A few weeks ago I mentioned in another thread that I was going to do the BillM Blues Junior mods. There was some interest so...
I managed it without causing any serious damage to self or amp and here’s how it went:Unplug Amp.
OK, no problem.Remove back cover, knobs, jacks.
The back cover is held on by 6 screws, it’s also a major part of the ground and shielding for the Blues Junior, so although you can operate the amp with the cover off, it’s not real happy about it. Knobs pull straight off with no problem. When removing the nuts that hold on the fat footswitch and speaker jacks, the speaker magnet will grab them right out of your wrench or nut driver. I also removed the power switch, giving me a little more room to remove the circuit board.Remove transformer connections.
This is a little tricky, particularly the connectors to the speaker jack daughter card since there are some thin wires going to the fat switch jack. You just have to wiggle them gently back and forth and eventually they come off. The connectors are color coded on the cream board amps, but I took notes anyway.Remove circuit board.
More screws; no problem. It’s a little tricky getting the top of the board out past the edge of the chassis; gentle but firm is the way to go (as in so many things
). Output Transformer.
The output transformer is mounted to the back of the chassis, so that comes out (4 screws), then the two holding the transformer down. You have to wiggle the connectors around a bit to get the leads out through the grommets and do the same getting the new leads back through. BillM’s T020 transformer comes with two washers that lift the transformer a bit so the leads can pass under it to the holes in the chassis. You’ll be amazed at how wimpy the original transformer looks beside the T020.Power Supply Stiffening.
This involves a second capacitor in parallel with the existing one. Being lazy and frankly afraid to drill through the circuit board, I took the easy way out and carefully soldered the new cap to the original’s leads. A dab of silicone adhesive under the new one and between the two caps eliminates problems from vibration.
At this point I hooked everything up and tried it out, just to make sure everything was still working. With no shielding, there was quite a bit of hum but it sounded OK, so power off, unplug, wait a minute for it to discharge, discharge to ground just to be safe and…Tone Mods.
Three items here: short the center and left terminals of the mids pot together with a little piece of wire and replace the mid and bass filter capacitors. Just cut the old caps out and use a solder sucker to get the little bit of bent over lead out of each hole. This isn’t hard, but the traces on the BJr circuit board are incredibly thin and it would be very easy to burn through them. The new caps just drop into the existing holes; bend the leads to hold them in place, solder, trim them off, and you’re done.Adjustable Bias.
I didn’t do this because as noted I was a little nervous about drilling through the circuit board. I’m planning to replace the output tubes with JJ EL84s in the future, and I’ll do it at that point.Re-tension tube sockets.
This was actually a little trickier than it looks. The sockets are sort of awkward to get to and in my case some of the connector springs were pressed solidly
to the sides, making it hard to get a little screwdriver in. The springs that hold the tube retainer on also seemed a little stretched, so I replaced them. If you have a BJr that occasionally makes a weird noise or has a farty sound in the bass until it's warmed up, your power tube sockets need to be tightened up.
Buttoned everything back up, and, just like everyone says, the difference in sound was like night and day. The flabby low end is completely gone as is amp hum at reasonable bedroom volumes. I can totally scoop the mids and clarity is better across the board. Everything is “as advertised”. Rather than subject you to my playing, BillM has just posted a video review of the mods by Anthony Stauffer which covers everything: http://billmaudio.com/wp/
Anthony doesn't really get into the completely clean tones but he's a Strat guy, so you can get some idea of what the BJr might sound like for Alligator era Jerry sounds. I think you can get a really decent Gibson era Bobby sound. I'm using an Ibanez AF-105 and it sounds darn good to me. If you do early rock, classic country or rock-a-billy type stuff a modded BJr is probably the most bang for your buck in a small amp - but honestly, it's probably not the best choice for general purpose Dead.
It’s been many years since I fooled around with electronic stuff and it was pretty enjoyable to do the mods. If I was gigging this amp, I’d put a more efficient speaker in (see the BillM website for his comments) and since I have the T020, adding external speaker is not out of the question either.
Now that I've got the tools - I'm thinking that an MXR Dist. + clone is up next