Noisy Pro Junior

[sean200000]

I'm trying to fix up an old version 1 Fender Pro Junior that has basically been sitting in a cupboard for 20 years. It is really noisy, with the volume above about 2 its basically unusable because of a loud water-rushing type sound.

So far I have tried:
Removing the top middle screw on the back panel (this helped a lot with the base level hum)
Twisting the green heater wires (made no difference)
Replacing the first filter capacitor with a 100uf capacitor and replaced all the other filter capacitors (maybe a little improvement)
Swapped both preamp tubes with a number of different 12AX7 and 12AY7s (no change)
With the first preamp tube removed the noise goes away, but its not the tube as I have tried different ones in the same spot.

I know the Pro Junior has a reputation as a noisy amp which i'm fine with but this one is way noisier than it should be.

Any ideas?

 

[SnidelyWhiplash]

If you have it open, please post some pics. There are a lot of very good techs here who could help diagnose problems.

 

[dougstrum]

You have done the filter caps and swapped tubes, good steps.
Do you have retainers for the el84s?
Have you cleaned the sockets are the pins tight?
Look for loose components on the board. Start with the solder joints of your new filter caps.

 

[sean200000]

Here it is. Excuse the messy solder joints, i'll try and reflow some of them.

 

[InstantCoffeeBlue]

Let me know what you find. I have a blonde one from the mid 90s or so and it is frustratingly picky about tubes. Right now, the best combination I've found is JJ 12AY7 in V1, 7025EH in V2 - still have some hum, but this combo eliminated the "rushing water" sound, so this is the one I'm sticking with for now. I recently had mine serviced as well.

 

[Nicko_Lps]

Any ideas?

Did you try swapping out Tubes and test it out?
Ive seen a youtube repairsman stating this amp is running EL84's really hot and wears them out pretty fast.

 

[dougstrum]

If you change R29 in the bias supply from 15k to 20k it will get those el84's lasting longer and running cooler without sacrificing any tone.

 

[lammie200]

I sold the one that I had but I changed the output transformer to Mercury Magnetics and used lower gain preamp tubes in the same family as the stock tubes. It was still plenty loud and it was probably the best sounding Pro Jr. that anyone could want.

 

[sean200000]

Let me know what you find. I have a blonde one from the mid 90s or so and it is frustratingly picky about tubes. Right now, the best combination I've found is JJ 12AY7 in V1, 7025EH in V2 - still have some hum, but this combo eliminated the "rushing water" sound, so this is the one I'm sticking with for now. I recently had mine serviced as well.

Thanks, I will definitely try that combination

 

[sean200000]

If you change R29 in the bias supply from 15k to 20k it will get those el84's lasting longer and running cooler without sacrificing any tone.

Yes i've already changed that resistor, it was the first thing I did

 

[sean200000]

I sold the one that I had but I changed the output transformer to Mercury Magnetics and used lower gain preamp tubes in the same family as the stock tubes. It was still plenty loud and it was probably the best sounding Pro Jr. that anyone could want.

What preamp tubes did you use?

 

[LightningPhil]

You could try replacing all the resistors in the input section with metal film type.

Gain can be reduced by removing the cathode bypass capacitor rather than using lower gain tubes. Try removing the one in the second gain stage first. The amp should still have plenty of gain. But if you want to recover some of it, then there's a resistor in series with the volume pot that can be reduced in value considerably. (if i'm remembering the circuit properly)

 

[vampwizzard]

water rushing is usually a preamp tube. try replacing them replace the power filter caps. Then we start troubleshooting.

 

[lammie200]

What preamp tubes did you use?

12AY7

 

[InstantCoffeeBlue]

Thanks, I will definitely try that combination

Hope it works for ya! Mine has had filter caps replaced, heater wires + lead dress tidied up, and a bias pot added, and even still, with the wrong tubes the noise can render it almost unusable. It really seems to come down mostly to this particular design being very fussy about preamp tubes, and even when you get it figured out, it's still going to have a noise floor significantly higher than a maintained BF or SF Fender. Like I said, mine still has a bit of hum but I've got it to where it's nothing you'd notice on a bandstand, and honestly the amp sounds so good it's worth it.

 

[sean200000]

You could try replacing all the resistors in the input section with metal film type.

Gain can be reduced by removing the cathode bypass capacitor rather than using lower gain tubes. Try removing the one in the second gain stage first. The amp should still have plenty of gain. But if you want to recover some of it, then there's a resistor in series with the volume pot that can be reduced in value considerably. (if i'm remembering the circuit properly)

I don't think it has cathode bypass capacitors, according to the schematics they added one in later versions parallel to R4.

When you say resistors in the input section do you mean from the input to the grid of the first preamp tube or all the way to the power tubes?

 

[sean200000]

Hope it works for ya! Mine has had filter caps replaced, heater wires + lead dress tidied up, and a bias pot added, and even still, with the wrong tubes the noise can render it almost unusable. It really seems to come down mostly to this particular design being very fussy about preamp tubes, and even when you get it figured out, it's still going to have a noise floor significantly higher than a maintained BF or SF Fender. Like I said, mine still has a bit of hum but I've got it to where it's nothing you'd notice on a bandstand, and honestly the amp sounds so good it's worth it.

Yeah my SF Princeton is almost 50 years old with original caps and its dead quiet, I wish they were all like that.

 

[InstantCoffeeBlue]

Yeah my SF Princeton is almost 50 years old with original caps and its dead quiet, I wish they were all like that.

My 75 Princeton has had some caps replaced, but yeah - silent at idle.

 

[LightningPhil]

I don't think it has cathode bypass capacitors, according to the schematics they added one in later versions parallel to R4.

When you say resistors in the input section do you mean from the input to the grid of the first preamp tube or all the way to the power tubes?

My one must be a later version then.

All resistors can be a source of noise. Wire wound are least noisy, then metal film. I generally use these in all small signal applications.

If the noise in your amp is random/pink/white spectrum, blame the resistors.

Totally possible to keep the origionals if carefully removed, when swapping in newer ones for an experiment.

 

[sean200000]

My 75 Princeton has had some caps replaced, but yeah - silent at idle.

Actually mine may have had the filter caps replaced but it would have been a long time ago before I owned it.