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. I supply a 330K resistor in the kits, which provides a bit more reverb drive.
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.
Reverb tone/distortion suggestion:
I was thoroughly impressed by the improvements in my green board Woody Blues Junior, but I did find that the reverb was somewhat distorted either by signal overload or an excess of high frequencies causing harmonic distortions in the reverb springs. I found that turning the tone down on the guitar to reduce the highs had a dramatic effect on the cleanliness of the reverb. This lead me to believe that the problem was probably caused by excess highs in the reverb tank as opposed to general signal overload. It just happens to be that I didn’t have a 680K resistor on hand to replace R43 however I did have a 300K and a 390k on hand which I placed in series to get the needed value. (at least close enough for rock, country and blues). The junction point of these two resistors was an ideal spot to try and do some tone experimenting with the reverb. I pulled out my old capacitance substitution box and connected it between ground the resistor junction to see if a suitable capacitance will clean up the reverb distortion. Indeed this proved to be very worthwhile. I found that placing a 2.2 nf cap between the resistor junction and ground provided a remarkable clean up of the reverb signal without a substantial loss to the reverb signal.
Hi Bill, I’ve done the reverb kit in my ’91 green board Blues Junior. I dont changed eitehr the capacitor nor the resistor. So the hum and noise is gone and all is in function. Only in case I turn the volume to 12 the reverb will missed a bit. May be if I would use a 330K resistor instead of the 470 K (R56) the result will be better. Thanks a lot. Matthias
Hi Bill:
The reverb on my creamboard FBJr died. After replacing it with another tank, I found the original to be the problem. The replacement is from a 1980′s Musicman RP112-65 combo. The trouble is that now the reverb is very noisy. Unusably noisy. I don’t use a lot of reverb to begin with, just enough to add ‘life’ to the sound.
Is it possible that I put the wires on backwards (I don’t want to swap them until I know that I won’t damage a ‘vintage’ tank.), or is this tank bad also? Which tank would be a good replacement?
Soon, I’ll be ordering the stand-by switch and your new transformer. There’s a guy on e-Bay selling matched tubes for the FBJr that I’m considering, either him or the Amp Doctor.
Thanks for your help,
Ron
That tank is probably the wrong impedance for the BJr. Use the stock Accutronics 8EB2C1B tank or the replacement Ruby or MOD tank available from Mojo Musical or Antique Electronics. You can also see how to repair the usual problem with the tank here: http://billmaudio.com/wp/?page_id=46.
The red wire goes towards the middle of the amp. The black is under V1.
If you’re interested in improving the tone, start with the basic mods. Upgrading the transformer doesn’t help much if the amp is strangled by the stock electronics. There’s no magic about where you buy your tubes. A pair of JJ EL84s from Mojo, Tubestore, Antique Electronics, etc. are the same. Matched 12AX7s are a waste of money.
Hi Bill. After installing this reverb mod on my green board, I felt the reverb sounded more ditorted as i increased preamp overdrive (in a brittle way, not a true reflection). And still a bit noticable with pre amp down and main volume up. Upon reading the first comments by Dieter Billinger on 1/1/09 I’m wondering if you’ve also tried his resistor and cap mod to the reverb circuit? Upon looking for R43 on schematics it appears to say (910K) but is that a standard value? Am I looking at the wrong resistor? Can you help “clear this up”?
I have not tried Dieter’s mod. R43 on the green board is R37 on the cream board. Fender increased the value to 1MΩ. The difference is not really noticeable.
Thanks Bill. I do love the re-routed ‘pre-master’ reverb path making the balance much better while I change settings between full master and low master. Perhaps a linear pot change will be next for the reverb.