Weekend Mod Project: BillM Blues Jr Mods, Part 2

Last week I posted about my decision to jump headfirst into the world of amp mods, performing some simple tweaks to my Fender Blues Jr using a popular kit from online Jr guru Bill Machrone. This included the “TwinStack” mod, replacement orange drop tone capacitors, power supply stiffening capacitor, and adjustable bias trim pot, as well as a presence control and more robust input jack. Sounds complicated, but in reality the mods involve only a handful of components. What could go wrong?

I have plenty of experience with a soldering iron, but this was my first foray into the innards of a tube amp.  I was a little hesitant going in, based on the very legitimate warnings I had read about the dangers of poking around amp circuits. In the interest of keeping readers safe, I think it’s important to reiterate:

***IMPORTANT PUBLIC SERVICE ANNOUNCEMENT*** These mods are relatively simple, but by no means trivial. If you skip basic precautions, it’s entirely possible to hurt (even kill) yourself and/or damage the amp. I don’t want to see any injuries or broken gear on account of New.Old.Stock. When in doubt, consult a qualified tech! Bill will even install the mods for a very reasonable fee.

If you go it on your own, invest in a good multimeter and make sure to watch (and then re-watch) Bill’s video about properly discharging the capacitors.

The real challenge in performing these mods has much to do with the mass-produced nature of the Blues Jr. Fender never intended for the amp to be serviced at home (or even really by a professional past 5 or 10 years of use); this was obvious given the difficulty I encountered in simply pulling out the PCB, which requires disconnecting a number of subcomponents and applying a surprising amount of force to pull it clear of the chassis. 

Rather than walk you step by step through the install process, which you can read about on Bill’s website, I thought I would offer a few nuggets of advice based on my experience. Most of it is common sense, but easy to overlook when you’re eagerly modding:

  • Take your time, take your time, take your time. And then some more time. Amp mods reward patience and attention to detail. If you start getting frustrated, put the soldering iron down for a little while and reread Bill’s excellent instructions. I rushed through the mods on my first pass, doing some minor PCB damage and encountering excessive noise after buttoning up the amp. I actually set the project aside for a couple weeks. After revisiting the amp in a recomposed mental state, I easily worked through the issues.

Essential tool kit: multimeter, drill, soldering iron, jumper cables, wire ties, Bill’s installation instructions, wire clippers/strippers, screwdrivers, exacto knife, and solder sucker.

  • The PCB is delicate, especially the printed circuit traces. A low-temp soldering iron (preferably adjustable) and solder sucker are non-negotiable tools for doing a good job. Even with my soldering experience, I managed to lift off some trace after over-eagerly pulling out a capacitor that wasn’t completely de-soldered. It was a rookie mistake, but fortunately Bill’s site has helpful instructions for addressing common soldering errors like mine.
  • Make sure you tidy up the internal wiring as Bill recommends, twisting colored wire pairs together and dressing the ribbon cable tube leads; it’s easy to overlook when you’re nearing the end of the project. I didn’t do this on the first pass, and the noise increase was appreciable. After following instructions on the second pass, the amp was quiet again.
  • If you elect for the presence control, make sure to use an appropriate drill bit on the control panel (Bill recommends a step bit). This was the one area where I skimped on appropriate tools, and the hole I drilled with a conventional twist bit was serviceable but pretty rough. Also make sure to cover the amp’s innards to minimize contamination from the thousands of little metal chips you’ll create. You might even consider (carefully) tidying up the inside with a shop vac. The metal chips can create electrical shorts.

  • The hairiest part of the basic mods (at least for me) was the tube bias control, which requires drilling into the PCB and installing a tiny trim pot. Bill supplies an appropriate bit, but it’s tiny and easy to break; take your time and be prepared with a backup. Setting the bias requires measuring voltage with the amp plugged in and turned on, which sounds scary to the uninitiated. I recommend using a good set of alligator clip leads on the multimeter, which you can leave in place (see pic) before turning on the amp — meaning you don’t have to go poking around a charged circuit to take the measurements. You can then shut off the amp, drain the capacitors, and adjust the trim pot, repeating until you’ve reached the desired voltage. This was actually a lot simpler than it sounded.

OK then, so what about the tone? Suffice to say, the results were more dramatic (in a positive way) than I expected. Before undertaking the mods, I had replaced the stock speaker with an Eminence Cannabis Rex; the upgrade appreciably enhanced the bass and smoothed some of the harsh treble, but it was a subtle change at best (and probably inaudible to anyone in the audience). Coupled with the BillM mods, however, I was suddenly playing through a very different amp.

Most of the before/after audio clips you find posted on YouTube don’t do the mods justice. I’ve read users compare the resulting amp to a “poor man’s Princeton,” which is probably accurate. The modded amp is considerably warmer, rounder, and more responsive to dynamic playing technique; there’s more than enough 3-dimensional bass to go around, and the high end is no longer grating. The tone stack mods, paired with the presence control, offer considerably more sculpting flexibility. The original Blues Jr tone remains on tap, but you can also dial in a much mellower amp appropriate for jazz (meeting my needs), or — on the opposite end of the spectrum — a spanky, sweet Bakersfield tone that harkens back to Fender’s early 1960s golden era. Both single coils and humbuckers sound great, though I have to say the modded amp really rewards a good Strat.

If you have a Blues Jr collecting dust, you might consider the BillM mods before trading it away. There’s a lot of potential lurking inside, and Bill Machrone can help you unleash it. While I wouldn’t recommend the mods to a repair/soldering novice, the DIY route lends additional self-satisfaction every time you plug in and hear the pleasing tones coming out of your amp.

Follow link to Part 1.

Mod closeup: Obvious changes include the orange drop capacitors (upper center and lower right), big black power supply capacitor (upper left), bias trim pot (blue shape lower right of gray capacitors), presence control (behind yellow piece of electrical tape), and input jack (just peaking out upper right).

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