Billm's Blues Junior Head Unit
I've often thought that a Blues Junior head would be useful as well as fun. One of the problems with the combo layout is very limited room for additional knobs and jacks as I add mods. If figured I could take the standard chassis, remount the tubes to the large surface that holds the transformers, and then I could use the space where the tubes used to be as a back panel. According to my measurements, the tube sub-board would just fit between the main circuit board and the back of the chassis.
|I started with a stock Revision C green board Blues Junior, made in 2000. It had been used, but not abused.|
|The amp bore some scars and showed some inept servicing. The reverb wire clip was reinstalled backwards. Some unknown liquid had been spilled on the reverb tank, but it cleaned up easily and worked fine. Someone had gorilla-tightened the reverb hold-down screws, squashing the rubber bushings.|
|I work on enough of these amps that I made myself a knob lifter. It's just a piece of aluminum bar stock with a cutout milled for the shaft and a taper filed on the fork. The back is cushioned with rubber tape to prevent scratches.|
|The first order of business was to install the basic Billm mods--adjustable bias, tone stack, reverb mod--and put the amp back together to see if there were any other problems that should be addressed before proceeding with heavier mods. When removing resistors, as in the bias mod here, it's important to melt the solder, then straighten the bent-over lead gently with a jeweler's screwdriver or an X-Acto blade. If you just heat the joint and pull the lead through without straightening it, it'll tear the solder pad off the circuit board.|
|Since I do a lot of adjustable bias mods, I made myself a drill guide out of 1/4 inch Lexan. I stick it to the circuit board with double-sided tape; it goes one way for the green board, the opposite way for the cream board. It gets the pins positioned perfectly every time.|
|Here's the trial fit of the circuit board in the amp in head orientation. The measurements I'd made said it would fit, but you can never be too sure!|
|Once I was sure of the fit, I made a template for the socket holes. I taped aluminum flashing over the existing holes, then used a center punch to mark the small holes and made an impression of the large holes by pressing the flashing with my thumb. I then transferred the marks to the top surface of the chassis.|
|I then clamped the chassis in my benchtop milling
machine and used a hole saw to cut the large holes and drilled the small
holes conventionally. A drill press would have done fine, but the mill
guaranteed that everything would be in a straight line. The hole saw is
less work than a chassis punch since you have to drill a hole anyway for
the punch's center bolt.
|Here's what the chassis looks like with the tubes
in the new location. It'll make a very tidy head unit. The output
transformer is a little close to one of the output tubes, but at the
lower bias setting, it doesn't get significantly warmer than it did with
the old layout. The head unit will have better air circulation than the
combo configuration did. But first, more mods!
The pot in the lower right corner is an experimental reverb dwell control. So far, it works great!