Pro Jr Build - Lowering Gain (and Noise)

[emptyburns]

Newly registered long-time reader here!

I not too long ago completed a Pro Jr hand-wired rebuild using the hoffman board. I used the stock PT and an Allen TO-20 OT. My son wired up and soldered the board, and I did the chassis work to get it up and running.

The donor amp only worked for about 5 minutes after I bought it (many PCB pads and traces were lifted due to poor DIY "upgrades"), so I don't really remember what it's supposed to sound like. This amp sounds very good with a lot of nice crunch to it, but that's about all it has. It breaks up immediately with humbuckers, and pretty quickly with single coils. It also has a good deal of hiss/noise, but I believe that's a common problem with the stock amp too.

I double and triple checked everything, and all seems to be done right. Voltages check out to within 3-5% of those labeled on the schematic, and it's biased up to just over 60%. There are no obvious problem spots when chop-sticking around, and different tubes don't seem to change much. Layout is clean with short wires and good lead dress. Volume pot is a proper 250k audio taper.

Is this amount of gain/headroom I should expect? If I wanted to lower the gain, any tips? I'm probably going to try a split load on the plate of V2-B like a 6G3. I'll also try some lower gain tubes, but a 5751 in either slot didn't seem to do much.

Thanks for any tips or suggestions

[Latole]

"..It also has a good deal of hiss/noise, but I believe that's a common problem with the stock amp too. "
-emptyburns

No, amp should be quiet with nothing plug in.

Look for wires dress issue, poor or missing ground.

Good pictures of the wiring may help.

[emptyburns]

Thanks for taking a look. I would like to improve the noise/hiss, but the headroom is the primary concern or area for improvement right now.

Earth ground is bottom left of the transformer. Power section is grounded to the upper right lug of the transformer. I have a ground bus on the back of the pots for the preamp, connected to the input jack. I do *not* have the pots isolated from the chassis (fiber washers ordered), but if I leave them hanging from the chassis, it does not improve the noise, so I don't think that's the main issue.

[dude]

Separate the PS grounds from the preamp grounds. Ground the PS caps , PI caps to a PT lug or a chassis ground near the PT. The buss on  the back of the pots should go to the ground on the input jack, pick up the pre amp cap ground along the way too, close to the input jack. Don't worry about isolating the pots, also for good measure run another wire from from the input jack ground to the chassis near the cathode caps for the first stage, ground the cathodes of the first 12AX7 here too.

[emptyburns]

Thanks for the reply, dude! I have the grounds separated that way, and just did a quick double check with the dmm on continuity. All checks out. This is also how it's set up on the layout: https://el34world.com/Hoffman/files/Hoffman_Pro_Jr.pdf

[shooter]

your filament wires should be moved against the metal lip, laying them against the metal, that get's them far away from signal wires.
put the pots back, they are already "isolated" from their housing, the metal housing acts as a shield for the guts.


the board looks pretty splattered, take some denatured alcohol and a tooth brush, those splats have a nasty way of finding just the right place to bread gremlins.

[emptyburns]

Thanks shooter, good feedback. I scrubbed it down and it’s pretty shiny. It was mostly flux but a couple bits of solder.

I didn’t leave the pots hanging, it was just a test to see if I could find a ground loop.

I subbed out a 5751 in v1 and improved the headroom situation. Still pretty noisy thought. The noise reduces when I pull v1, and goes silent when I pull v2 (PI), so it’s not in the power section. 

Thanks for all advice! If anyone else has built this, any tips?

[JB]

The originals of these are noisy.  I've got one I bought in '94 not long after they came out and I've done all manner of mods to it to make it quieter.  I've considered putting in a Hoffman board in the hope it would reduce the remaining noise and you're worrying me now! 

What sort of noise are you left with - hiss, hum (50/60 or 100/120?) buzzing?
You heater wiring could definitely move in next to the chassis lip as stated.  But if it's hiss, or PSU hum that's not going to help.

[Latole]

The wire should be as short as possible.
Particularly the wires of the lamp grids.
The wires should be as close as possible to the metal frame. **
They should not be the least close to each other if there is a parallel.
The wires must cross in 90 degrees

Your filaments should be kept away from all other threads; here raised.

Like this Fender amp:

[bmccowan]

All of the suggestions seem like good ones. Assuming that you have tried swapping all tubes, in addition to V1&2? If so and if none of the other changes get you there, I would set up a listening amp. Since you will be chasing hiss/hum, you do not need to generate a signal, you would just need to get it humming and probe the signal chain and hopefully locate where the noise is being generated. If you do not have a listening probe, there are good instructions from Doug. I would not accept defeat based on a belief that it's just a noisy model. If the noise is loud enough to be annoying, something is not right IMO.
And - I've not heard a Pro Jr. but looking at the schematic, I don't see any reason that it would be a super high gain amp. NFB would be the first place I'd look to make sure it was right, and maybe reducing R27 to increase the feedback.

[vampwizzard]

the videos of these pro juniors show that its really quick to break up and that its really dependent on the tone control.

Really broken up with the volume at noon: https://www.youtube.com/watch?v=TOCvbQjz6Ns

Similar, but with more walkthrough the settings: https://www.youtube.com/watch?v=JnasQD1qF54


The PCB versions of these were notoriously hummy. There should be ways to mitigate it though especially with what the other folks have suggested here.

[emptyburns]

Thanks everyone - I really appreciate all the replies and help. I've spent a few more hours on it last night and this morning without much improvement.

Filament wires have been adjusted against the chassis. Solder joints have been examined with a magnifying glass and touched up if they look suspect.
Grid wires have been adjusted, making sure that they're either fully away from other wires, or crossing at 90deg.

I've swapped out at least 3 different (new) tubes in v1 and v2, and three different el84 sets.

I also tried clipping in a bypass cap across v1a and v1b cathodes to see if there was any signal being picked up there, but there was very little change.

I think I'll build a listening amp, as this isn't the first time, nor will it be the last time I struggle with noise.

Thanks again, and any other tips (or experience with building this) would be appreciated

[scstill]

I have two PJs.
One is US tweed one is Mex black
I like playing them stereo out of pedal board
Both are rather noisy, but I like the way they sound and don't notice while playing.

I have done all the classic PJ noise reduction mods with limted success.
I had purchased a couple of 100uf filter caps but never put them in.
Somewhere along the line I heard that this might help.

I did add an Lpad to the tweed so I can get natural power tube overdrive at low volumes. Lpad is rated at 50w but still gets warm. Does sound really good with this. But when Lpad is past "1" it gets really loud again, thinking its linear so if anyone knows where I can get an audio taper would appreciate advice. The PJ needs to be above 6 or so to start overdrive which is really loud. Lpad tames this so the wife and neighbors are still friendly...
https://www.parts-express.com/parts-express-speaker-l-pad-attenuator-50w-mono-1-shaft-8-ohm--260-255

[ac427v]

You have been very persistent with this amp. You used good quality components for low noise. The solution has to be close! I don't see any obvious build problems in your photos. Never played or worked on a ProJr. But those owner reports cause me to think the excessive noise is due to the design and not your construction techniques. Guitar mps have the least hum and hiss when the design makes the maximum possible clean gain from the first stage. That minimizes the need for signal gain (and resulting noise) in the following stages. The ProJr does not follow that philosophy. If is was my baby, I would add a 22uf cathode bypass cap to R4 in the first stage, and change the value of the R11 cathode resistor in the second stage to something in the range of 3.3k to 10k. Basically making the Marshall-style "cold clipper" stage. If those values result in too much bass distortion, I would change the C1 coupling cap to .002uf. Then tweak to get the clean/distortion balance right for your preferences.

[emptyburns]

Thank you ac427v. I'm convinced it's something stupid staring me in the face. I think I'll walk away from it for a few days/weeks and look again with fresh eyes (and resolve!).

I like your idea for potential modifications. The tone of the amp right now (noise aside) is quite good with the 5751, but it'd be fun to try something different. Totally understand about clean gain in the first stage - I'm a former EE/RF engineer (turned software guy), and low noise and high gain in the first stage was definitely a primary design goal...

[JB]

If you look at the Fender schematics, later versions of the Pro Junior have a 47uF cathode cap on V1A.  I've tried this on mine, made it switchable.  It's useful for earlier breakup with single coils but I find it too much with humbuckers or P90's.   

I found a hum balance pot on the heater supply helpful for noise reduction.  The theory is with these that a fair amount of the noise is rectifier based and gets in via the heater circuit.  However you'd expect that to be layout related so may or may not be of use for you with a Hoffman board.