Noisy Pro Junior

sean200000

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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.
Ok thanks, i'll check them out
 

LightningPhil

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After a bit of digging about, nope, my pro junior is quite old.

Here's a link to some heavy mods made to it:

Not noisy at all now, but also not a pro junior anymore.
 

vampwizzard

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So, looking at the photos and rereading the thread: did you replace Illinois filter caps with Illinois instead of going with something like f&t?

The next thing after filter caps I’d look at is reflowing the solder joints on the tubes, and potentially spraying them out with contact cleaner and a brush and retensioning the pins.

From there, I’d be looking at the other electrolytics that are in there. The design itself is noisy because they sacrificed noise floor for their gain staging. Lower value V1s can help but they are all noisy via design flaw.
 

sean200000

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did you replace Illinois filter caps with Illinois instead of going with something like f&t?
No they are Evatco branded caps which is a retailer here in Australia. They are one the cheaper side definitely but at least they are new.

The next thing after filter caps I’d look at is reflowing the solder joints on the tubes, and potentially spraying them out with contact cleaner and a brush and retensioning the pins.
Yes, i'll try cleaning out the tube sockets next.

Lower value V1s can help but they are all noisy via design flaw.
I did try a 12AY7 i both V1 and V2 but it didnt seem to make much difference. I'll try getting a 7025EH as recommended earlier in the thread.
 

Leewar

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I too had a projr that was very noisy. Looks like the same early version. It is now quiet as a mouse (same stock board). Although looks like somone has done some mods on your board. Here is what I did to quiet the beast:
1. Replaced the filter caps. Used F&T.
2. Replaced the power tubes and check the preamp tubes. The power tubes were the cause of most of the noise in my case.
3. Twisted the heater wires (green). See that has been done.
4. Twisted the B+ (red) wires.

I would also replace the filter caps with the correct value and quantity. I would check for other mods, I see a silver mica cap that is non standard. Not saying that is an issue but there may be something else that was done.

If all else fails you can go to https://el34world.com/schematics.htm#Hoffman_Pro_Junior_Conversion and build a handwired verion.
 

LightningPhil

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If the schematic reflects your amp...

Suggest removing C22, C2 and R6 then replacing R5 with 47k to 100k metal film.

Replace R4 and R11 with wire wound of same value.

Replace R1, R2, R3, R9 and R10 with metal film of same value. Making sure R3 and R10 can handle the power.

All gain stages have noise. The pro junior seems to amplify the input massively, then also massively attenuate it before amplifying it a huge amount again. So any noise takes that ride too. By reducing the gain of the first stage (removing the bypass cap), some of this can be avoided. The lower noise resistors will likely help too.

For a bit more mains filtering, increasing the value of C17 and C18 may have some effect also.

Further reducing gain is easy. R4 and R11 can be increased in value a bit, increasing from 1.5k to 3.3k could well drop gain by about 4.5 dB. On both, this would be 9dB and perhaps be taking things too far.

BTW, removing the C22 cathode bypass cap likely reduces gain by 5dB too.

Dropping the gain quite a bit will mean less noise and being able to turn the volume knob further up. It's inexpensive and totally reversable.
 

sean200000

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If the schematic reflects your amp...

Suggest removing C22, C2 and R6 then replacing R5 with 47k to 100k metal film.

Replace R4 and R11 with wire wound of same value.

Replace R1, R2, R3, R9 and R10 with metal film of same value. Making sure R3 and R10 can handle the power.

All gain stages have noise. The pro junior seems to amplify the input massively, then also massively attenuate it before amplifying it a huge amount again. So any noise takes that ride too. By reducing the gain of the first stage (removing the bypass cap), some of this can be avoided. The lower noise resistors will likely help too.

For a bit more mains filtering, increasing the value of C17 and C18 may have some effect also.

Further reducing gain is easy. R4 and R11 can be increased in value a bit, increasing from 1.5k to 3.3k could well drop gain by about 4.5 dB. On both, this would be 9dB and perhaps be taking things too far.

BTW, removing the C22 cathode bypass cap likely reduces gain by 5dB too.

Dropping the gain quite a bit will mean less noise and being able to turn the volume knob further up. It's inexpensive and totally reversable.
Awesome, thanks!
 

OldPup

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Lots of guessing going on here. You really narrowed the problem down when you said that with V1 removed the noise goes away. This means the problem is in the circuitry connected to that tube. So the first two gain stages. Only look at components to tweak or replace there. I’d start by cleaning the V1 tube pins and socket. A friend of mine had an unplayable 1968 Bassman and all I did to get it functioning well again is clean tube pins and sockets and tighten up nuts to be sure of good mechanical ground connections.
 

tubeToaster

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I remember years ago one of the noise fixes on the Pro Junior was moving the purple negative feedback wire. It didn’t have to move much to make a difference. You might want to give it a try. It worked on mine.
 

mojodelic

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Just out of curiosity is the noise present with the back off (as in your first picture)? Not sure about PJs but BJs had a lot of problems with ribbon cable lead dress which caused noise. I'll spare you my potentially lethal plate voltage sermon; sounds like you know what you're doing but with the back off, well...
 

mretrain99

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I’ve got an early-‘90s blonde tolex Pro Junior that I’ve never had maintenanced at all, and it was really noisy, but I replaced the tubes with a pre-matched JJ tube set for Pro Junior/Blues Junior, and luckily that did the trick for me. And that was right before the Ukraine war, lucky timing.
 

Wiley

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I (still) have one from the first year, 1994 I think, with the blue alnico speaker. It was an impossible amp ready to jump out of its tweed. I first tried a 12AY7 in V1 which did little or nothing. I did find this video, eventually, learned about the quirks of the amp, had the mod done by a tech and now it's playable and very nice sounding. The bias trim pot allows a ton of flexibility with new power tubes, single coil vs. humbuckers, etc.

 

sean200000

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Just out of curiosity is the noise present with the back off (as in your first picture)? Not sure about PJs but BJs had a lot of problems with ribbon cable lead dress which caused noise. I'll spare you my potentially lethal plate voltage sermon; sounds like you know what you're doing but with the back off, well...
I did spend some time moving the cables around with the back off but I couldnt hear a difference. I also twisted the heater wires which didnt seem to help either.
 

sean200000

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I (still) have one from the first year, 1994 I think, with the blue alnico speaker. It was an impossible amp ready to jump out of its tweed. I first tried a 12AY7 in V1 which did little or nothing. I did find this video, eventually, learned about the quirks of the amp, had the mod done by a tech and now it's playable and very nice sounding. The bias trim pot allows a ton of flexibility with new power tubes, single coil vs. humbuckers, etc.


Yes I watched the video but I ended using a fixed 20K resistor for R29 as recommended by a number of people.
 

sean200000

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OK, so after replacing R1, R2, R3, R9 and R10 with metal films, increasing the size of R4 and R11, as well as cleaning the tube sockets and reflowing their solder joints its sounding ... much better!

Thanks so much to everyone for your help
 




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