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Post subject: Unexpected New Amp Day!
Posted: Sat Mar 03, 2012 11:25 pm
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Working at a guitar store does have it's perks. :) Today a Fender Pro Junior came in the shop used and I was able to purchase it for $85. I just made that much at a gig last night, so I figured I might as well pull the trigger on it. I really need something for traveling, and for low volume gigs and band practice where a 40 watt tube amp is overkill. I really don't like SS practice amps (with the exception of the Fender Mustangs and some Roland Cubes), so this is perfect!

The reason I got it so cheaply is that there is a very loud, very noticeable hum whenever it's on. You can even hear it while you're playing and it has nothing to do with regular 60-cycle hum. I played it with guitars that have humbuckers and it's every bit as loud. It's in addition to normal 60-cycle hum. The only time I've ever heard anything similar was when the tubes on my singer/rhythm guitar player's Deluxe started giving out. She had a microphonic tube, once they were changed it cut down on the hum considerably. The previous owner of my Pro Junior said that it was about 8 years old. That, coupled with the fact that Blues Juniors and Pro Juniors are known to run very hot, leads me to believe it might be a tube issue. How would I be able to tell for sure (and would I be able to tell for sure) before I buy new tubes?
I'm usually really tolerant of things like 60-cycle hum, but this hum is ridiculous. Also, it increases as you turn up the volume and the tone, if that's gives you any hint about what it might be.

Also, I've heard about tons of mods that people do to Blues Juniors and Pro Juniors. Some of them intrigue me, like giving it more headroom, more dynamic response (speaker change), and adjusting it so that it doesn't run so hot and fry the tubes. Has anyone had any experience with these kinds of mods? I'm planning on having a reputable amp tech actually do them, that thing about how messing with tube amps can kill you is enough to keep me away.

Finally, here's a pic! I haven't cleaned it up yet.
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Post subject: Re: Unexpected New Amp Day!
Posted: Sat Mar 03, 2012 11:58 pm
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Could be a bad solder joint or a problem with the tube heater circuitry. If new tubes don't fix it, take it to a pro amp tech.

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Post subject: Re: Unexpected New Amp Day!
Posted: Sun Mar 04, 2012 12:58 am
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I put a Vintage 30 in my Blues Junior and it freakin rips. Sometimes its something as simple as a bad receptacle. An open ground most likely. Or a bad power strip.


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Post subject: Re: Unexpected New Amp Day!
Posted: Sun Mar 04, 2012 2:39 am
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First, the really good news. Most of the tubes are probably gonna check out to still be good. If you have access to a tube tester at the music store, use it test the tubes.

Sometimes the preamp tubes will get microphonic and generate a slight hum as well when going bad, but not an incredibly loud hum like you describe.

One thing you can try without a tube tester. With the amp on and set at normal playing volume, tap the glass casing of each tube with a pencil eraser. If it sounds like you are hitting one with a hammer that is a microphonic tube and you probably are going to wind up replacing that one.

I would not replace the tubes without checking them on a real tube tester first though since they could be perfectly good and new tubes aren't cheap. Also a technician needs to replace the power tubes as they may need to be biased.

New tubes are not always better than perfectly good stock tubes either, especially preamp tubes. Certain makes and types of old used but still good tubes command bigger money than new production tubes!

I agree that it could be a bad solder joint or broken ground somewhere. However it could also be a bad filter capacitor or rectifier in the power supply. If any AC gets into where there should only be DC that is usually a hum recipe and also can damage the tubes. Checking capacitors is for sure something to leave to a skilled electronics tech if you are not a skilled electronics tech.

A tube amp tech doesn't have to be at a music store either. A neighborhood TV/Radio repair shop (a vanishing breed I know) will have a tube tester and all the other bench gear to check it too. I used a TV guy here for repair work but he recently passed away in his 50's of a sudden heart attack. I haven't found anyone else as qualified yet either. The music store in town that works on amps only works on amps for the original owner to whom they actually sold it new, so I have to find a new tech.

You are probably looking at another $75 to $100 or maybe more in labor plus whatever the parts cost to have someone check it out and fix it. Caps are cheap. The rectifier isn't much usually.

Another option is to find a local community college or technical school with an electronics repair class. They are always looking for stuff to fix. You get labor free usually but have to pay for parts in this sort of arrangement. Also it takes way longer as the instructor has to supervise everything. I actually got a couple of amps fixed this way over the years, but not lately.


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Post subject: Re: Unexpected New Amp Day!
Posted: Sun Mar 04, 2012 2:37 pm
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Thanks for all the info, brotherdave. I've been doing some more research on Pro Juniors and it looks like excessive humming is a common problem. It looks like I'm going to have to have an amp tech check it out.

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Post subject: Re: Unexpected New Amp Day!
Posted: Sun Mar 04, 2012 3:47 pm
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Jeff Beck plays the Pro Jr!!!!

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Post subject: Re: Unexpected New Amp Day!
Posted: Sun Mar 04, 2012 6:50 pm
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I posted some suggestions in your other thread in the lounge, but I'll had
this here - - good reading if nothing else. :wink:

viewtopic.php?f=13&t=56947

..and welcome to the club! :D

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Post subject: Re: Unexpected New Amp Day!
Posted: Sun Mar 04, 2012 7:16 pm
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Miami Mike wrote:
I posted some suggestions in your other thread in the lounge, but I'll had
this here - - good reading if nothing else. :wink:

viewtopic.php?f=13&t=56947

..and welcome to the club! :D


Very cool! I'll check it out right after dinner.

I read somewhere on a forum that removing the top center screw holding the chassis in place reduced the hum a bit. I tried it, and I think it worked! To be sure, I had my Dad listen to the before and after, without telling him what I did, and he immediately said that the difference was that the hum was quieter. It's not perfect, but with the screw removed it's tolerable at volumes below half. I'm more optimistic about it now. I think I'm going to look into what mods I'd like to have done and then take to a tech. :)

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Post subject: Re: Unexpected New Amp Day!
Posted: Mon Mar 05, 2012 7:47 pm
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i bought a used pro junior a couple months ago,while playing it the volume would change from low to higher and sometimes the amp would feedback a little, the tubes seemed loose all the time and needed to be reseated quite often. so i took it to a fender certified repair shop and was told the tube sockets on some of these amps could be replaced at fenders cost because they used a very poor quality tube socket. while they had it open i had them replace at my cost the input jack which also is not of great quality I wouldn`t do much of any modes to this amp because I think it sounds pretty good as is. I did put a n eminence copperhead speaker in it, or you could try the ragun cajun its a thoss up and get some decent tubes. get a quiet 12ax7 tube for V1 and a balanced 12 AT7 for V2 this seems to be the tube that made my PJ quiet and now its a really good sounding amp.


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Post subject: Re: Unexpected New Amp Day!
Posted: Tue Mar 06, 2012 11:40 pm
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mats54 wrote:
i bought a used pro junior a couple months ago,while playing it the volume would change from low to higher and sometimes the amp would feedback a little, the tubes seemed loose all the time and needed to be reseated quite often. so i took it to a fender certified repair shop and was told the tube sockets on some of these amps could be replaced at fenders cost because they used a very poor quality tube socket. while they had it open i had them replace at my cost the input jack which also is not of great quality I wouldn`t do much of any modes to this amp because I think it sounds pretty good as is. I did put a n eminence copperhead speaker in it, or you could try the ragun cajun its a thoss up and get some decent tubes. get a quiet 12ax7 tube for V1 and a balanced 12 AT7 for V2 this seems to be the tube that made my PJ quiet and now its a really good sounding amp.


Thanks for posting that! You've given me some more good ideas to look into.

I actually managed to get rid of about 70% of the hum at low volumes. The tone pot was loose, and when I pushed it to one side by accident it became immediately quieter. I have no idea why, but I jammed a piece of foam against the tone knob to hold it in place and I'm using it to practice with. A temporary fix, but at least I'm enjoying the amp now! :)

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