the volume pot.  I will find it when he brings it back, I was just wondering if anyone had ever ran into a volume pot acting this way.  I have known lots of people who have done the stokes and paul c mods and have never heard any complaints of hiss.

Have you ever encountered an oscillation which will disappear completely when the volume is between 200 and 700 ohms?  I mean it is dead quiet until you turn it almost all the way down and all the way up.  In-between is quite, very quiet.

BTW, I did not mod mine.  I did change a few cap values and it is working fine.  I am using a 2 10's, Celestion Gold and Vintage 30.  It is plenty loud for a princeton.  I already have a blackface deluxe.

[HotBluePlates]

If I touch the wiper of the volume pot with my meter, it stops. 

It almost sounds like a bad ground on that pot, as though when you probe with the meter you're providing a ground reference that otherwise wasn't there.

This is where a listening amp would really help. You could use one to see if the hiss is present ahead of the tone circuit or only at the volume pot wiper.

[Ed_Chambley]

If I touch the wiper of the volume pot with my meter, it stops. 

It almost sounds like a bad ground on that pot, as though when you probe with the meter you're providing a ground reference that otherwise wasn't there.

If I remember correctly, probing the wiper is the same as the Control Grid on v1b.  I think there is a direct wire to that pin.  Since the mods do not change the plate voltage on v1, I question the plate resistor being the problem.  Still could be as they are old.  Doesn't explain why it has a hiss at zero volume.  If turning up the pot reduces the hiss the my thinking is a resistance problem.  I may be way off here, but I plan to check grounds and resistors.  I also need to check all the voltages to see if anything has changed since initially doing the mods.  Reversing the PI mods only takes a few minutes.  The only thing I can think of here is the 4m7 as I used a 1/2 watt and the rest of the amp looks like it has 1 watt resistors.  Well a mixture of different types.

The pot is what is got me scratching my head.  I have never heard a hiss increase at zero volume.

[HotBluePlates]

My thinking is that if there was a good-quality ground at the bottom end of that pot, then with the wiper all the way down any hiss at that grid or before would be shunted to ground.

You also mention having changed the pot, and if this is an old amp, you're unlikely to have a 40-year old pot with the same defect as a new pot.

*If* it's an old amp, even if you changed the pot, you likely didn't mess with where that pot had a wire soldered to the brass strip, or mess with the brass strip itself (which would require all the pots to be unbolted from the chassis).

However, I'm not confident that it is a ground problem, which is why I brought up the listening amp, so you wouldn't go trying things in the dark.

[Ed_Chambley]

I have been needing to make a listening amp anyway.  I have a solid state amp I put aside for doing this, just haven't done it yet.  Never needed one til now.  It is a simple thing to make.  Anyway, I will find it and report back what it was.

This is an old build, not really a fender chassis.  It doesn't have a brass plate, just a steel chassis.  It is wired the same, just in a square chassis mounted in the top of a cabinet large enough for a 12 inch speaker.  It sort of looks like an old peavey amp as it is all black.  It has a stock eyelet board, but the chassis is huge.  It has CC, CF and metal oxide resistors.  Really the hiss is not that bad, but the amp sounds so clean when dimed, I want to get it quiet for the guy.  He doesn't care as much about as I do.  Anytime something weird comes along, I have to find out what it is.

It reminds me of an amp I built out of an old 50's small console tv.  I flipped it over and removed the legs.  Mounted a chassis where the old one was and put a baffle where the tube was.  Kept the maple wood.  Turned out nice as it is a deep cabinet closed back with a 2.5" ported hole.  Guy that bought it from me uses it all the time.  It is a conversation piece.

[Ed_Chambley]

Ran a new teflon wire from the wiper under the board to v1b.  Silenced the volume pot.  Moving it around made no change.  Don't really know why or how it was causing a hiss as that section of the amp has no change in voltage.  It was running right next to the plate resistor, but the resistor is fine.  I guess sometimes you never know the cause.