23 April 2014

The Bill M Mod Kit Saved my Blues Jr.

I was looking for a Blues Jr to mod and the cheapest one I found was $300 and it was already sold. The rest were $400 or more. Then I saw a Hot Rod Deluxe for $300, so I bought it because it was so cheap and I figured I could mod that. But it was way too loud to play at home and there weren’t too many interesting-sounding or necessary mods to do to it.

I put an ad on craigslist offering to trade it for a Blues Jr and someone responded. So now I have my Blues Jr.

It needed work. The input jack desperately needed to be replaced. The plastic threads were completely stripped and it wiggled a lot when you inserted or removed the cord. The footswitch input has the same problem (I haven't fixed that yet).

It sounded harsh and trebly. Ice picky, even. The highs were way too loud and the bass response was lacking. It was worthless for playing through. However, I wanted to learn more about the Blues Jr, so it was not worthless in that regard.

To fix these things, I ordered Bill M’s cream board mod for the Blues Jr. (first mod listed on his Mod Kits and Services page. My previous Blues Jr. was a 1995 green board, which sounded much better to me (especially after doing a couple of Bill M's mods to it, particularly the reverb fix). 

The first thing I did, even before removing the board is the Twin Stack mod (see Bill M's detailed instructions). I jumpered the two left lugs of the Mid control. It worked, but still not a fan of the sound. Even with the bass turned way up, it’s much too trebly and harsh.

This amp already had three new 22uF 450V caps, which must have been replaced as part of the repair that was done. R47 had previously burned out, leaving a substantial scorch mark on the PCB. It was replaced with a higher wattage resistor and placed away from the board an inch or so, presumably to keep it cooler. The 100uF  filter cap is still original, but that will be replaced with the Bill M mod. 

I considered the standby switches, but they seem to be unecessary, especially for how often I play. If I gigged regularly, I would have bought that as well. For the price and the ease of installation, it seems like a good investment.

I replaced the original GT EL84 power tubes, which were 10 years old or so, with a matched set of JJ EL84s for $30. I had some EHX 12AY7, 12AT7 tubes on hand, so I replaced V3 with the 12AY7 and V1 with 12AT7. The amp doesn't break up as quick, which suits me. The overdrive sounds okay, but still harsh.

Bill M's parts arrived pretty quickly. He has a notice on his website that it could take one to three weeks. I expected the worst, but was very pleasantly surprised. 

After removing the board (slow and steady wins the race here—see Bill M's instructions), I replaced the input jack and the Mid and Bass caps. Since I had the board out, I also redid the Twin Stack mod by placing a jumper on the solder side of the board. I didn't need to—having it on the component side of the board bugged me a little bit for some reason. Then I replaced a couple of other caps and jumpered another.

I replaced the 100uF filter cap and noticed that the three other 22uF filter caps were a mess. Well, I had noticed it before, but the mess that the previous repair left behind was really bugging me at this point. They used radial caps instead of axial and then flung hot glue on it. It looked awful (pic below), so I ordered three axial caps as well (from ebay, because I was in a real rush this time). 

There is also a big jumper installed behind the big resistor (previous repair job—pic below). It works, so I figure it's best not to mess with it.

I installed the bias trimpot. The drill bit was smaller than my drill would hold, so I used the smallest bit I had to start the hole, then finished with the bit included in the mod kit, rotating it with my fingers. Not perfect, but it worked out, fortunately.

I installed a small cap on R30 (reduces phase inverter oscillation—see Bill M's description). 

Finally, the 22uF caps came, and I installed them with no problems. 

I put everything back together, biased it per BillM’s instructions. Pretty easy, and I think it looks much better:

Here are the voltage readings on the power tubes, before and after the mod:
Tube Pin
JJ EL84 Before Mods
JJ EL84 After Mods
2 (Grid 1)
-10.5 V
-12.97 V
7 (Plate)
322 V
329 V
9 (Grid 2)
288 V
308 V

With a voltage drop of 2.4 V, I came up with 7.9-ish watts idle dissipation per tube, which will keep the tubes much cooler.

I put the back cover on and tried it. It sounds great. Bill M's mods turned what was to me an unusable amp into one that I now quite enjoy playing through. And a great thing about his mods is that he explains them. You don't just follow instructions and replace parts—he explains why you're replacing the parts. Thank you Bill M!