Pro Junior Mods for Non-Dummies

My amplifier secret weapon is a Fender Pro Junior.  I’m not entirely sure if they’re still in production - mine’s a `94, probably the first series.  The speaker went fairly early on and was replaced with a Mojotone 10″.

The amplifier is stupid simple, even for guitar players.  One input, one volume, one tone, 15 watts.  What happens when this amp gets cranked is heaven for the ears.  Unfortunately, as most of us know, even 15 watts tends to be LOUD, sometimes with a band too.

There have been numerous attempts over the years to attain cranked tone at reasonable levels but most have failed miserably (justifiably so).  I tried out a Weber Mass (attenuator), which does a semi-decent job but still kills the top end.

It was time to do some modding.

Unfortunately I really don’t have much of an idea what I’m doing electronically.  I can assemble and modify but lack any design or serious mod skills.  With the very kind assistance of some nice folks over at the FUNWITHTUBES Yahoogroup, I got the ideas turned into serious suggestions.  There’s a frightening amount of knowledge there.  You can’t exactly ask anyone how to play with tube circuitry.

There were three areas on which to work:

1. Triode/Pentode switching: this is what manufacturers sometimes call a half-power switch.  In reality it nullifies one of the tube grids.
2. A sag resistor:  simulates the power supply’s behavior under heavy load
3. Cathode bypass capacitor: there isn’t one on either half of the preamp tube.

In English, these mods will make the amp distort at a lower level.  This distortion is intentional, as opposed to the stereo business, where distortion is evil and engineered out of the design.  Guitar players love distortion.  Jimi Hendrix used to stand in front of a wall of Marshall amps, cranked past Painful.  Being the master he was, he could control them, where we would just stand there, hands over our ears, and go deaf anyway.  The point is that they sound great cranked but the level is way too much for mere mortals to bear.

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STANDARD DISCLAIMER

I feel stupid saying this because if you play around inside tube amps, you already know it.  This particular amp has just north of three hundred twenty volts floating around inside of it.  Three hundred twenty volts has a certain negative impact upon humans (and guitar players).

Trust me - don’t ask how I know.

You must discharge the power supply capacitors, then discharge them again several times, even if the amp hasn’t been played in a month.  This shit can and might kill you, ok?

Playing with electricity is best left to people who know how to play with electricity, or at least those of us brave or stupid enough to not care.  If you have any doubts, ask someone else to do it for you.

Doubts are the universe’s way of bringing Important Stuff<tm> to your attention.

This post is for people who have done this sort of thing before.  Don’t even bother if you haven’t.  There are no step-by-step instructions.  It is not difficult for people who do this normally.

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So in preparation, you had better fetch the schematic.  It will be considerably more difficult without it.

SAG: Good for amps, bad for boobs

We’ll start with something relatively easy: the sag resistor.  In short, you want to put a 1K resistor between B+ and the center tap of the output transformer.  This is relatively easy to identify, as it’s the only red wire with a connector attached to the circuit board.  I used a 20W resistor, as that’s what I had handy.  A 1/4w resistor will let the Magic Smoke out and the amp won’t work anymore.

Remember: electronics work on Magic Smoke.  As soon as you let the Magic Smoke out, they don’t work.

I am going to install a switch to short out the resistor if I decide to keep the mod.  You may want to lower the value of the resistor to change the effect.

LESS TUBE ELEMENTS = MORE TONE

I put a triode/pentode switch on my Marshall years ago but it wasn’t too aurally pleasing so I removed it.  I figured now might be a good time to try again.

Of course I couldn’t remember exactly how to wire it so the FUNWITHTUBES guys straightened me out before I let the Magic Smoke out of the Pro Jr.

Pin 9 of both EL84’s goes to 100ohm resistors (R24, R25) and to `Y’ in the power supply section.  This is the stock pentode operation.  The DPDT switch will break the connection before Y and shunt it to pin 7 of each tube, the plate.  VOILA - a triode.

I put this together with the switch hanging out the back of the amp, as I wanted to listen before I went drilling holes in the chassis.  Twist the wires and route them away from anything else.  The right way to do this is to remove the pcb, unsolder the inside end of R24+25, and solder a wire to run to the switch (only one wire is necessary here).  Instead of running the other switch lead to the tubes, you can run them to the point where the output xfmr connects to the pcb (brown and blue wires).

VERY EXPENSIVE MOD HERE

I haven’t done this to the Pro Jr yet but I have done it to others.  Adding a bypass cap to the cathode resistor can add some serious zing to the tone.  There are no bypass caps on either of the first two stages.  IIRC, Marshall uses .68uf and Fender uses 22uf bypass caps.  You can put a switch in to choose which cap or no cap.  Three or more sounds from one switch.  This mod can cost lots of money if you have a professional do it for you.

Start with the first stage.  If you like it, try the second stage too.  Make it permanent or use a switch.  Try it at normal volumes to best hear what effect it has.

YOU’VE SLEPT WIV A LADY… WHASSIT LIKE?

I brought the amp out of the lab into the living room to check out the sonic effects.  This was my first mistake. Everything was hanging out all over the place because I didn’t know if I was going to like what I heard and I didn’t want to go drilling for something non-permanent.

I listened first as a control.  What really shocked me was the normal tone.  Not that it sounded any different - just the fact that there was sound.  Normally this would require much screaming and many trips back to the lab before there was any sound in the first place.  Success has been so rare lately that it confounds me rather than impressing.

Power off.  Insert B+ resistor.  Power on.

Strum.  bloooooOOOOOOOMMMMMM.

Yeah, that’s some serious sag.  I was told it would distort too but it wasn’t giving up too much of that, at least yet.  Still - it was good.

Uh-oh….  I had just developed help.

Stupid me.  I was kneeling by my small amp, playing the guitar.  I should have known….

Here comes MARSHALL!  Not the amp - the dog.  Marshall is helpful.  Marshall will help Dad work on his amp.  Marshall is always helpful and always helping.  He started by situating himself four inches from the wires that were hanging out the back of the amp.  The effect was somewhat electrifying (for me).  I quickly pulled the plug and moved the amp so Marshall’s help wouldn’t be so… helpful.

Then I switched to triode mode.

Nice.

Much grungier at lower volumes.  Very responsive to the guitar’s volume control.

Not enough time to really put it through its paces but enough time to know I’m going to get out the drill and make some switch holes in the chassis.

Here’s a bonus: clip the purple wire.  This is the omnipresent negative feedback wire.  Amps always sound better to me without any NFB.  YMMV.

Let me know what you come up with.