Improving Green Board Reverb
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 cream board redesign. The green board circuit picks up and reinserts the signal after the Master Volume control, so that any hum or noise in the reverb circuit is fully amplified by the power stage. Apart from noise, this is a poor design because if you turn the master volume up or down you change the drive to the reverb tank. This changes the proportion of the reverb in the post-master signal, so you generally have to adjust the reverb if you make any significant changes in the master volume.
The newer circuit picks off the signal and reinserts it before the Master Volume, so the reverb changes with everything else and the proportion of the reverb in the final mix doesn’t change. It also uses a different dual op amp, with different component values, but the circuitry is essentially the same.
This modification updates the green board reverb so that it picks up the signal at the same point as the cream board and inserts it back into the signal chain at the same point before the Master Volume. The result is much quieter reverb operation, stronger reverb, and better reverb tone.
Although Fender used a different op amp, a 4560, when they revised the circuit there’s nothing intrinsically wrong with the TL072 in the green board Blues Junior. Some early “fixes” for the Blues Junior’s reverb attempted to change either the gain or the frequency response of the reverb circuit, but these early mods have proven to be ineffective; no one does them anymore.
I provide the proper capacitor, with the lead already attached, plus the resistor in the green board basic mods kit. As you can see from the photo, you simply unsolder C20, insert the free leg into the right hole of the C20 location, and connect the other end of the lead to the right side of R40. Then replace R56 with the provided 330K resistor, running it from the top hole to the left side of R40. It’s easiest to do this while you have the tone stack caps removed or before you replace them with the tone stack caps provided in the green board basic mods kit.
If you want to roll your own, the 470pF cap should be a 1 kilovolt (KV) ceramic because the 1KV caps are made to a higher quality standard that affects audio performance. Alternatively, you can use a 470pF 250V or 500V silver mica capacitor. I supply a 1KV ceramic in the basic green board kit.
You can use the existing 470K resistor in R56, but connected across to R40, but the new one in the kit gives you more lead length to work with..
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.
One 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 I mod green board Blues Juniors, I always replace the input jack with a Switchcraft all-metal input jack (see kits). I bypass the feedback-prone circuit board traces entirely and wire it directly to the preamp input resistor with shielded coaxial wire. I highly recommend the Switchcraft input jack with all green board mods.
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.