A Vox-like Voice for Blues Junior

These modifications were created by Günther, a Fender Discussion Pages member from Germany. He has made his Blues Junior into a loud, bright, gig-worthy machine that sounds like a Vox AC-30 and can stand up to a drummer. His mods emphasize the BJr's clean tones; he uses a boost pedal for overdrive.

See my page on Blues Junior circuit board removal for instructions on removing the circuit board. You don't have to remove it completely to reach the back of the board.

I've added some notes on the theory or background behind some of the mods. - Bill

Günther's mods:
Some important things at the beginning:

You have to know what you are doing. That means you have to know how to solder, how to build up an electronic device, how to read a schematic, and how to work on tube circuits with their high voltage (discharging caps etc.). If you do not know anything about these things, give the job to somebody else who does.

Open up the amp and do your job only one time, that means do everything you ever want to do only once, to avoid broken connections etc. that can result from dismounting the PCB.

Try to understand why I mention a part to mod and what it will give for a result in tone, I will do my best to explain, but what I think is best may not to be for you.

Buy each part you need before beginning to mod; you can get most parts from Rubytubes

So let's start: I brought a BJr because I wanted a small amp (20 years ago I had a Mesa Boogie that worked fine). So I looked for a true tube amp with a standard vintage circuit. (The BJr has only one FET-transistor for the FAT switch; that is OK because it is not in the signal path.) I have to gig with this amp, and I use it only for clean-sound with a Fender Stratocaster. For overdrive sound I use a Marshall pedal. The stock BJr sounds good, but it is too small sounding and not loud enough. So what is loud? OK, take a VOX AC30, it has 30 watts and two 12" Celestion speakers. Play it with the loudest clean sound possible. To get the same amount of volume with a stock BJr, you need about 4 of them. That is partially because of the less efficient speaker of the BJr. If you change the BJr speaker to the same Celestion you will need two BJrs to get the same loudness. That's the loudness you need to play clean solo guitar (Dire Straits, for example) together with a drummer of normal loudness and have a little headroom. Also don't believe that this amp is a versatile amp where you can get a dozens of different sounds, it has one sound -- that's all!!


Speaker
I decided to put in a better speaker, the best choice was an Electro Voice EVM 12L. This speaker does not have a distinctive sound of its own like a Celestion Blue Dog, but it is the loudest speaker you can get (as twice as loud as a normal Celestion) and works fine in a BJr. I brought a used one on eBay for $70. This speaker has a 4-inch voice coil and a huge magnet. It does not fit into a stock BJr; you have to pull the speaker baffle out and turn it over so the hole is on the opposite side. (remove the grille cover, the nuts, etc.) and then it works. On the grille cover I glued a round piece of cardboard (diameter 3") in front of the center of the speaker to avoid the high frequency beam. On the sides of the amp I mounted Fender tilt-back legs (14"), so you don't need a stand. I tried a stand-- too much treble. Don't forget amp corners! I covered the reverb spring with a piece of thick carpet. This cleans up the reverb and the whole amp a little. (Bill Machrone presented a reverb modification in the Fender Discussion Pages. I tried it, he had done a good job, it works fine but I don't like it. It's a personal taste; I like the stock reverb more.) Because this amp is a small but heavy beast, I removed the grip and a leather grip took place.

The Electro-Voice EVM 12L is discontinued, although you might be able to find one on the used market, as Günther did. The EVM 12L has a published sensitivity of 100dB at 1 watt at 1 meter. Fender doesn't publish the specs on the Eminence-built speaker in the BJr, but it's similar to other Eminence speakers with a sensitivity of 96 and 97dB. So a speaker with 100dB sensitivity will sound twice as loud at a given power level. That means your amp isn't working as hard to produce a given volume output, and thus stays more in its clean range. 

If you need the Blues Junior to be louder (3dB more speaker sensitivity is the same as ten times more amp power), there are other speakers with sensitivity similar to the EVM 12L. - Bill

Now it's time to remove the PCB and make some changes. The part numbers are for the green-board Blues Junior.

Power Supply
Remove C15, C17, C18, C19 (power supply) and replace these caps with Ruby Caps. Add another 47uF to C15 in parallel.  Add to each of the 4 caps
a silver mica 250pF in parallel. Replace C32 with the same value Sprague Orange Drop. You can replace the four diodes CR1-CR4 with FREDs [fast-recovery epitaxial diodes], but I think it is not necessary. These mods to the power supply will result in better bass response, but will not change the sound at all. It is a quality improvement only.

Using premium-grade caps may make the amp more reliable for heavy gigging, although there are no recorded failures of Blues Junior filter caps that I've heard of.

Increasing the size of the cap that feeds the output stage (doubling, in this case) theoretically "stiffens" the power supply, eliminating sag, and provides extra current for loud bass notes. I tested my green-board BJr with an additional 47µF capacitor, and I heard a very slight increase in clean tone on the bass strings--with the volume at 4 and the master volume at 12. It was a very subtle change. Adding 250pF caps in parallel to the large filter caps may help to reduce high-frequency noise and hiss. FREDs switch faster than normal silicon diodes and theoretically generate less noise. Whether the difference is audible is highly debatable, since the difference is down in the nanoseconds range. Separating and twisting the AC leads has more effect on hiss and hum.  All caps in the power supply section should be rated at 500V. All others should be 400V to maintain factory safety margins. - Bill

Preamp
Replace R1 with 68K - I have seen this value in older Fender amps and decided to use it too; I don't know what it does on sound. Replace C3 with Ruby 22uF/25V if there is a 22uF. If your amp has a 47uF, you have to replace it with 2x 22uF/25V (Ruby) in parallel). Replace C4 with Ruby 22uF/25V. Replace C1 and C16 both with Orange Drop 0.003uF + Silver Mica 250 pF in parallel. Replace C25 with same value (250pF) Silver Mica. Replace C19 with Orange Drop 0,033uF + Silver Mica 250pF parallel. Replace C5, C6, C7, C8, C9 with Orange Drops, C33 with Silver Mica Replacing these Caps will open up the sound and give smoother highs and more depth to the tone; it will not change the overall sound of this Amp.

R1 is the input resistor. 68K attenuates the input a little bit more than the stock 10K, but won't make any difference in tone. The 68K resistor is better at rejecting radio interference. Silver mica caps are expensive, but generally regarded as the highest-quality caps you can get for the signal chain. Orange Drops are premium caps for the larger values. Can you actually hear a difference? Probably not. -Bill

Negative Feedback Loop/Presence
Change R 27 to 100K, R26 to 0K (replace with link) and R16 with 1K This opens feedback a little and give a bit more presence, these are the same values Fender uses in the Pro Junior. Overall it will give the tone more life.

Brightness
Change C2 to 250pF silver mica. this will add more highs to the clean sound with settings below 5 of the volume pot. Change C35 to 680pF Silver Mica (if not avaliable, 500pF+250pF in parallel) to get more sparkling highs. These two mods will not change the overall sound, but will give you more "sparkling Fender highs", you cannot get this tone by just cranking up the treble pot. Of all the mods, I think these are the most important.

C2 is the treble bypass capacitor for the volume control. The stock value is 100pF. Changing it to 250pF will bleed more trebles and high midranges around the the volume control, but only at low volumes. C35 is mounted right under the master volume control, and it bleeds highs to ground. The stock value, 1500pF, gives the green-board BJr its characteristic dark tone. The smaller capacitor makes it much brighter sounding, like the newer cream board. As Günther says, you can't get the same sound with the treble pot. If you want the old sound, try my switchable voicing mod that retains the warm, creamy sound and lets you switch to a brighter tone. - Bill

Tone Stack
Replace the versatile BJ tone stack with the less versatile Twin (Champ) tone stack. It will result in a deeper bass. With this mod, you don´t have to use the tone controls any longer to get a good sound, normally you can leave them on middle position. You are able to dial in more treble, can remove bass - but not give much more, remove some mids and give a little more. As I said it´s less versatile, but I think the overall tone is much much better, in the middle of Vox AC30 and Twin Reverb. I tested it only with the Electro Voice speaker, so I can't say if it works with other speakers too. So here is the mod of the tone stack: Replace R6 with 91K (more Brownface sound), replace C11 (0.022uF) with 0.1uF Orange Drop, replace C13 (0.022uF) with 0.047uF Orange Drop, add a resistor 22K parallel to the mid pot and make a link between the center of the mid pot and the pin of the Mid Pot that is connected to the Bass Pot.

My tone stack mod is somewhat simpler, replacing just two capacitors to get more emphasis in the bass and more available midrange. It retains a large adjustment range, and passes a little more signal than the stock circuit. Smaller values of the slope resistor (R6) steer more signal towards the bass and mids; larger values emphasize the treble. -Bill

Reverb Driver IC
Remove the IC for the reverb Circiut, replace it with a socket and then put a NE5532 into the socket. The NE 5532 is less noisy and sounds better, can't say if there is a hearable difference, but does not cost much.

Fender switched from the TL072 to a 4560 in the cream-board revision. The 4560 is supposed to have better current-handling capacity. I haven't checked to see how the 5532 compares. - Bill

Speaker Jack Resistor
On the Speaker Out PCB there is a Link (on the PCB) that shorts if there is no Speaker connected. Open this link and solder a 8 Ohm/20Watt (or 2x 15Ohm&10Watt parallel), so that this resistor takes place when there is no speaker connected.

Shorting the jack is standard practice for tube amps. The output transformer gets a little warm if you run it without a speaker plugged in, but it won't damage the amp. - Bill

Resoldering
Resolder the complete PCB to make sure that there are no dead connections.

Tube Recommendations
For tube V1 the GE 5751 will give a rich and deep 2-dimensional tone, for V2 use Phillips JAN 12AX7, for V3 Phillips JAN 12AT7, for Output (V4,V5) a matched pair of JJ EL84 (36 up to 40mA) The JJ/Tesla Tubes work fine in this Amp, make an improvement over the Fender tubes, and are not expensive.

Now you have got a BJr that is grown up, is nearly as loud as a Vox AC30 and sounds like a AC30-TwinReverb.

For PA or Recording you can connect and mount a H&K RedBox into this Amp, for me that works.

Overdrive Tone
And now overdrive sound. The BJr has only one channel, so you can't switch between clean and overdrive. My decision was to get the best clean sound possible and do the rest with a stomp box. A classic crunch or overdrive sound you can get with a Marshall Amp, so I decided to use a BluesBreaker II. This pedal has a true bypass and is easy to use, but requires some mods to sound really good. You have to replace the IC with a TL072, and the two clipping LEDs with MOSFET diodes (MOSFET transistor + germanium diode, take a look at Geofex.com, I use BF170 and AA112). For the BB2 you have to take two MOSFET diodes in series to replace one LED. Now take this modded BB2 and set the volume of this pedal to a level where it pushes the BJr a bit. Then adjust the amount of distortion to your need. It is important to push the BJr a little with this Pedal, otherwise it sounds to fuzzy. For a more modern Marshall sound you can take the Marshall Jackhammer (for me it has to many knobs), you don't need to mod it, but also you have to push the BJ a little.

Typical settings with BJ and Stratocaster: Volume 4, Treble 6, Mid 6, Bass 6, Master 12. The whole rest is done by your fingers and the controls on your guitar.

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