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.

reverbmod1

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.

shieldedjack1 metalfoil1

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.


28 Comments

  1. Dieter Billinger says:

    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.

    • TP says:

      Thats a good suggestion. I always found the reverb of the BJr too shrill with too much treble . I tried a 1,5nF across R38 (cream board) to drive the reverb with a little less high frequencies, but what I found to be even more effective: a 2,2nF cap across the reverb poti smooths out the reverb signal itself and sounds more pleasing to my ears. Anyway I left the bypass cap C23 in the circuit, would be probably even more effect to cut that out too.
      BTW: I have a Limited Edition BJr with the wooden sunburst enclosure from 2003, and I had to change also all power supply filter caps. Two of them leaked and shorted the supply voltage after a while.

      Many thanks to you, Bill for your passion and sharing so much usefull information about this nice little amp…

  2. 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

  3. Ron Ditty (P-strat63) says:

    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

    • bill says:

      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.

  4. Glenn Heller says:

    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”?

    • bill says:

      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.

  5. Glenn Heller says:

    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.

  6. Greg Mauser says:

    Hi Bill,
    I didn’t see the answer I was looking for in the previous posts. In the green board reverb mod, can you elaborate on the difference of the reverb sound when using the 330K
    instead of the 470K for R56? I’m asking because I put the Ruby tank in and it sounds REALLY good but my ears are not as finely tuned as yours and
    I have not tested at all levels yet.

    Thanks so much,
    Greg

  7. NAUSICAA says:

    Bill, is it neccessary (or at least preferable) to sheild the input jack even if I keep my stck plastic one? Or, does it only make sense to shield the jack if its all-metal?

  8. greekscramble says:

    Hi Bill,
    Are there instructions for bypassing the reverb all together? My chicken head knob has broken off of the reverb, and it is pinned at 12. I never use reverb, and I don’t want it.

    Thanks in advance.

  9. Colins says:

    Hi

    Just came across this page looking for information on my BJ III reverb feedback and It kinda sounds like the same thing, feedback/distortion when the reverb is set around 10 and above. It just seems to happen when I hit a couple high strings together. Is there anything similar I can do to the BJ III to quiet it down cuz its kinda ugly.

    Thanks for any help!
    Colin

    • bill says:

      You may just have a saggy spring in your reverb tank. Try isolating the tank with some heavy cloth. If that helps, you may want to get a reverb bag. Or it may make more sense to have the dealer replace the tank.

  10. tim2b98 says:

    Hi, Just recieved the Basic kit that came with a few extra parts that the instructions did not mention. I found this page and now I know what the 470pf cap and resistor (I recieved a 430k) are for. But I still don’t know what the blue 100pf cap is for? …Tim

  11. brooking says:

    Hi Bill,

    I have a question about the internal feedback you mention in relation to the shielding of the input socket. I have just aquired my first Blues Junior and it exhibits the following issue, both before, and now after I have fitted a Switchcraft input socket. (I have also just comleted your Reverb mod and lowered the bias; mine is a REV C Green board).

    The problem is I get squealing feedback when the treble and volume (pre-amp) are set high, typically greater than 10.

    First I shielded the input and now I have wired in a Switchcraft socket in place of the original part but still the feedback occurs. I used shielded audio cable from the socket and picked up the connection to R1 at the adjacent wire link. Do I need to completly isolate the “feedback prone traces” by cutting them? At the moment, although by-passed this trace is still connected.

    Also noticed today that the feedback seems sensitive to which pickup I use. On my Tele humbucker neck pickup I can go to higher treble/vol settings without so much feedback but if I switch to the single-coil bridge pickup the squeal kicks in earlier.

    Any ideas, or do I need to go searching for another source of the feedback/ oscillation at high treble & vol settings?

    Thanks,

    brooking

    • bill says:

      Yes, you definitely have to remove the jumper that ties the input trace to the grid resistor. Otherwise, the trace is just a big antenna. When I install a Switchcraft jack, I also orient the spring away from the treble and volume controls to further reduce capacitive coupling.

  12. Hello everyone,
    I have a 2009 Tweed Blues Junior cream board.
    I have the problem of reverb that saturates after the final stage. I saw that there is
    modified on the green board. that changes can I do for the cream board? Thanks for your help.
    Paul.

    • bill says:

      No, the modification makes the green board operate like the current cream board.

      If you want less reverb sensitivity, change the reverb control to audio taper so it comes on more gradually.

  13. snakehips says:

    Hi there !

    I have a question about replacing a reverb tank – and which one you would suggest to go for.
    I have a modded Fender Blues Jnr Mk3 (by RAT Valve, UK) – see this link :
    http://www.ratvalveamps.com/fender-blues-junior

    Mine is the “Red October” red combo, pictured.
    It has a “Power Scaling” half power switch and Silicone Diode / Valve Rectifier option switch too, and variable boost footswitch (also with Reverb on/off) amongst other improvements.

    I’ve been wanting more bass and made some “easy” changes – all before I found your website – perhaps I might have done some things differently ? !!
    I use acoustic guitars with old Dearmond “monkey-on-a-stick” pickups from the 1940/50’s – and they are a bit thin on the bass side – so I was looking for a way to fill out the sound more, on the lower end.

    So, I had a new larger cabinet made to house the Blues Jnr chassis – so I could fit a 15″ speaker. I went for a Jenson P15N alnico speaker. I gave all the relevant dimensions to the cabinet maker – but unfortunately, the cab was still made not deep enough. The chassis fits but the speaker magnet gets in the way of the EL84 valves fitting into their sockets. Doh !
    So, I changed speaker for a shallower Jenson C15N speaker – and all fits together fine.
    Unfortunately, since I did all that, my Reverb was sounding a bit “boingy/clangy”.
    I removed it and found one of the stock 3 springs was broken at one end. Was advised by the RAT Valve guy that while it could be repaired, it is very tricky to do and I would be as well getting a new Reverb can – and a NOS US made one if possible.

    That’s the long-winded story so far, leading up to my actual question.
    As my new cabinet is bigger, it is actually a bit over 18 inches wide inside the cabinet – and as such, wide enough to fit a full length reverb unit I think.
    I’m in the UK and a good source of NOS Accutronic reverbs has apparently dried up of NOS items.

    Your site somewhere mentions a standard Fender Blues Deluxe reverb unit is compatible (if it fits, which it should, in my case). The RAT Valve tech I bought the amp from is directing me to a choice of two units that are described as upgrades for Vox AC30 CC’s – one with a medium and one with long reverb decay :

    Upon asking the German supplier that used to have NOS US reverbs for advice, they have suggested that any of these would be suitable :

    Dear Richard,

    the Blues Junior calls for an “EB” Type regarding impedance and a “C” for grounding, so You can use:

    NOS Accutronics: nothing left, I regret.

    Accutronics current production:

    http://www.tubeampdoctor.com/en/shop_Reverb_Cans_Accutronics_NEW_Producti/RA_4EB2C1B_Accutronics_Reverb_Can_3475

    http://www.tubeampdoctor.com/en/shop_Reverb_Cans_Accutronics_NEW_Producti/RA_4EB3C1B_Accutronics_Reverb_Can_3477

    http://www.tubeampdoctor.com/en/shop_Reverb_Cans_TAD_Reverb_Cans/4EB2C1B_TAD_Reverb_Can_2_springs_2200

    http://www.tubeampdoctor.com/en/shop_Reverb_Cans_TAD_Reverb_Cans/4EB3C1B_TAD_Reverb_Can_2_springs_2201

    You can also use the 3 spring type:

    http://www.tubeampdoctor.com/en/shop_Reverb_Cans_Accutronics_NEW_Producti/RA_9EB2C1B_Accutronics_Reverb_Can_3479

    http://www.tubeampdoctor.com/en/shop_Reverb_Cans_TAD_Reverb_Cans/9EB2C1B_TAD_Reverb_Can_3_springs_2215

    So, does anyone out there have any good recomendations for my amp ?
    Anyone I have asked so far would want a full length reverb where possible.
    Medium or Long decay – I don’t know – and I suppose it is all about personal taste.
    Many thanks in advance for any advice.

    • bill says:

      Go for a MOD, Belton, or Ruby 4EB2C1B for medium or a 4EB3C1B for long. We’ve experimented with the 9EB2C1B 3-spring tank, and I think it sounds too thick.

  14. rosshulio says:

    HI Bill,

    I recently did the green board reverb mod and it improved the reverb a little and doesn’t have the awful noise it had before. It still doesn’t sound as nice as the reverb on my friends vr2/cream board. Is it possible to mod the circuit to be exactly the same as the never version?

    Thanks for your help!
    Ross

    • bill says:

      Green boards don’t sound like cream boards–there are many small differences in the reverb circuits, too many to change. There are also small differences in the preamp circuitry. The board layout itself is different, so the small interactions among components and signal paths gives each its unique tone. Try to cherish each for what it is. :)

  15. kentf14 says:

    Did the reverb mod last night on my green board and couldn’t be happier. With the addition of a new MOD tank, the reverb range is opened up significantly… and it’s nearly silent compared to the massive hum present in the stock config.

    Thanks for the kit and tips!

  16. SteveC says:

    I’ve had Bill’s reverb mod (and almost all of his others) plus a Ruby tank in my green board BJ for years; I love the amp but have never liked the reverb, even without the hum and weird interaction with the master volume. I thought maybe I just didn’t like reverb, or perhaps it was the digital driver or the short tank. But I recently got and restored a ’66 Princeton Reverb, and was surprised by how great its reverb sounds — by comparison, the BJ’s was way too metallic and trebly. Since the rest of the amp pretty much stands up to the PR, I came back here to see if there might be a reverb tone fix, and saw Dieter’s and TP’s suggestion of a high bleed cap to ground. So this morning I put in TP’s 2.2 nF (.0022 mF) cap from the reverb pot’s input pin to its ground pin. Wow! Rich, pure, balanced reverb that sounds every bit as good as the PR’s! The treble to bass mix now is just the same with and without reverb, and it gives an even smoother slope to Bill’s audio taper control pot, so its even easier to dial it in. Now it gives a touch of reverb at 3 (like the PR at 1), a moderate amount at 6 (PR at 3), a lot at 9 (PR at 5), and pure drench at 12 (PR at 7). I see a lot of BJ bashing on websites due to its tinny reverb, especially compared to the PRRI. From my experience with this one amp, this seems like an easy and effective fix, on top of the much better signal management that Bill’s basic green board reverb mod provides.

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