# Help Eliminate Pro Junior Hum



## vadsy

I've always wanted to try a Pro Jr and bought one recently but as good as it sounds cranked up it idles with a fair amount of hum. I mostly want to leave it stock because I don't need another project right now but I'm thinking of swapping the tubes, they look well used and have some odd crackles to them, and adding a trim pot to lower the bias voltage since these amps seem to be running unnecessarily high according to some. I hope those two things lower the hiss and extend the life of the next set of tubes, outside of that I don't really know if anything else could be done, wire dressing doesn't really seem to be an issue since not a lot of loose wire is run inside of these. 

Does anyone have similar experiences with these amps or have suggestions on replacing possible components that could be the noisy culprits? I'm waiting to order a trim pot in case something else is suggested for replacement on the PCB. Any help or suggestions would be greatly appreciated.

Thank you.


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## vadsy

Further thinking about this and further trying to justify my laziness I'm considering pulling the chassis and shipping it off to someone to get a few mods done, possibly beyond what I first considered. Something along the lines of this but possibly not all out...

http://ivanrichards.com.au/2013/03/03/mods-upgrades-to-the-pro-junior-amp/

I know a few do it in regards to the Pro Jr, Keith in Calgary offers some upgrades, anyone else you guys might suggest? Is anyone on this Forum up for it? I'll get the chassis to Fedex today even.


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## WCGill

Crazy as it sounds, remove the back panel and see if it reduces the hum.


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## vadsy

I'll give that a go, what's the reasoning behind that super fancy maneuver?


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## zdogma

I vaguely recall it has to do with a screw in the back panel that touches something it shouldn't. I found mine had really noisy preamps tubes (Sovtek 12AX7WA) I put new Tung sols in and they were significantly quieter.


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## vadsy

Yikes, screws touching things inside of anything where voltage is present can't be good. I know the tubes in this guy are in pretty rough shape so that is definitely on the list.


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## ampaholic

I can't recall if the Pro Junior is the same layout however I did a couple of mods for a friend on his Blues Junior and when I had the board reinstalled I was getting a hum and some really weird bias fluctuations. It took me a long time to figure it out but there is a ribbon connector that joins 2 boards together and I didn't have it back in place properly. (I think I had the ribbon bent out rather than in).
Once I put that in place properly the hum disappeared.


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## vadsy

Funny about the back panel, supposedly a screw that is too long can attribute to random noise issues and a few folks around the net have found cutting it shorter or removing it can help a fair bit. 

Is anyone familiar with the Fromel kits? Would someone happen to know or be able to assist me in tracking down a bill of materials from one? I would like to see if it's cheaper to source the parts locally or at least within Canada or this Forum.

- - - Updated - - -

I was also incorrect that wire dressing is not a big deal within this amp, as mentioned here and elsewhere on the net, the ribbon cable and transformer wires need to be neatly run and tidied up.


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## StevieMac

Having just read the entire thread, I'd remove that (potentially) offending screw and then start subbing in new preamp tubes...and see how that leaves you feeling. If you're still not happy, try the trim pot and bias up a new set of output tubes and THEN see if you think further mods are required. You just might find that you like the amp enough without further modification.

Steve


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## WCGill

I worked on a friend's Pro Jr several years back that hummed, but only with the back panel on. Eventually I figured out it was the middle screw that was very close to the tone pot. Removing just that screw eliminated the hum. I cut the screw so it barely caught the fastener, reinstalled it, no hum. I posted this on one of the amp boards in response to another query shortly after. Not sure if I was the first, but I'm a legend in my own mind.


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## vadsy

WCGill said:


> I worked on a friend's Pro Jr several years back that hummed, but only with the back panel on. Eventually I figured out it was the middle screw that was very close to the tone pot. Removing just that screw eliminated the hum. I cut the screw so it barely caught the fastener, reinstalled it, no hum. I posted this on one of the amp boards in response to another query shortly after. Not sure if I was the first, but I'm a legend in my own mind.


We can track down the date to what I found posted about the screw and compare to when you had your friends amp, or I can forget about it and let you live on a legend!

I think I might take some of the advice given here and approach with a less head first attitude which is already different from earlier this afternoon. I think I can attribute that to being left unsupervised. 

Bill, in your professional opinion, do you think this amp is biased too hot? I like mods but am usually too lazy or busy to do them, still, this one just won't let me rest in peace. If the screw solves the hum issue and I pop in a new set of tubes and like the sound, would the tubes last or die quicker trying to be awesome? Should I at least throw in a trim pot?

Vadim


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## parkhead

these amps also hum badly with bad or mismatched el84's start with fresh toobz


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## WCGill

Without measuring, can't say whether it's biased too hot or not Vadim. I always like adjustable bias (trim pot) because all EL84's bias up differently.


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## nonreverb

Checked the output with a scope? Try it with nothing plugged in. You may have parasitic oscillation problem. Some Blues JR's are known to have this issue.


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## jb welder

The position of the ribbon cables in that whole line of Fender amps is super critical in terms of hum/noise.
There is some discussion of it (in the Blues Junior) here:
http://billmaudio.com/wp/?page_id=115


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## b-nads

After the issues I had with mine, my answer would be - sledge-hammer. Ultimately, I solved it by posting and selling it on Kijiji. ;-)

Good luck.


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## vadsy

Your post made me think immediately of this ...

[video=youtube;hqyc37aOqT0]http://www.youtube.com/watch?v=hqyc37aOqT0[/video]


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## blam

vadsy said:


> Your post made me think immediately of this ...


your post made me think immediately of miley cyrus' wrecking ball video. I will spare everyone and not post a video


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## nonreverb

Another thing I forgot to mention....has the amp got those goofy blue Lelon electrolytic caps in it? If it does, get'em outta there. They're garbage and most inevitably die. While yer in there, look for a halo of brown stuff around the positive leg of all the electrolytics. Fender has had intermittent problems with the caps over the past 10 years or so primarily with the Lelon's but with others as well.


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## vadsy

So, I finally got around to doing a bit of work on the amp. Trying to see if it will work for my needs without much investment, counting my time as free because this is fun and a great learning experience.

Pulled the screws, no difference. Pulled the back right off, nothing that I could hear. I made sure the ribbon wire was running nicely with gradual curves but it didn't seem to make much difference. I de-soldered the heater wires, twisted and shortened them a bit and as I could tell not a whole lot of improvement in the noise. I replaced the power tubes with a fresh set and it seemed to help, the odd crackles and pops disappeared and it got a tad bit quieter. I decided to replace the bias resistor, I think that's what I did, R29 from a 15K to a 22K and the noise dropped right off with the amp still sounding pretty nice. Good early on cleans and a great breakup anywhere after 6. I didn't measure the initial bias voltage but it's supposed to be in the 10 to 11 volt area and now it reads just over 14 volts. I found a suggestion that 12 volts is a good setting but don't have a 1/2 watt resistor to try for that range. I have a 19K 1/4 watt but first need to figure out if it can handle it. What I need is a proper trimpot, which no one locally seems to stock.


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## dtsaudio

Replacing R29 has reduced the current draw significantly. That means less ripple, and less noise. Also the half wave rectifier used for the bias probably has significant ripple, which you have also reduced. I would replace those two 10uf caps in the bias circuit. That may eliminate your hum completely and allow you to bring the bias voltage back down. Also as nonreverb stated, the main caps probably need replacing. I've found them to be quite poor.


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## vadsy

I considered doing everything at once, including the main caps, but thought that if I can't get the floor noise to go away with a simple solution I wouldn't waste time and money. It seems to be fairly quiet now and I'll consider doing a few other things to improve it maybe but mostly to tinker with it. 

What's the reasoning for replacing the bias caps? Do you think they are spent or just bad quality? 

I thought this amp had a full wave bridge rectifier? If I was asking because I don't know any better, is 14 volts running the amp too cool?


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## dtsaudio

I would hazard a guess that the bias filter caps are spent, which is why the hum with the smaller resistor. Putting it on a scope would confirm.
14V may not be too cool, if you like the sound. However since there is no direct way of measuring the output tubes current you really don't know where you are.
The high voltage is a bridge, filament runs AC, the bias is a half wave. Common way of doing things.


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## nonreverb

As a rule, I don't find the bias filter caps bad on those amps. There are the exceptions however and changing them might be the cure in this case . Having said that, the main caps are usually the real culprit. Most of the Jr series and HR series use a mishmash of different companies caps. That's where I found trouble. They do it for the singular purpose of saving money. That's not to say that there aren't more than just the B+ caps that could be bad. However, in my experience, the most troublesome caps are by far, the high voltage ones. Do yourself a favour and check them while you're in there. If the bias caps are indeed bad, there's a very good chance there are more.


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## Lincoln

vadsy said:


> I know a few do it in regards to the Pro Jr, Keith in Calgary offers some upgrades, anyone else you guys might suggest? Is anyone on this Forum up for it? I'll get the chassis to Fedex today even.


If you're still thinking about sending it out, 
I don't think you could go wrong with C4 Sound Works in north east Edmonton. Chuck Frank. Walking into his shop for the first time is like you've died and gone to heaven.


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## vadsy

dtsaudio said:


> I would hazard a guess that the bias filter caps are spent, which is why the hum with the smaller resistor. Putting it on a scope would confirm.
> 14V may not be too cool, if you like the sound. However since there is no direct way of measuring the output tubes current you really don't know where you are.
> The high voltage is a bridge, filament runs AC, the bias is a half wave. Common way of doing things.


Thank you for the info and education, really appreciate it!



nonreverb said:


> As a rule, I don't find the bias filter caps bad on those amps. There are the exceptions however and changing them might be the cure in this case . Having said that, the main caps are usually the real culprit. Most of the Jr series and HR series use a mishmash of different companies caps. That's where I found trouble. They do it for the singular purpose of saving money. That's not to say that there aren't more than just the B+ caps that could be bad. However, in my experience, the most troublesome caps are by far, the high voltage ones. Do yourself a favour and check them while you're in there. If the bias caps are indeed bad, there's a very good chance there are more.


I completely agree about the main filter caps. I've switched them out on a couple of other project amps I've done and it's made a noticeable difference, not a total cure but better for sure. I must have a terrible soft spot for these new-ish little Fenders because a few years ago I started modding a Blues Jr. and replacing some key components with better quality stuff has eliminated pretty much all noise at this point. I think the Blues was a quieter platform to start with though.



Lincoln said:


> If you're still thinking about sending it out,
> I don't think you could go wrong with C4 Sound Works in north east Edmonton. Chuck Frank. Walking into his shop for the first time is like you've died and gone to heaven.


I'm not really thinking of sending it out anymore, it's easy enough to do myself less the parts of course, I just thought with how busy this season is I wouldn't have time but... I've made time for the important things. 

Keith didn't strike me as someone I wanted to spend the money on. At first he thought I was talking about the Blues Jr when I clearly mentioned Pro and when I asked him what he was able to offer he just threw out that he did the Fromel mods and a price, no real details even though I asked. Fromel offers a few different kits so at least explain yourself, maybe he's busy enough that he doesn't need my business. I'm thinking of getting a kit from Fromel but wish they included a trimpot for the bias. As for Chuck, great guy, has worked on and looked over plenty of my stuff. I've spent hours hanging out trying to learn a few things from him. I've already contemplated calling him twice on this but want to have a better idea of what needs doing or if the amp is even worth it at all before going to see him, he can up sell me way quick. I find him good in a pinch or when I need something rare/specific/vintage but otherwise he can be a tad bit pricey although I've been very happy with his services in the past. To praise the guy, I've taken in an amp and sat down trying tube after tube until finding one that works just right and my only charge is for the tube, not bad when you think about it. The guy has also lent me stuff to take home and try, pretty cool in my opinion.

@dtsaudio and nonreverb;
If I was to source and replace some components, besides the filter and bias caps, what else would you recommend doing? I've looked at the kits and some guys say half of it is unnecessary and a waste of time and money.


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## vadsy

I think this thread needs and update with a happy ending.

As previously stated I have a soft spot for these little troublesome Fenders. So after all the talk and discussion I decided I would just buy a kit and do the work myself, that would be something I wouldn't get back if I was unhappy and decided to sell unlike pulling a speaker and tubes reverting things back. I was willing to gamble on a $70 a Fromel Supreme kit which seemed to include much of the guts in question of replacement. I also was interested in a speaker swap, it got to me and I was wondering about the grass being greener on the other side, so I talked to our very own WCGill about a 25 watt 10F150T Weber and some new tubes. All the parts showed up over the Christmas break and I had time to pull the PCB and break in the new soldering iron. The Fromel kit came with some great instructions and a few mods that I definitely prefer over stock, he actually takes a whole page to go over the changes about to be made. The modding/upgrading took me a couple of hours and after the speaker swap and new tubes the work was done well withing a moderately paced evening. 

I first fired up the amp with the old tubes, the noise was lowered but it wasn't until I was comfortable with the first stage of testing that I popped in the new tubes and realized the importance of good/fresh set and a balanced phase inverter, at least that's my guess. Things got way quieter and I got way happier, also relieved in a way. The amp still sounds fairly boxy but its expected and I like it in this particular instance, plus it cuts through the mix incredibly well. It also is way quieter, sorry I already mentioned that and so to continue, defined nicely on the highs and lows without sounding harsh, more "pulled together" across the sound spectrum and I found the ice pick highs are gone and the break up is substantially more pleasant with the new speaker. I took a chance buying this amp and had my doubts initially but after some minor mods I'm convinced I'll hold on to it, at least until I get a Princeton. I took some picts of the process, the only thing missing is I've added glue in between most of the newly installed caps to keep them secure.

*Thanks to all for the help, suggestions, education and general discussion!*

Comes with a few extra pieces to give some choice on where to go with mods and a short screw for the back panel, remember all that talk?








30 watts on the left, 25 on the right, the magnet size difference really surprised my. Is bigger better? In this case, it is.








PCB before...








PCB after...


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## WCGill

Nice work! Now for a Twin.


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## vadsy

Not that this thread is current or valid in regards to hum anymore but it did turn into a mod my amp thread, so to continue with updates.

Instead of working on my kids play structure this morning I decided to swap the output transformer on my Pro Jr. A change where the amp sounds less boxy and has a fuller high end. Very pleased. Check out the size difference.
















Now to try some more speakers.


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## StevieMac

LOVE LOVE LOVE that Allen T0-20 for smallish amps like the PJ, BJ, PR, etc. Without fail, it provides BIG tonal improvement and is my single favourite mod for those little amps. The Allen TO-40 has done wonders to many a Fender 40 watter as well (pretty much ALL of them but the Super, which already has big iron). As an added bonus, no drilling is required for as it's designed to fit existing mount holes. Simply superb!


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## Lincoln

Wow, that's quite the upgrade OT alright!!


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## Lincoln

I can't seem to find a link to the "Vadsy Pro JR speaker shootout 2014". 


Must be something wrong with my browser :sSig_DOH:


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## vadsy

I wish I had one to share with you, sorry, but the good thing is nothing wrong with your browser. I actually have time tonight to do some speaker swapping but no way to record anything. Just texted my studio guy and he says he won't have time until next week but by then I hope to have your loaners back to you. Thanks again.


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## blam

this thread makes me want to run out and get a pro jr and mod it.....damn you vadsy


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## Lincoln

blam said:


> this thread makes me want to run out and get a pro jr and mod it.....damn you vadsy



even my unskilled eyes can see the differences between the before & after board shots. It looks good! I bet it sounds great!!


you NEED one :smiley-faces-75:


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## Lincoln

vadsy said:


> I wish I had one to share with you, sorry, but the good thing is nothing wrong with your browser. I actually have time tonight to do some speaker swapping but no way to record anything. Just texted my studio guy and he says he won't have time until next week but by then I hope to have your loaners back to you. Thanks again.


It's all good, just pullin your chain. :sFun_dancing:

I am curious though


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## vadsy

blam said:


> this thread makes me want to run out and get a pro jr and mod it.....damn you vadsy


Trade you something, kidding, not yet anyways? Or if you'd like just borrow the little guy for a week or two, seriously.

- - - Updated - - -



Lincoln said:


> It's all good, just pullin your chain. :sFun_dancing:
> 
> I am curious though


I know you are. I just got home and plan on swapping as soon as the kids are settled in the shade. I'll keep you posted.

- - - Updated - - -



Lincoln said:


> even my unskilled eyes can see the differences between the before & after board shots. It looks good! I bet it sounds great!!
> 
> 
> you NEED one :smiley-faces-75:


Here is the last gut shot, glue in the possible buzz spots and wires dressed.


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## blam

vadsy said:


> Trade you something, kidding, not yet anyways? Or if you'd like just borrow the little guy for a week or two, seriously.


oooh, don't tempt me. i appreciate the offer, but i hardly touch my guitar in the summer. plus i have barely had a chance to play with my CDR since I got it because the band has only gotten together twice since april.


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## Riff Ron

vadsy said:


> I've always wanted to try a Pro Jr and bought one recently but as good as it sounds cranked up it idles with a fair amount of hum. I mostly want to leave it stock because I don't need another project right now but I'm thinking of swapping the tubes, they look well used and have some odd crackles to them, and adding a trim pot to lower the bias voltage since these amps seem to be running unnecessarily high according to some. I hope those two things lower the hiss and extend the life of the next set of tubes, outside of that I don't really know if anything else could be done, wire dressing doesn't really seem to be an issue since not a lot of loose wire is run inside of these.
> 
> Does anyone have similar experiences with these amps or have suggestions on replacing possible components that could be the noisy culprits? I'm waiting to order a trim pot in case something else is suggested for replacement on the PCB. Any help or suggestions would be greatly appreciated.
> 
> Thank you.


Cleaning up the lead dress and replacing V1 with a NOS RCA 12AX7 did the hum fix for me. Next step is to add the Fromel Complete Mod kit. I also moved the amp chassis to a MojoTone Pine Tweed cabinet that houses a 12" speaker (WGS Veteran 30) and this turned it into a gigging amp. I also copper tape shielded the wood back where it meets the chassis to form a complete Faraday shield. A simple test to determine that V1 is the culprit (other than swapping it out) is to ground R9 (labeled TP5) that goes to the grid of V1A (pin 2). Any of the big electrolytic caps negative leads are a good ground point. If the hum does not go away, then V1 needs to be replaced. You may need to try several to find a good one. They are all not created equal.


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## greco

@Riff Ron Welcome to the forum. 

I hope you noticed that this thread is almost 6 years old. 
However, there are typically many amp/electronics based threads in the forum at any given time.

Looking forward to reading your posts and threads. 

ENJOY!


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## Wardo

For what it’s worth and for posterity.

I have the Mark IV Tweed version which I think I bought about two years ago. I just plugged a Firebird into it and cranked everything to ten. There really no hum, it’s more like just the sizzle you would expect from an amp that’s cranked and It’s not that loud. At about half volume there’s really no extraneous noise coming from it. So it looks like the newer ones have been fixed.


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## vadsy

this thread/posts are welcome as I think this is a fun amp to own but I have since moved on from mine. I believe someone else here just got one and posted about it, maybe they could benefit from the info.

I got rid of the hum but I don't remember what fixed it exactly. I did replace the tubes after all the other mods and changes with a calibrated set pre and power. If I was to still have this little powerhouse I would throw him in a larger cab for sure but do my best to compare the 10" and 12" speakers as I think some of its mojo comes from the smaller speaker. 

lastly, I wish this amp came as a kit to build for us regular folk


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## Wardo

Agree on the smaller speaker.


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