Fender Blues Junior Voicing and Reverb Mods

(Revised 8/17/03)
When the Blues Junior went into production in 1995, Some owners were quick to complain that the amp was dark and muddy sounding. The reverb was generally agreed to be noisy and muddy, too, and it was very susceptible to electrical interference. Even a nearby wall outlet could induce a disturbing amount of hum.

After hearing many complaints from members of the Fender Discussion Pages, the company almost admitted to a product flaw, then stood its ground... and totally revamped the reverb circuit and when it did a complete circuit board re-layout in early 2001.

I've seen a number of proposed fixes for the dark tone and the noisy reverb, but I think Fender did the best job of redefining the Blues Junior's tone. What follows is my personal quest to bring an old Blues Junior (designated Rev B on the schematic and circuit board) up to the new BJr's performance level. I have a new Blues Junior and recently bought a used Rev B BJr on eBay so I could experiment with it.

The new circuit board/new Rev A amp uses a different reverb drive and recovery op amp (the only solid-state component in the signal path), and the component values in the reverb circuit are somewhat different as a result. But the most significant difference is that the Rev B thru D circuit picks up the signal and reinserts it after the Master Volume control, so that any noise in the reverb circuit is fully amplified by the power stage, no matter whether you turn down the Master Volume or not. In effect, the old reverb level control is an independent volume control. The later (2001) circuit picks off the signal and reinserts it before the Master Volume, so that the amount of reverb is always proportional to the overall volume of the amp.

Making the Modifications
Changing C4 on the Rev B board from 22μF to 47μF made a big improvement in tone, closer to the current design, but the newer, cream-colored circuit board is still brighter-sounding. The difference appears to be in the layout of the board. Differences in capacitive coupling can make a big difference in tone. This modification is not necessary if you have rev C or D, or one of the newer, cream-colored circuit boards.

Changing this capacitor is a complete pain in the butt because of the way the Blues Junior is constructed. But if you have a Rev B board it's well worth you while to do so. If C3 is already 47μF you don't need to make this change or the next one--you already have a Rev C or D board.

CAUTION!! Unplug the amp and discharge all of the power supply capacitors using an insulated grounded jumper or clip lead! Clip one end to the chassis and touch the positive lead of every one of the large capacitors. Do it a couple of times—they can hold a residual charge. (I shouldn't tell you that the Blues Junior actually self-discharges rather quickly; you should always discharge the power supply caps as a matter of habit.)

You don't have to take the amp chassis out of the case, but you do have to loosen the circuit board. See Removing the Blues Junior Circuit Board for full information.

You don't have to remove the circuit card completely, and in fact, you don't want to--it's tethered to the tube breakaway card, to the power transformer, the output transformer and a couple of other random wires. Just get enough slack to pull it down far enough to clear the pot shafts from their holes. Make sure you're not stressing any wires or connectors.

You can cut some of the tie wraps that hold various bundles of wire over the circuit board, but you’re making more work for yourself when you put the amp back together. If you’re planning to do more mods to the amp, go ahead and cut the tie wraps—you can always neaten things up when you’re all done. But if modifying this cathode capacitor is all you ever intend to do, I’d say that you should do it quick and dirty: Get a new 22μF capacitor, hook the leads under the leads of the existing 22μF capacitor (keeping polarity the same), and solder them so that the new capacitor piggybacks on the old one. The resulting 44μF isn’t quite as much as the 47μF, but it’s close enough, and it’s a heck of a lot less work. It’s an ugly way to make a mod, but excusable under the circumstances!

Here's the 47μF capacitor in place of the stock one at C4. The original one had axial leads, but all I had on hand was this radial lead version. It doesn't matter whether the cap is lying down or standing up. Make sure you get the polarity right!

Second Modification: Moving FAT circuitry to second preamp stage
Rev B, as mentioned above, has the FAT circuitry on the first preamp stage, while the current Blues Junior has it on the second stage. If the capacitor on your circuit board labeled C3 is 47μF, you don't need to make this change.

I didn’t expect a big difference in tone, but it was possible that the more brightly-voiced first stage would sound better through the FAT-switched second stage. This is a relatively easy change to make, because both stages are in V1, the first 12AX7. All that’s required is to switch the circuitry connected to pin 8 to pin 3 and vice versa.

The tubes in the Blues Junior are mounted on a breakaway board, with short, thick ribbon cables connecting the breakaway board to the main circuit board. The board is held in place by eight screws, but they come out and go in quickly, making access to the board much easier than the main circuit board.

It’s easy to identify the ribbon conductors going to pins 3 and 8 (the innermost pair) and to slit the ribbon between them with a hobby knife, without cutting into the insulation. I did this, and was able to unsolder the ribbon cable terminations at the breakaway board. I then crossed the wires leading to pins 3 and 8 and resoldered them.

The difference in clean tone was minimal, although cranked tone, with the volume turned up, seemed a bit fuller, more authoritative.

Although this mod is easy, it’s probably not worth the effort. The Fender designers may have had a specific reason to move the FAT circuit to V1B, but it’s such a subtle change that you may not want to to bother with. it.

UPDATE: I thought about this one some more, and I realized that the first mod increases the bandwidth of the first preamp stage, giving it more high end without substantially affecting the bass. The FAT circuit boosts the bass and midrange response of the stage, so it makes sense to pass the full-fidelity signal to the second stage and then distort it, rather than sending a distorted signal to a boost stage that could then further distort it. I suppose I should have passed some sweep tones through the amp or modeled the stages in PSPICE, but I didn't. After the fact, I'm thrilled with the way it sounds, and that's what counts.

Updating Green-board Reverb to Current Design
Most people agree that the reverb in the early Blues Junior stinks, and most people blame it on the short Accutronics tank. Fender heard a lot of complaints on the Fender Discussion Pages board, but admitted to no faults. Then, without fanfare, it totally redesigned the reverb circuit in the 2001 redesign. The Rev B thru D circuit picks up the signal and reinserts it after the Master Volume control, so that any hum or noise in the reverb circuit is fully amplified by the power stage. The newer circuit picks off the signal and reinserts it before the Master Volume. It also uses a different dual op amp, with completely different component values, but the circuitry is essentially the same. This modification moves the pickup/reinsertion to the same point before the Master Volume as in the newer circuit, mixing the dry and reverb signals around a 430K resistor before the Master Volume.

When Fender revised the circuit, they went to a completely different dual op amp, but there's nothing intrinsically wrong with the TL072 in the Rev B. Blues Junior. Most of the "fixes" for the Blues Junior's reverb that have been published previously change either the gain or the frequency response of either the drive side of the op amp or the recovery side. To my mind, the reverb is actually overdriving the output stage, so this is the last thing that you want to do. My fix changes no components, but moves the reverb capture and insertion earlier in the signal chain--before the Master Volume instead of after.

To do this mod, you do have to loosen enough things to get the pot shafts clear of the chassis, so you can get to the back of the circuit card. Be very sure that you're not breaking any wires, cutting insulation, or stressing the circuit card when you pull it out. See Removing the Blues Junior Circuit Board for full information.

Unsolder C20 (right under the FAT switch). Replace C20 (330pF) with a 470pF ceramic capacitor. The larger capacitor will couple more of the signal into the reverb circuit and will also allow slightly lower frequencies into the reverb.

Bend one leg of C20 out, and crimp a piece of stripped, insulated hookup wire to the bent leg. Make sure you have a good mechanical connection, then solder it. Route the wire down to to R40, where it joins C16. Trim the wire to length, strip it, and insert the bare wire under the right side of R40. Crimp tight and solder. Reinsert the other leg of C20 into the right hole and solder from the back. C20 is nonpolarized; it doesn't matter which way the leads go.

Unsolder R56 (below and to the left of R40). Straighten one leg. Attach a small piece of uninsulated hookup wire, crimp, and solder. Hook the free end under the left side of R40, crimp and solder. Reinsert the other end into the upper hole for R56 and solder from the back. Alternatively, you can use a new resistor of the same value, so you don't have to use that short piece of hookup wire.

One More Step
If you play your BJr with the volume control at 10 and higher, you are likely to hear a high, ringing tone that sounds like feedback when a guitar is plugged in. It is feedback, and it's caused by the poor layout of the old green circuit board, which has no ground planes or other shielding. Some of the signal from the preamp circuitry leaks back into the amp through the input jack, and causes the feedback.

The cure is simply to shield the jack. You can use adhesive metal foil tape (not duct tape, real aluminum tape). Here's what the shielded jack looks like, and here's the pattern I cut out of the foil. The "fingers" go on both sides of the threaded portion of the jack, and ground the foil against the inside of the top of the chassis when you reinstall the circuit board.

When you get everything back together, you'll find that the reverb sounds cleaner and brighter, and has virtually none of the previous sensitivity to hum. Turning up the Reverb control no longer swamps the rest of the signal; you can use its full range. The Master Volume control will now vary the amount of reverb along with the dry signal.

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