Bill-M Modified Blues Junior Amp



I have been using Blues Juniors for several years on pro gigs, session work, and in open mic situations. The BJ is a good, reliable little amp, but like many other guitarists, I wished that the amp had a bit more gain, better tone, and that it
Blues Jr
didn’t sound “boxy”. I had heard and read so much about Bill-M, AKA Bill Machrone and his Blues Junior modifications, that I decided to call him and put his claims to the test. Bill assessed my needs, and then explained his modifications in detail. Following is a brief narrative on the evolution of my “green board” (green circuit board) Blues Junior and some of my personal recommendations.

The cumulative result of Bill’s modifications was amazing. Even with the EL-84 tubes, the original cabinet and speaker, the difference in the amp was dramatic. The tone controls were more responsive, and rendered a more expanded EQ. Touch sensitivity was greatly improved, and the overall sound and tone were much more articulate. Overdrive settings sounded more pleasing, and sustain was improved.

Next, I decided to experiment with different cabinets, and ordered both Tweed Deluxe and Tweed Super cabinets modified for the Blues Junior from Mojo Music Supply. The Deluxe cabinet rendered a noticeable improvement in the amp’s overall response, tone and efficiency, but speaker choices were limited due to the depth of the cabinet. The Super cabinet was deep enough to accept a wider range of 12” speakers, and I was able to install An Eminence Red Fang speaker with its elongated Alnico magnet in the Super cabinet with room to spare. If you intend to use any of the larger / longer magnet speakers like the Red Fang, Greenback – Anniversary etc., then the Super cabinet is the way to go.

I experimented with the Ruby Reverb Tank and although the reverb was deep and lush, I missed the sharper, “surfy” 60’s tone of the OEM Accutronics tank. The 250k Audio reverb control mod works fine, but it was easier and quicker for me to dial in reverb setting in dimly lit club conditions with the stock control and I had Bill re-install it.

My friend Matt Daniels and I visited with Bill in March 2010 to preview a new Blues Junior design and we were impressed enough with his 6V6 / 5881 tube conversion to order the modification for both our amps. I noticed previously that in a medium size club setting, the EL84 tubes couldn’t deliver the gain needed to cut through the mix, and that my amp would distort and sound muddy at higher volumes.
Blues Jr Tubes
The 6V6 / 5881 conversion did the trick. My amp now has more clean gain and headroom than with the EL84s, and we installed a 12AY7 preamp tube in the phase inverter to smooth things out and tame the strong midrange of the JJ6V6 power tubes . 12AT7 and 5751 tubes can also be substituted for the 12AX7s in V1 (preamp) and V3 (phase inverter) positions, allowing the player to tailor the amp to personal preference. My amp sounds nice and snappy, and it is perfectly tuned for work with Telecasters and Strats. It does not sound muddy with P-90s and humbuckers unless the amp is driven hard. Also, the clean boost mod sounds punchy and more articulate with the larger power tubes. We experimented as well with EH6V6s which yielded a very vintage tone, with slightly less gain, and a flatter bass and midrange response. I am waiting for both TAD and Tungsol 6V6s to arrive in order to further tone test the amp, and I will post my findings accordingly.

blues jr Mod and Comparison

Comparison of the BillM Mod Cabinet with a Stock Blues Jr


Following are the mods I had performed on my green board Blues Junior :

Green Board:
Orange drop tone stack capacitors, adjustable bias trim pot, a pair of power supply stiffening capacitors, The adjustable bias is a high-rel, precision, sealed, 25-turn trim pot for setting bias accurately. The power supply stiffening capacitors double the size of the main filter stage, halve the ripple going to the output tubes, and provide solid bass tones. The green board mod also includes a new reverb coupling capacitor and resistor to overcome the hum- and noise-prone reverb circuit on the green board. The mod updates the reverb so it performs identically to the current cream board.
Presence Control:
Potentiometer will be mounted on control deck in front of Fat switch. Adjusts from mellow, restrained tone to bright and aggressive. Connects to circuit board with Mogami low-noise coaxial cable, includes pull-up switch for use with Clean Boost.
Clean Boost module:
This 1x1" circuit module installs inside the amp and gives you a volume boost that's about the same as that of the Fat switch, without the fatness. Its volume adds to that of the Fat switch, and can be wired to go on with the Fat switch or by itself on a pull-up on the presence control (if you choose the presence option).
Footswitch control:
Converts Fat switch jack to stereo so 2-button footswitch (not included) can control both Fat switch and Clean Boost separately. Footswitch overrides panel settings when plugged in.
Twin Stack:
This inexpensive mod changes your mids control from normal Blues Junior/Hot Rod operation to a Twin Reverb-style stack. It lets you drop out all of the mids for more incisive treble tone or more scoop. Reduces midbass heaviness and muddiness. The normal Blues Junior mids kick in at 5 and up, so all stock tones are available, too.
250K Audio Taper Reverb Control:
Changed back to stock potentiometerl --- Just my personal preference
Metal replacement input jack:
Switchcraft jack with isolating washers to prevent ground loop hum. Connects to circuit board with Mogami low-noise coaxial cable.
Auxiliary speaker jack:
Plug in an additional 8-ohm cabinet for bigger tone. High-quality Switchcraft jack.
High-voltage preamp mod:
Brighter, crisper tone, better headroom This mod replaces four resistors in the power supply and preamp and brings the voltages up to a more typical Fender Blackface level. Highly recommended for green board amps. It works for cream boards too, especially if you’re looking for a brighter tone with more clean headroom.
Heyboer 22 watt Output transformer designed by Allen Amplification for Bill-M The amp becomes a cross between a good BF Deluxe Reverb and a Blues Junior
March, 2010 : 6V6 Conversion JJ 6V6 tubes Stand by switch :


For more, here's a Link to Bil's Site - Billm Audio