Blues Jr. Modifications
Making a Great Amp Even Better
Caution: Tube amplifiers can contain lethal voltages even when shut off and unplugged. Do not attempt to modify, repair or even inspect the chassis of a tube amplifier unless you are trained to do so, understand and use all of the safety precautions involved and have someone trained in CPR and First Aid present in case you become incapacitated by an inadvertent contact with high voltage.
After about 18 months of owning a Blues Junior I did find a flaw that I felt needed to be dealt with. At home volumes I was very happy with it but at gig volumes it almost sounded distorted. I listened carefully and determined that it was simply a case of the amp being boxy sounding, a conspiracy of a small cabinet, a fairly large speaker and a tone stack design that failed to take these factors into account. At the Fender Discussion Pages I found a thread dedicated to the Blues Jr and it was there that I learned of the fabled Bill M modifications, a set of simple upgrades that made the Blues Junior a real winner. I ended up sending my amp to Bill Machrone himself to have the work done for me. The mods included his tone-stack modification, a cool-down of the output tube bias, a cut/boost presence control and a Trace Elliot output transformer.
The Blues Jr is not quite of the same sonic family as a blackface Fender out of the box but it responds very well to a few simple mods. The Bill Machrone tone-stack mod along with his Twin-stack mod work wonders and the parts cost is less than what I left as a tip at breakfast this morning. I have to reluctantly agree with the critics of this amp that it doesn't do very well with the Fender blackface sound out of the box but my modified Blues Jr. sounds much better and is much closer to the sound of a blackface Fender.
Bill M also has a presence control mod which is very simple to perform and adds incredible amounts of flexibility. You can go from Tweed territory to the blackface's backyard with the twist of a knob. Like most presence controls it adds a variable resistor to the negative feedback circuit and can have a very big effect on the behavior of the amplifier. With a high resistance in the negative feedback circuit the behavior is somewhat similar to a tweed Fender amp. The headroom is low and the feel is pretty loose. The higher you turn the control the lower this resistance becomes and the amp begins to behave more like a typical blackface which is to say it tightens up a lot and stays clean to a much higher volume. I was skeptical about doing this mod at first but now I see it as one of the best moves I ever made with this amp.
Another, very cost-effective modification was the installation of a replacement spring reverb unit. The replacement was a 3EB2C1B Ruby Tubes Reverb Tank which I bought from Mojo Musical Supply. The results were astounding.
It's no secret that the stock reverb on a Blues Junior is not its strongest point. It's adequate but not up to the standards of the classic blackface era Fender amps. This replacement spring unit takes a huge step in the right direction. This is a long delay tank which lives up to its billing. The reverb stays effective from the lowest levels all the way up to 12.
One other simple "mod" that I've done has been the substitution of a 12AY7 tube in V1 and a 12AT7 in V3. These really helped make the amp more tractable and increase the clean headroom.
I wouldn't argue that even all of these modifications leapfrog the Blue Junior beyond the Deluxe Reverb but they do make it into a very decent amp, quite up to the task of small to medium gigs.
