Help w/Pro Jr diagnosis

[chaz498]

I am pretty much an amp newbie, but being here in China means doing it yourself or risking the local "service". I can discharge and test the caps safely, have basic electronics theory knowledge, and am passable with a soldering iron, but have no electronics repair experience. Here's the history of the amp so far...

Bought new but started a loud hum/hiss that forced the owner (my playing buddy) to send it in for "repairs". The shop apparently changed the pots (but managed to screw up the tone pot), and changed the filter caps. It worked for about an hour then the original problem returned. When I looked at it one of the power tubes appeared blown (blackened inside near the top) so I changed all the tubes for ones that I know are good. I opened it up to see if there were any obvious visible problems, which there weren't, did the heater wire twist/dress, replaced both pots with original Fender ones, and buttoned it back up. At first all seemed good - some hiss that does not change with movement of the volume pot - but it sounds great. Then my mobile rang and the amp picked it up from over 4' away but really loud! When I turned it on this morning there was noticeable hum which I found was caused by the humbuckers on my LP that was plugged in about a foot away to the side. A Strat causes the same but quieter, and it changes volume as you move the guitar around the amp. Killing the signal from the guitar by using the tuner stopped the hum.

That's about all the info I can give for now, so any ideas? Sorry for the long post but hopefully giving as much info as possible will help you point me in the right direction. Thanks folks!

[Mesotech]

First off, I would eleminate the obvious. From the symptoms you've described, I would first ensure that you are using a proper instrument cable between the amp and guitar, and not a speaker cable. What you've described seems to be a non shielded cable issue. By disconnecting the guitar from the circuit, if the amp still hummed, it would be a problem in the amp.

I've had similar, but not exactly the same issues when my Blues Jr. was plugged into a faulty power source. Moving the amp to another outlet corrected the problem.

[chaz498]

Thanks but I'm using George L's so that's not the issue, and I am using the same outlets I use for my other amps (Blues Jr and Bad Cat) which don't have any problems.

My reason for posting, which perhaps I didn't really make clear, is that I can't help thinking that the earlier problems and the current issues are related, e.g. is there a component failure that would cause the interference, and also lead to the blown tube and/or hum? How do you go about troubleshooting this kind of issue, or do you just keep using it till it dies again then find out what is wrong (not my preferred avenue obviously)?

I love working on things myself so see this as a good opportunity to learn something.

[JJ Gross]

Cell phones in close proximity to tube amps make noise when they connect or ring. I found that out when I figured out that my drummer was leaving his cell phone on during gigs.

Like he's gonna stop playing and take a call at a gig?!

I just made him turn the thing off before we do sound check. After that he can turn it on when he needs to make a call (offstage) and answer messages on break if he needs to. You really need about 20 feet between a tube amp and a cell phone to keep the signal switching noise from coming through the amp. After all - cell phones are little radio stations and operate on microwaves. They will get through cable shielding at close range.

Drummers ... I swear. lol