5e3 weird scratchy pot problem...

[Phil_Tre]

Ok, it's a weird one:

I have build a 5e3 about a year ago, working perfectly, it's the Tube Depot kit.
Problem is, when I turn the bright ch volume pot, I can hear a scratchy noise,
like a scratchy pot, but only from 1 to 1 3/4. Also, if there is no signal from the
guitar, than no noise at all. Only when turning the pot from 1 to 2 and plucking
strings.

It's not the pot because I change it to try, I measure no dc voltage at the input
jack (from the guitar cable). Not the tubes, have test every socket with new tubes
multiple times. So what could it be ? Bad coupling cap ? I'm lost...

Thanks for your help !

Philippe

Please, no comments on my Blackface style filament in a Tweedish amp 
The circuit is almost stock, except it sports the Deluxe Reverb transfos.

[PRR]

> I measure no dc voltage at the input jack

But the scratch is at the pot. *Carefully* check for DC at all three pot terminals.

Most likely cause is a leaky cap from plate to pot.

Another possibility is leaky circuit-board or damp flux on the board. (Or dead vermin, but that's obviously not the case in your clean build.)

Another strange one is a leaky *grid* on the *next* tube. Easily checked.

[stratele52]

+1 PRR ,a leaky cap

[sluckey]

Problem is, when I turn the bright ch volume pot, I can hear a scratchy noise...

Also, if there is no signal from the guitar, than no noise at all.

Only when turning the pot from 1 to 2 and plucking strings.

So, it's only scratchy when you turn the pot while plucking. Otherwise, it's fine. How often do you pluck 'n turn at the same time?

[Willabe]

How often do you pluck 'n turn at the same time?

       

[HotBluePlates]

... when I turn the bright ch volume pot, I can hear a scratchy noise, like a scratchy pot, but only from 1 to 1 3/4. Also, if there is no signal from the guitar, than no noise at all. Only when turning the pot from 1 to 2 and plucking strings.

I'm gonna vote for a bad pot, as I'd think d.c. leakage would make it scratchy over the full travel.

Play louder than volume on 1-2. Don't play while turning the pot from 1-2.

Or replace the pot.

[stratele52]

Or replace the pot.

He write ; "It's not the pot because I change it to try,"

[Phil_Tre]

Ok guys, I will check again for DC leak...

It's not a big problem, just annoying when I try
to adjust for low volume... 5e3 volume pots are
very finicky: from 1 to 3 = whisper to scream !!

Thanks !

Phil.

[HotBluePlates]

He write ; "It's not the pot because I change it to try,"

I know, and at first I thought it must be leakage. But what causes a pot to only be scatchy over a very short portion of its travel?

It's not a big problem, just annoying when I try to adjust for low volume... 5e3 volume pots are very finicky: from 1 to 3 = whisper to scream !!

You might want a smaller amp; I'd suggest a 5F2 Princeton, as it is basically a Champ but has a tone control just like the 5E3.

[stratele52]

very finicky: from 1 to 3 = whisper to scream !!

Thanks !

Phil.

You probably have a " B " pot , linear and you need a "A" pot , logarytmic or audio pot . "B" pot  work like you are in full power in first quarter turn .

[stratele52]

. But what causes a pot to only be scatchy over a very short portion of its travel?

 

I see that before and it was a leaky cap

[Phil_Tre]

very finicky: from 1 to 3 = whisper to scream !!

Thanks !

Phil.

You probably have a " B " pot , linear and you need a "A" pot , logarytmic or audio pot . "B" pot  work like you are in full power in first quarter turn .

No, those are audio pots... That's what you get using a 12AX7 in v1 in a 5e3 instead of a 12AY7...

[Phil_Tre]

He write ; "It's not the pot because I change it to try,"

I know, and at first I thought it must be leakage. But what causes a pot to only be scatchy over a very short portion of its travel?

It's not a big problem, just annoying when I try to adjust for low volume... 5e3 volume pots are very finicky: from 1 to 3 = whisper to scream !!

You might want a smaller amp; I'd suggest a 5F2 Princeton, as it is basically a Champ but has a tone control just like the 5E3.

Yes, I will probably build a Tweed Princeton for my next amp. I already built a Tweed Vibrolux and a Tweed Deluxe so maybe a
Black Face next time as I wanted a clean platform for effects, maybe a 1/2 power Twin 

[stratele52]

Yes it is a Audio pot . Schematic here

https://d1sjrnpi226dnf.cloudfront.net/spree/attached_files/tweeddeluxe_schematic.pdf?1382030448

[Phil_Tre]

Yes it is a Audio pot . Schematic here

https://d1sjrnpi226dnf.cloudfront.net/spree/attached_files/tweeddeluxe_schematic.pdf?1382030448

language barrier here, sorry... Yes, Audio pots were installed in my amp. It's just that when using
a 12AX7 in preamp in a 5e3, you got a lot of gain as soon as you turn the pots... It's the nature
of the beast. It's not as bad with the specified 12AY7...

[stratele52]

Yes it is a Audio pot . Schematic here

https://d1sjrnpi226dnf.cloudfront.net/spree/attached_files/tweeddeluxe_schematic.pdf?1382030448

language barrier here, sorry... Yes, Audio pots were installed in my amp. It's just that when using
a 12AX7 in preamp in a 5e3, you got a lot of gain as soon as you turn the pots... It's the nature
of the beast. It's not as bad with the specified 12AY7...

+1