10 January 2015

How to Install a Fender Blues Junior Bias Trimpot Without Drilling Holes in the Board

Where Fender Blues Junior mods are concerned, Bill M of billmaudio.com is truly a player on the world stage (see my previous appreciation of him and his mods here). The info and kits he provides on his site have been invaluable to me and countless others from around the world. So it is only with due respect (and sometimes awe) that I disagree with him on one issue: the need to drill holes in the board to mount a trimpot for bias adjustment.

His point is that bent leads on a trimpot may cause the pot to be wobbly, which in turn can cause the leads to break. First, the trimmer I used is not wobbly even with bent leads. Second, if you can use hot glue to secure other components on the board (electrolytic caps, multi-wire cables), you can use it here if you want. Again, there is no need to with the pot I used (but see my final note in the last paragraph). This is not the same pot that is included in Bill M kit (same specs, narrower housing), so I have included a close-up shot of it so you can find it. I got mine on eBay—a Bourns 3296 variable resistor, 50K ohm, 25-turn (Fig. 1).
Fig. 1. Here is the trimpot I installed in my Blues Junior. It is a little narrower than the one provided in Bill M's kits.
I recently installed this trimpot in my Blues Junior III (details vary among different BJr versions—check billmaudio.com for details). To avoid drilling a hole in the board (which I have done once before, and very nearly botched it), I bent the middle lead of the pot straight out and spliced a piece of wire to it (Fig. 2). I bent the outer leads slightly inward and down (note: you really only need to bend one lead, but I have an often unnecessary habit of centering things). I pressed the pot to the board and soldered it in place (Fig. 3).

Fig. 2. Ready to install: trimpot with extra wire spliced onto the middle lead and outer leads bent slightly inward

Fig. 3. Installed trimpot.
Because this trimpot housing has little nubs/standoffs on the edges (visible in my beautifully rendered and incredibly realistic diagram in Fig. 4 below), there is space underneath for the lead to fit between the housing and the board. As a result, when the trimpot is mounted, it is snug, not at all wobbly, and perhaps most important, reversible. Further, it was easy and not even remotely nerve-wracking.

Fig. 4. This is what a diagram looks like when it is created in Word and Paint. But you get the idea.
As a final note, though—and I can't stress this enough—I would personally take Blues Junior advice from Bill M before I took it from me. I am only sharing this as an alternative for people who aren't willing to drill a hole in the board. If that's a dealbreaker, this alternative might work for you.


ALIKEN said...

Awesome advice, thank you for sharing! Any other advice for modifications would be greatly appreciated. I just picked up a blues jr.& want to hook it up!

smrbates said...

Thanks for checking out the page. Did you see this one too? http://momentfx.blogspot.com/2014/04/the-bill-m-mod-kit-saved-my-blues-jr.html?