Pro Jr amp hum problem

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by MAZ, Apr 18, 2005.

  1. MAZ

    MAZ Member

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    I just got a Pro Jr and I have a problem that maybe one of you guys can help me out with-

    the amp was shipped to me, and was packed well, no cosmetic damage at all.

    But when I turn it on, it has a decently loud buzz (even with vol. on 0 and nothing plugged in the input). It also passes signal (quietly) at volume set at 0.

    Then as I raise the volume, at around .5 the signal gets a bit quieter and then gets louder as normal. The amp sounds great otherwise. When playing I don't notice the buzz.

    I've reseated the tubes, and swapped out the EL84's and the 12AX7's with known good tubes, but no change.

    As I raise the volume up to 10, I hear the normal amp noise I would expect- some hiss, a little hum, but different sounding. There is no change in the "problem" hum at different volumes of the amp.

    The pot is a bit scratchy too, could be a bad vol pot (which is why it passes signal at 0) or just bad connection of the pot since it's not point to point...

    One final thing- the EL84's get screamin' hot after only a few minutes. I'm pretty sure that's normal though, I have another EL84 amp and they are raging after a short time too.

    any thoughts? Thanks!
     
  2. Greggy

    Greggy Member

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    MAZ, I seem to recall hearing how the earlier pro jrs. hummed because a screw on the back panel was made too long and touched an interior part creating a ground loop or something similar. Not sure if this applies to your amp, but you may want to research it on the web. This issue is discussed on the Harmony Central amp review I believe.
     
  3. Greggy

    Greggy Member

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    After further review, seems that on "some" models, the middle screw on the back panel contacts one of the pots, creating a ground loop. Back out the screw a couple of turns and it's fixed. It's in the HC reviews. Hope that helps.
     
  4. sandpiper

    sandpiper Silver Supporting Member

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    MAZ,

    I also have a Pro Junior that hums all the time, which sucks because my main purpose for the amp was recording. I'll try that screw thing, hope it works!
     
  5. MAZ

    MAZ Member

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    Yeah, I found the screw trick- it didn't work!

    I switched the preamp tube to a Mullard 12AT7 and the hum is pretty much gone. I actually think I'll like having a lower gain, so I can use pedals with it easier.

    But I'd still like to know how to clean it up more with the AX7's in there as well.

    From all the research I've done on the web about it, it seems that it's pretty much a crap shoot with these amps.

    I want to use mine for recording as well, so it's got to be pretty quiet!

    Thanks for these replies. If anyone else has info or ideas, I'd love to hear them!
     
  6. claptonisgood

    claptonisgood Member

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    i bought a new Pro Jr last year with the intention on making it a project amp...One big problem i found was that the filter caps that are stock are really poor quality. Off the top of my head, i think they were showing something in the area of several volts of AC ripple. I replaced them with Sprague Atoms and the ripple went well below a volt. The Atoms are much larger and required some imagination and hot melt glue to install.
     
  7. MAZ

    MAZ Member

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    I wondered about that- did it quiet down as well?

    I think this amp is totally worth a few bucks of customization...
     
  8. claptonisgood

    claptonisgood Member

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    yes, it helped quite a bit in reducing the hum...with tube swapping, i got it to be virtually quiet...i also reverted the circuit back to the original early 90s spec...there is a lot of PS foolishness that was added as well as a change to the tone stack and a couple other minor points...i also added a bias control because Fender has the EL84s roasting for no really good reason...i built a 1X12 combo cab for it about the size of a tweed Deluxe, covered it in Marshall purple tolex with Marshall cane grill...very cool amp and loud!
     
  9. Third Stone

    Third Stone Member

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    Claptonisgood, hey man I would love to see a picture of your amp. I love my PJ and have considered doing something like putting it in a bigger cab to get it pushing a 12". If you have a pic and can post that would be great...or email it to me? I would really appreciate it. I think the PJ is an amazing little amp, and record with it almost daily(thru a 2x12 Germino cab.)
     
  10. esoteric pete

    esoteric pete Supporting Member

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    i looooooooove my pro jr.! great little amp, but it does hum sometimes...but mine isnt overbearing, so i put up with it.

    now you guys have me thinkin that i need to mod and customize it! damn you!!!
     
  11. tennisplayer

    tennisplayer Senior Member

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    I bought a Pro Jr. last September. At idle it hummed so loud. Dougs tubes suggested replacing the pre amp tubes with Electroharmonix which were known to be quiet. I did and it helped tremendously.

    Now, 8 months later the hum is back and loud as ever. How long do pre amp tubes last? Probably play it 3-4 times a week an hour or more each time.
     
  12. tennisplayer

    tennisplayer Senior Member

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    Alos, this amp loves pedals too. I put my PODXT in front of it and everyone is quite amazed at the tone from this little monster.
     
  13. tennisplayer

    tennisplayer Senior Member

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    OK, replying to myself......I replaced the pre amp tubes tonight with 2 new groove tubes and wa la. it's just a soft gentle hum now. Much better.
     
  14. claptonisgood

    claptonisgood Member

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    i thought i would update what i have just done to my Pro jr project (circuit mods and 1X12 combo cab)...i installed a GDS/Heyboer OT and the improvements are not subtle! Bigger, fatter and louder are the best ways to sum it up...also harmonically more complex; this is a no-brainer...i did the lazy man's install and it only took ten minutes...i cut the old xformer leads and spliced/shrink-tubed the GDS/Heyboer...the original Pro jr OT is affixed with 2 screws that directly screw into chassis threads...i used one of the original threaded holes and then drilled a slightly undersized hole for the second screw and self-tapped it in. The GDS/Heyboer is physically almost three times the size of the original and, obviously, much better made...it was interesting to note that the original was showing bigtime signs of distress and probably wasn't going to last much longer. I took this amp out to a smallish room local jam the other night and it hung with Classic 30s no problem. The final mod is the speaker: i have tried several and currently have a V30 in it but I should have my Emi Private Jack early in the week and can't wait...mmmm, but i may just have to try a Red Fang before i am "finished"...:D
     
  15. tennisplayer

    tennisplayer Senior Member

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    Clapton, I have no idea what you are talking about, but it sounds like those are great improvements. My next trip to Florida, I'll drop mine off to ya and you can doctor mine up!

    Put it in a Tweed cabinet with twins and make it rock.
     
  16. claptonisgood

    claptonisgood Member

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    i thought i would update...i put the Emi Private Jack in the amp and i am impressed...more harmonically complex than the V30 and it is well behaved when pushed...i have a Red Fang that should be here monday, so i will decide between the two then...i also wound up biasing a new pair of JJs right at 70% dissipation (8.2W) and it sounds so nice...also did a few preamp comparisons and decided on a JJ ECC83S in V1 and an EH 12AX7A in the PI...this amp is a little monster and i am very pleased...i am going to take it tomorrow and do side-by-sides with a Badcat Cub 15, a Goodsell Clairmont and Bucino and Dr Z...all good amps...i can't wait!
     
  17. claptonisgood

    claptonisgood Member

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    I wound up taking the amp out to jams with both speakers...for me, the winner was the Private Jack...with a 15W amp, it seems to have a more solid low-end response and wasn't farty, plus, with the low-wattage, the Red Fang compression licked in pretty early...this is not a bad thing, but, for me, i try to milk every bit of clean headroom i can and the PJ gave me that better than the RF...i am super impressed with the Private Jack...a terrific bargain...
     

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