Fender Blues Jr.....

[bsco]

Got a question for somebody....have a scratchy noise in this unit when the treble and, or the bass control is varied back and forth with no signal....when you input a signal such as a guitar, the treble control actually affects the volume as well as the bass response...the bass control also affects the volume and the treble response.....the mid control appears to be fine...I cleaned the controls.....replaced c6 in the tone ckt...same thing....dis-connected the end of C5 that connects to the treble pot...to check for DC out of the cap.....no DC.....both pots still make this scratchy noise.....anybody come across this type of problem??
Cheers
Bernie

[52 Bill]

...to check for DC out of the cap.....no DC...

Is there any DC on the input grid of V2B?

[bsco]

Thanks for the reply 52......there is about 1.7Vdc and it can be adjusted with the treble or bass control...I can bring it down to under a half volt but can't eliminate it completely with adjusting these two controls....

[TimmyP1955]

Clean it again. I've had a couple that took three cleanings to get them to shut up.

[bsco]

Clean it again. I've had a couple that took three cleanings to get them to shut up.

I did that twice....first I used a contact cleaner with lubricants...the second time I used De-Oxit.....I think it was De-Oxit 5......it made no difference at all......
Cheers
Thanks for the tip....by the way,this is new....this isn't used

[Jazz P Bass]

What is the plate & cathode voltage of V2B?

[52 Bill]

Try a different tube in V2?

[Enzo]

Doesn't matter how clean they are, pots with DC across them will make scratchy noises when turned. Get rid of the DC and they will be quiet.

[bsco]

Hi Jazz P...Checked those voltages..plate is 103V.....grid was at 1.35V and cathode at 1.32V....got to thinking when there was no DC bleeding from the caps....and 52 Bill made the suggestion to try another tube......so I did...and the problem went away....so....this brand new tube was causing a dc voltage to be leaked back to the grid element....when I re-checked the voltages, here is what I found...plate at 144V......grid at 0V and cathode at 1.32V......controls are dead quiet.....only goes to show.....the most weirdest and odd ball problems can be caused by the simplest of things....I never thought a tube would cause this problem......I learn something new every day.....thanks for all the help guys.....
Cheers.......

[Jazz P Bass]

You had to wonder where that grid voltage was coming from, if not the tone caps, then the tube itself.

I am glad that to hear that you got it straightened out.

[52 Bill]

In the olden days, you rarely found 12AX7 tubes that would leak like that, it was almost always leaky coupling caps. In the past few years, I've seen maybe 5-6 of them. Still not a huge number percentage wise, but enough to make me swap tubes as a quick test, after cleaning the pots.

Glad you got it figured out.

[bsco]

Thanks Jazz P.....I was wondering what would cause it....in the back of my mind I was leaning towards the tube...but was not sure.....and when 52 Bill suggested that, I realized that when looking at the schematic, there were not many components that would cause it...so thank you to you and 52 Bill......
Cheers.....

[bsco]

In the olden days, you rarely found 12AX7 tubes that would leak like that, it was almost always leaky coupling caps. In the past few years, I've seen maybe 5-6 of them. Still not a huge number percentage wise, but enough to make me swap tubes as a quick test, after cleaning the pots.

Glad you got it figured out.

Yes...now I know what to look for when I have that sort of noisy control problem with anything else.......Thank you very much......
Cheers

[g1]

In the olden days, you rarely found 12AX7 tubes that would leak like that, it was almost always leaky coupling caps.

My experience has been the same. The idea of an hugely over-conducting or outright shorted preamp tube is a new phenomenon for me. But I'm reading of it often enough to know it's no longer the extreme rare occurrence it once was.