A few (fairly) easy Blues Jr. Mods....

[duhvoodooman***]

I've been planning on doing some mods to my Blues Junior for several months, and finally got around to knocking 'em off last night. These are all covered on Bill Machrone's website. Some of you may already be aware that Bill, one of the editors & columnists for PC Magazine, is also a major guitar & amps mod guru and pretty much the "world's foremost authority" on Fender Blues Junior amp mods. He's very active at the Fender Discussion Page forums, and has this very informative web page dedicated to Blues Junior modding.

From the various mods he lists, I saw three that I wanted to do--two to improve the Blues Jr's tone (which has the reputation of being a little "boxy") and one to reduce the bias of the EL84 power tubes, which Fender sets extremely high. Here's a nutshell description of each:

• Twin Stack mod -- This is just absurdly simple, and extends the range of the amp's Middle control pot to mimic the way it works in a b/f Twin Reverb. The mod involves nothing more than soldering a small jumper across two of the contacts of the pot. You still have all of the tones that the stock setup has, but now you can turn the mids completely off when you turn the pot all the way down.
  
  
• Tone Stack mod -- Just slightly more difficult than the prior mod, this one involves replacing two caps in the tone stack with ones of different ratings. The effect is to bring up the bass and lower mids in better balance with the upper mids and highs, which the stock amp accentuates. This gets rid of much of that perceived boxy character and gives a fuller, more open sounding amp.
  
  The one complication here is that the solder pads on the Blues Jr board are quite flimsy, and you have to be very careful that they don't crinkle/lift while desoldering the original caps. I ran into some of that, though not enough to cause a problem. You'd think that Fender would use a more durable board, though.
  
  
• "Easy" Adjustable Bias mod -- Bill lists a more involved adjustable bias mod on his site, which allows you to independently set the bias of the two EL84 power tubes. This involves two trimpots and requires that several new holes be drilled in the amp's circuit board. However, since most of us buy matched pairs of output tubes anyway, the ability to independently adjust the two tubes' bias seemed like overkill to me. Fortunately, Bill also has an "easy" version of this mod using just a single trimpot and requiring that only two new holes be drilled in the board, and will provide the instructions upon request. This mod replaces two fixed resistors on the board with a 50K trimpot appropriately soldered into the circuit. I picked up a tiny little #60 drill bit at my local Ace hardware store and drilled the new holes exactly per Bill's excellent instructions. A couple of solder joints later, the pot was installed and the amp was ready to be re-biased. Bill's instructions cover exactly how to do that, as well. Piece of cake! You just need to be very careful, since you're measuring voltage points in excess of 330V DC with a DMM inside the chassis.

I finished up the mods and got the amp back together too late to test except at very low volume, but everything appears to be working perfectly. With the Twin tone stack mod, I can completely silence the amp now by turning down the three tone pots all the way, just like the Twin Reverb works. Tonight, I'll crank it up and see how it sounds at volume--maybe even record a clip!

Here's a photo showing the three installed mods:

[Tone2TheBone]

Voo,

My BJ had the cool bias mod done to it before I bought it. I recently did the twin stack mod (as you said very easy to do) and the mid control works much better in that you can turn the mids completely off and it sounds like the Twin Reverb amp with just the set mid tone down and adjustable bass and treble. When completely off it really does sound like a Twin only smaller and less loud. There is more room for adjustment on the mids from 0 to 4 like there never was before. Great mod. I had planned to do the tone stack mod but after discussing it with you I don't want to bother since I am pleased with the 2 mods I do have and the sound I'm getting with the new Warehouse Guitar Speakers Veteran 30 in the amp. I still get the famous Fender clean chime but now have the tight Celestion crunch with OD and distortion pedals with that new speaker. Good post Voodoo.

[duhvoodooman***]

My Blues Jr has the Jensen C12N speaker upgrade, which further accentuates the shimmery Fender top end. So I expect that the BillM Tone Stack mod should be a noticeable improvement for me, where it likely wouldn't make as big of a difference with that Celestion V30 clone.

[Tone2TheBone]

I'm anxious to hear the difference with your amp since I already knew what the BJ sounded like with the Jensen. Please do a couple of demos with the Strat neck pup. I'd like to hear the differences. I think any mod one does on the Blues Jr. greatly improves it. Even simple tube swaps and speaker swaps make huge differences.

[duhvoodooman***]

Here's a clip recorded on the Blues Junior with a variety of sounds. Recorded with my Strat in 4 sections:

#1 - Bridge/middle in parallel, clean
#2 - Bridge/middle in series, with ZYS for overdrive; SRV lick
#3 - Neck, clean
#4 - Bridge/middle in series, with tremolo pedal. What's more Fendery than surf music?

Tone stack settings were Middle at noon, Bass and Treble both around 2 o'clock.

http://www.box.net/shared/whdgejk4ko

[Tone2TheBone]

Ahhhhhh what a neck pup sound. Trem sounded kickin' Voo. I can clearly hear the C12N Jensen speaker in action but the bottom end on this speaker is much more defined and I'm sure the mods have facilitated this change. Sounds like a whole new amp my friend. Great job as usual!

[duhvoodooman***]

Here's a clip recorded on the Blues Junior with a variety of sounds. Recorded with my Strat in 4 sections:

#1 - Bridge/middle in parallel, clean
#2 - Bridge/middle in series, with ZYS for overdrive; SRV lick
#3 - Neck, clean
#4 - Bridge/middle in series, with tremolo pedal. What's more Fendery than surf music?

Tone stack settings were Middle at noon, Bass and Treble both around 2 o'clock.

http://www.box.net/shared/whdgejk4ko

BTW, the background hiss in that clip is NOT the amp--it's coming off the guitar & pedal chain. I forgot to turn on my Boss noise gate pedal before I started recording.

The Blues Jr. itself is very quiet.

[Commodore 64***]

I know this is old, but I'm bumping it because I just did the Tone Stack and Twin Stack mods too. I used 600v orange drops, because I didn't realize I had ordered them instead of the 100v ones. They are big but they fit, heh.

I also ordered a HUGE cap for power supply stiffening mod, but I didn't have the guts (or time) to drill 2 holes in my PCB. I wasn't quite sure where to do it, and I couldn't find a photo on Billm's site. So I still need to do that one. DVM, you wouldn't happen to have a photo of the Power Supply Stiffening Mod, would ya?

Also, I retubed with JJ High Gain package. So far I'm diggin' the results. I managed to jam the Fat Switch though, that thing is a pain in the ***, I'd like to remove it completely, as I never use it.

[duhvoodooman***]

I also ordered a HUGE cap for power supply stiffening mod, but I didn't have the guts (or time) to drill 2 holes in my PCB. I wasn't quite sure where to do it, and I couldn't find a photo on Billm's site. So I still need to do that one. DVM, you wouldn't happen to have a photo of the Power Supply Stiffening Mod, would ya?

At the risk of sounding like Ed McMahon, "How huge is it?"

Below are a couple of photos of the PS stiffening cap installed in my BJr. I used a 47 uf 450V cap that Tung sent me (you rock, T-man!! ) and just soldered it in parallel to the existing one. The effective capacitance of parallel caps is the sum of their individual capacitance values. The way I installed it, no board drilling was required. The two caps sit side by side, and I just bent the leads of the new cap over, hooked them around the leads of the original 47 uf cap, and soldered them there. I slid some green heat-shrink tubing onto the new cap leads for insulation, and fastened the cap body to the PCB with a big ol' glob of hot glue, so it wouldn't rattle. Very easy installation.

[Commodore 64***]

Mine's rated at 500v, so it's a little bigger than the existing, but your description is EXACTLY what I needed to know. I won't even have to remove the PCB then, to do this mod. Sweet

Also, would either of these trim pot work for the bias mod?



Pot - 3K Linear, Trim, 0.60" OVERALL Body DIA, PC Mount
3K, CTS.


Pot, 1K Linear, TRIM, 0.60" Overall Body Diameter, PC Mount
1K, CTS.


I ordered a couple of each with my last order, figuring they might be useful someday.

[sumitomo]

I'm ready for these mods now,I did an upgrade OT about 6 mts ago.I like to do a mod and play with it for awhile,the try another,oh yea thanks for the pics it helps me alot.Sumi

[duhvoodooman***]

Also, would either of these trim pot work for the bias mod?

No, that's not what you want to use for the bias mod, because that type of trimpot won't give you nearly fine enough control for dialing in the correct voltage. You need a multi-turn trimmer. Here's the one I used (visible near the bottom of the photo in the first post in this thread):

http://www.mouser.com/ProductDetail/...F8w7dO8xNvA%3d

[Commodore 64***]

Damn, this 500v 47uF is too big. I can't fit it in there. Which is fine, really, since I need to order a different trim pot. heh.

Also, my FAT Switch is busted. It doesn't stay in when pushed. I don't know anything about switches and I'm a bit overwhelmed by the selection at Mouser. I think I'd like a little toggle switch instead of the push button type. The one in the blues Jr. is a little plastic piece of crap.

Also, I think I used a 100v 0.015 orange drop in my tone stack mod...I know the .1 is a 600v. Anyways, should I have used a 600v, will this make a difference (I do have a 600v .015).