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FDP Forum / Fender Amps: 1985 - Current / Pro Junior Hum with Cable Inserted

Central Scrutinizer


Pete V
Mar 7th, 2017 10:53 PM   Edit   Profile  

I have a tweed Pro Junior, not sure the year, which I've had for a couple of years but haven't used for awhile. From reading postings on this site and others I get the impression this in an inherently noisy amp but I haven't come across my specific issue in any of the postings. I have no problem with the amount of hum present at idle, it is when I plug a cable into the input jack that it becomes unacceptable. If I plug in a short 1/4' pedal connector it squeals like a piglet. If I plug a standard length guitar cable in it hums very loudly. If I take the opposite end of the plugged in cable and wave it around near the components in the chassis the hum increases. If I wave it near the power tube nearest the preamp tubes I get that high pitched squeal again and if I do the same near the other tube the frequency response of the hum becomes more bass prominent. The filter caps were replaced a few years ago so I haven't troubleshooted for a bad cap yet. I have tried various tubes and it doesn't appear to be a bad tube causing this. I don't think this is the common scenario that can be fixed by fixing the lead dress etc,, as I said the hum level at idle is completely acceptable, it's only when something is plugged into the input jack. Any ideas?
- Pete V

(This message was last edited by Central Scrutinizer at 12:55 AM, Mar 8th, 2017)

Richard L


Mar 8th, 2017 03:15 AM   Edit   Profile  

With no cable in the input jack, the input is shorted by the jack socket, so no surprise that it's quiet. What happens when you plug the guitar cable into a guitar ? Noisy - quiet ?

Central Scrutinizer


Pete V
Mar 8th, 2017 05:37 AM   Edit   Profile  

The noise is still there with guitar attached. I should add that turning the volume down to zero eliminates the hum.

Central Scrutinizer


Pete V
Mar 8th, 2017 09:06 AM   Edit   Profile  

So it turns out I was wrong, plugging in an actual guitar makes a difference. I was fooled by the amount of hum I was getting from the single coils in my Tele and selecting the middle position did not eliminate the hum as I expected (which seems strange to me as the in between positions on my Strat do eliminate the hum). Bottom line is under normal circumstances it turns out there is no significant noise problem. Thanks!

Richard L


Mar 9th, 2017 02:57 AM   Edit   Profile  

Good news !!

Te 52

Laws of Physics

strictly enforced
Mar 9th, 2017 10:51 AM   Edit   Profile  

"...selecting the middle position did not eliminate the hum as I expected (which seems strange to me as the in between positions on my Strat do eliminate the hum)..."

You will only get hum-cancelling in the middle position if the two pickups are reverse-wound, reverse-polarity (RWRP). Most modern Teles are, but older ones or vintage-correct models may not be.

You can tell if the pickups are reverse polarity by holding a compass near each pickup. If they both read N or both read S, they are not reverse polarity and there is no easy way to make that pair hum-cancelling.

If one reads N and the other reads S, they are reverse polarity and should be hum-cancelling or can easily be wired to be hum-cancelling.

FDP Forum / Fender Amps: 1985 - Current / Pro Junior Hum with Cable Inserted

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