Wednesday, August 20, 2008

Harp Amp Project: Fender Pro Junior

The other day I spotted a MIM (Made In Mexico) Fender Pro Junior at a very fair price on Craigslist, so I snapped it up. I owned a Blues Junior amp a few years ago but did not like it much. I may have given up on it too early. So now I’ll take a shot at its simpler little brother, the Pro Junior.

Where the Blues Junior is a Master Volume amp with a FAT button and all manner of bells and whistles, the Pro Junior is bereft of features. It has two chicken head knobs: volume and tone. That’s it. One channel; no reverb. The speaker in the Pjr is a 10-incher, two inches smaller than in the Bjr,

What the two amps share in common is basic circuitry: 15 watts, two EL84 power tubes, a solid state rectifier, 12AX7 phase inverter, and a 12AX7 preamp tube. They also share the EL84’s tendency to sound trebly and to chime when driven hard. That is not a good sound for blues harp.

Be that as it may, the Pro Junior is a pretty popular harp amp. I’ve heard passably good tone honking out of these little amps at blues jams. Some of my harp buddies swear that this or that speaker swap is the magic bullet that transforms the Pjr into a monster little harp amp.

This particular Pro Junior looks to be completely original, and it is in excellent shape, the tolex and grill unmarked. The tubes are all Fender/Groove Tubes/Sovtek and are almost certainly the tubes that came with the amp from the factory. The speaker is the original Fender Special Design. I played the amp and it sounds like… well, it sounds like a stock Pro Jr.

I’m looking for suggestions here. The basic plan: I’ll replace all the tubes. For the preamp tube I’ll start with my favorite, the NOS JAN Philips 5751. In the power section I’m thinking of EL84s known for a bigger bottom end and less sizzle, such as the Electo-Harmonix or the new production Mullards. When replacing a matched pair of power tubes you should always replace the phase inverter as well. I’ll try both a new 12AX7 and the 12DW7 I still have lying around from my Twin Reverb project.

I need recommendations on the new speaker. I don’t want to go with my favorite, the Weber 10A125-O, for two reasons: It is rather expensive and I already use it in my silver face Champ (it sounds amazing). I also don’t want Jensen “vintage” or MOD, or the Weber Sig. I’m thinking maybe a used speaker, or something different. What have you tried that sounded good?

I also want to limit the mods to those that can be accomplished without being (or hiring) an EE. I’ll try to disconnect any Negative Feedback Circuit in the Pjr, but I don’t really want to dive into the complexity and expense of slinging a soldering iron all over it, unless the mods are REALLY simple. My target is to radically improve the harp tone of the Pro Junior with, say, a hundred dollars worth of parts and with me -- a lay tinkerer -- doing all the work.

I welcome any suggestions either in the comments below or via email to me at

Jump to Fender Pro Junior Harp Amp Project – Speakers


Anonymous said...

It is probably hard to say how much of a difference it makes but my older USA Blues Jr. sounds great stock especially with JT-30 crystal elements. All of the MIM ones that I have tried sounded thin and didn't sing at all. This included some of the fancy ones with upgraded speakers etc. The boards are definitely different in appearance and the speakers between the two seem to be different as well. You do have to have the settings just right though and forget the "Fat" switch, it is a waste of time for harp and guitar.

Rick Davis said...

Yeah, the FAT switch on the Blues Jr. should stand for Feedback All the Time. It's the worst feature on earth for a harp player.

sonny-boy 27 said...

I have to ask you a question...
What is the better harp amp between the fender blues junior(alot of feedback) and the fender pro junior(2 controls). I want a warm tone(something like like Musselwhite sound :P) and a good range of sound.
If the blues is better, am I supose to change 1 of the 12ax7 tube or what is the midification ? ...
And if the pro junior is better, is it better with an equalizer or a delay pedal (for an higher range of sound) ???
Thank you very much for your time ^^!