Other Blues Junior Mods

When the Blues Junior first appeared, amp gurus were quick to identify the deficient reverb. Likewise, not everyone was thrilled with its tone. Some of these mods have been kicking around almost as long as the BJr itself. These mods were gleaned from www.blueguitar.org. I haven't tested them, other than some of the changes to the reverb dwell.

Bigdaddystovepipe Mod
The bigdaddystovepipe mod (named after its author) is an early attempt at fixing the reverb. It only applies to green-board Blues Juniors (1995-early 2001).

1. R49 from 2K to 1K
2. R50 from 330K to 470K
3. R43 from 910K to 680K
4. R44 from 120K to 180K

The first two changes increase the gain of the reverb recovery op amp. The second two increase the drive on the reverb driver op amp. They also increase the reverb dwell, changing its sound somewhat.

I've only tried half of it (increasing the reverb dwell with R43/44), which works fine, but a pot works even better, so you have adjustable dwell. But the TL072 op amp has somewhat limited drive capability, which is, as I understand it, why Fender switched to the 4560 in the cream-board amps. Switching to the new op amp might be a good idea if you're contemplating adjustable dwell. But there's no doubt that driving the reverb harder increases the signal-to-noise ratio, and gives you cleaner, stronger reverb.

The other half of the mod, R49/50, is somewhat problematic. It increases the gain on the reverb recovery, which, unlike the new design, dumps directly into the power amp stage, independent of the master volume control. Thus any noise that the recovery driver picks up is amplified full blast, with only the reverb level to control it. If you have a quiet reverb and good shielding, you're in good shape. If your amp is noisy, you've only made it worse.

Also, since the reverb level is independent of the amp master volume level, you can get some very wet (or very ugly) sounds out of the amp that are impossible with the cream-board reverb. Most players with master volume amps like the convenience of setting the reverb-to-clean level with the reverb control, then varying both with the master volume. If you change the master volume setting on an unmodified green-board BJr, you need to change the reverb setting too if you want to keep the same proportion of reverb.

Some green-board BJr players realized this from the start, and some never got the hang of it, which is why I think Fender revised the circuit so that the MV controls everything. I prefer it that way, which is why my mod makes the MV work like the cream boards, but not everyone does.

Dave Stork Mods
A friend stopped by and asked for a simple, reversible mod to juice up the preamp in his Blues Jr. and increase the reverb depth slightly. He's an apartment dweller and he wants to be able to get a hotter crunch tone at low volume without ruining the amp's ability to produce a good clean tone. Here's what I came up with on short notice:

1. Change R3, R10, and R39  from 100K to 150K.
2. Change R30 from 10K to 150K
3. Change R5 from 130K to 220K
4. Change R44from 120 to 220K.
5. Add 22uF/25V across R38.

After the mod, the preamp produced a nice bluesy crunch even with single coils. The dynamic character is good... it cleans up nicely as you let up on the pick or turn down the guitar volume. The increase in reverb depth is subtle, but I didn't want to increase R44 beyond 220K lest the reverb driver be driven into clipping.

Blues Jr. owners who are looking to try something different might want to give this mod a try. It's easy to do (if you're
used to working with PC boards) and is easily reversed if you don't like it.

When posting critiques, please bear in mind that this mod was requested, designed, installed and tested in the space
of one hour :-)

Comments: R3, R10 and R39 are the plate resistors on V1 and V2. Making them larger increases the gain. This has to cut into clean headroom, but if you want the amp to overdrive hard, this is one way to do it. Increasing R30, the output resistor from the tone stack, may provide a better impedance match into V2, the tone recovery preamp stage. Likewise, adding the 22uF cap across R38 increases the gain of V2. R5 sets the maximum drive from the volume control into the second preamp stage (V1A). The larger resistance puts more voltage on the grid of V1A, driving it harder. These changes are more dramatic than Günther's in changing the character of the Blues Junior. -Bill

Randy/Tone Lizard Amps Mods
And here are some Blues Jr. tips from Randy of Tone Lizard Amplifiers:

1. Of course increasing R4 might send the reverb driver into clipping. Do you want more reverb depth? Increase R50. Try 390K or 470K.
2. Put in a 250K master (use the part # for the treble control). You can debate about removing R40.
3. Jumper R18. Debate about the value of R17.

The phase inverter looks like a spot for a few other ideas:
4. Reduce R13 to 10K
5. Reduce R16 to 100 ohms
6. R26 is the presence control as it were. Play with the value or! short it out for full effect.

I guarantee these will work, and better. Please, this is no battle of the techs, just a tip from someone who has 'been there, done that!'

Rooster's Mods
Here's the "One Wire" mod.
You need to find the two resistors that attach the 6.3 AC voltage to the circuit ground plane. These would be R54 and R55 on the newer board, a pair of 1/2 watt 47 ohm fireproof resistors. Use your ohm meter, determine the end that is attached to ground (zero ohms vs. 47 ohms?, in other words). (You are going to do this without removing the board, BTW.) Cut the ends that attach to ground, leaving yourself as much lead as possible, raise them straight up, standing on the one end still soldered into the circuit board - together, as a pair. You are going to solder a single piece of stranded 22-24 gauge wire to this PAIR of resistors standing off the board, (strip a 1/4" of sheathing off the wire to make your solder point at the resistors).
Next, get a #10 chassis lug, attach it to one of the power tranny lugs, or drill another hole in the chassis near there if you like, solder the other end of this wire to it. You're done -the 'one wire mod'.

What this does is remove the 6.3 AC filament ground path from the PC board, something all BF and SF Fender amps employ - and for good 'GOOD' amp building reasons.

Here's the "Out of Balance" mod.
Clip the resistor R29, the 9.1K 1/2 watter out of the circuit - but leave the leads on top of the board as long as possible! Now carefully solder a jumper of single stranded wire - like one lead off a fresh 1/2 watt resistor, for example? - across this newly made gap. Of course, you can leave the resistor in and just jumper around it, too, and maybe this is the best method.

FWIW, this resistor is a new thing in phase inverter design - perhaps an attempt by Fender to help the little BJr stay remarkably clean as it is cranked? Way too clean in my book!

I have copied this circuit into other amps to 'see' what it sounds like, in circuits that I know. Whew, peeling paint! ........Oh, the other amp that shares this design is the Pro Jr, the only two guitar amps I know of - in the entire world - that have used this additional resistor.

In a BF circuit, you will find a similar end product here, although it will take the Phase inverter even further out of balance with an 82K resistor instead of the 91K you will be left with. But I figure this is way better than stock, close enough for me.

Comments: Hum control in amps like the Blues Junior depends on having a balanced filament line. If one side of the center-tapped filaments in the 12AX7s drew significantly more power than the other, it would create hum. The resistors negate this effect by having more resistance (and being closely matched) than the filaments. Some amps use a hum balance pot of around 100 ohms for this to precisely null out the hum from tubes that may not be close. The One Wire mod does move the AC ground off the board, which certainly can't hurt, but the amount of hum should be minimal. The other way to address filament hum is to float the AC filaments at a DC level higher than the bias voltage. This prevents electrons that boil off the filament from migrating to the cathode. After retwisting the AC wires coming off the transformer. though, I can't find much hum in the BJr.

The Out of Balance mod may be a good way to increase power stage distortion, and get some more of those older Fender sounds. I don't think it's advisable if you want clean headroom, but it might go well with Dave Stork's high-gain/distortion-oriented mods, above. Note the changes to the phase inverter that Randy suggests, above. There's lots of room to experiment. - Bill
 

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