duhvoodooman
April 22nd, 2008, 11:12 AM
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 (http://home.comcast.net/~machrone/bluesjunior.htm) 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:
http://www.thefret.net/imagehosting/74480e1c5f1da68.jpg
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:
http://www.thefret.net/imagehosting/74480e1c5f1da68.jpg