Pro Junior hum

[Robert]

I love my Pro Junior. What a sweet tone it has. I have a Tweed version from a few years back.

I just got tubes replace a while back, and it's been great. However, since the last little while, it has started to hum abnormally loud. The tubes look alright. I'm no amp tech though. It sounds like normal, I think, it just way more hum than there used to be.

I also noticed when touching one of the power tubes, there is a little bit crackling happening. Perhaps a bad socket?

Any ideas?

[Spudman]

Just pull each tube, spray the legs with some cleaner, wipe it off if necessary then re-install the tube. It sounds like what happens when the tube is lose or good contact is not being made.

[cebreez]

Does the hum get louder when you turn up the volume? Could be a dirty input jack. Other places I would look are corrosion on your grounding points and bad filter caps. Beyond that you are looking at possibly a transformer going bad. Of course its not unheard of to get a bad tube. If you can scrounge a like tube then try replacing each one and see if the hum goes away. Oh... and what Spudman said. Loose tubes can definitely be a noise problem.

[Robert]

The hum actually gets quieter if I turn up the amp more.

[cebreez]

Im still thinking Caps. Especially in the power supply. But now Im only going on what I've read. Without electrocuting yourself. I would touch or move everything I could safely to rule out connection issues. Then again... thinking that the hum gets quieter as you turn the volume up then a preamp tube could still be an issue since in most of the tube amps I've seen the volume comes after the preamp circuit. jimp would definitely be your man on this one.

[jim p]

Went to the Fender site and did not see the schematic for the Pro Jr posted. So figure it would be similar to the Blues Jr. and by the look of it all amps have the input grounded without a cable plugged in. So I would see if you have the problem with no guitar plugged in and the volume control set at different levels. Because by your post above the noise drops with volume setting, so it sounds like you may have a bad volume pot. You could use a pot cleaner on the volume pot like this http://store.caig.com/s.nl/sc.2/category.293/.f me I use WD40 which by just saying will cause some people fits, but I find it works. Who knows what is the secret sauce in the DeoxIT stuff anyway relative to WD40. But if the problem is contact wear on the resistive element in the volume pot the final solution would be to replace the volume pot. You should look for bad wire connections from and two the volume pot along with the solder joints to make sure the problem is not there while you are checking things out. Also the solder joints on the tube sockets and components on the PCB. If you think the problem is related to a tube and you have multiple 12AX7 preamp tubes you can also swap the location of the tubes to see if this changes the problem. If the problem changes then it would look to be a tube that is bad. I did find the Pro Jr. schematic looking a little more and see that there are two 12AX7 tubes one the two preamp sections and the other the phase splitter. You can even run the amp with tube V1 removed which is the preamp section if the amp is quiet all your noise problems are in the preamp section. I think your problem will probably be the volume pot by what you have posted.

[Robert]

I'll see what I can find out. The hum is present without a guitar plugged in. It gets quieter as I turn up a bit (say 4-5), then back to the "default hum" from 6 - 10.

[jim p]

If the hum is going in and out with volume settings it may not be the volume pot. It could be hum picked up in the preamp section is canceling out hum in the power stage at certain settings. You can operate the amp with both 12AX7 tubes removed if you have any noise then it would be in the output tube section. Keep in mind to be careful with the high voltages in a tube amp. By the schematic I found posted there is a bleeder resistor in the amp so after the amp is off for a minute or so the voltages should be low but measuring with a voltmeter to check is best for safety. Plugging in and out tubes is relatively safe just keep fingers away from the tube sockets and pins.

[kiteman]

Have any spare tubes Robert? Swap 'em out and see.

[Ch0jin]

I'm betting a broken ground trace or a failing filter cap, but aside from that, PLEASE PLEASE post back when you fix it Robert!

I find the amount of "how to fix it" suggestions generally outnumber the "well this is what fixed it" posts by a huge margin. I know for me personally, most of my fix it knowledge is well out of date as I downed tools professionally more than 10 years ago. It'd be nice to know how many of my guesses were actually correct!

In the spirit of my request..

Recently my Peavey Ultra was humming intermittently and reverb not engaging, also intermittently. Caps looked OK but solder joints were weak. I re-flowed all the filter caps and gave the reverb relay section a re-flow as well and that fixed both issues.

[Commodore 64]

Here's a schematic for the Pro Jr.

http://www.lucky-cricket.com/files/pro_jr.gif


Also, BillM has few mods to reduce hum and hiss. Maybe these will give you a place to start: http://home.comcast.net/~machrone/bjr/pjmods.htm