Bill M instructions for Blues Jr

Discussion in 'Shock Brother's DIY Amps' started by Tom Freeman, May 11, 2016.

  1. Tom Freeman

    Tom Freeman TDPRI Member

    Posts:
    17
    Joined:
    May 11, 2016
    Location:
    Toronto
    Hey, so I've been reading this forum for ages, and I finally decided it's time to join the site as a real member (been a tele player for almost all the time I've played electric, so might as well ha ha).

    Anyways, I have a Blues Jr that sounds good, but I think could be made to sound better. I've already replaced the speaker with a C-rex, and upgraded the reverb tank, and the difference has been substantial. I also plan to upgrade the tubes in the next little while. However, I want to see what I can modify in the actual circuit to get the sound I'm looking for. I know some people will say to just leave it alone, but that's out of the question - I'm switching into an electronics technician program in college this summer as soon as I finish a couple university courses, specifically so that I can get all the knowledge I need for working on amps, so I'm going to get into tinkering with circuits one way or another lol. I've already had experience wiring guitar circuits the past couple years, and got into pedal circuits a little while ago (in fact, I've started building pedals from scratch).

    So I was wanting to try the Bill M mods for the Blues Jr, specifically his basic mods and the upgraded input jack, standby switch, and master and reverb pots. If I liked them, I was going to then try out a few of his transformer mods to increase the wattage of the amp for more headroom. However, when I was ready to pull the trigger and buy the basic mod kit from his site, I saw that his online store has been shut down, with little hint as to when it will be open again...

    Now, I want to support Bill and pay him if I can, but I also want to work on my amp sooner rather than later. Since I know that I can probably buy the components from the kits online myself, all I really need are the directions. I've been scouring the internet for many weeks looking for them, and I've come up with posts from several forums that sort of explain what to do, but they all seem to be missing information. For now, I just want a confirmation on what exact parts to use for the basic mod kit, and the actual directions that come with the kit (specifically installing and setting the bias trimpot). If anyone is willing to help me out, I would really appreciate it.

    Thanks
     
  2. King Fan

    King Fan Friend of Leo's

    Posts:
    4,784
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    Welcome! I'm not able to steer you to instructions for the mods. I'll bet tons of folks here have done them. There are some pretty arcane (and old) amp forums out there, some of which Google can't crawl readily, that may contain more details. I'll wait for the smarter folks here to check in.

    I haven't been down the BJ road, but know someone who travelled it for years and found Billm a great source of info. Supporting Bill's work is a great idea, but he'd had some serious health problems (haven't kept up there, either, my bad) so sourcing the parts may not be disrespectful.

    For the simpler mods, Bill's blog should tell you what you need to know to get started. For example the standby switch mod is described here

    http://billmaudio.com/wp/?page_id=327

    and if you look up that Carling off-standby-on switch several suppliers (?Mouser ?TubeDepot ?AES ?Carling) link wiring diagrams for that switch, and lots of other builds incorporate it into their amps. (You'll also note Bill says on the page that the mod isn't really needed on a small amp like the BJ!)

    Finally, can't help but mention, with all respect for the BJ, before you spend big bucks, my BJ-modding friend eventually found the BJ too small a canvas, and went on to build, repair, restore, and modify the older Fender amps the BJ was built to sound like.
     
    Tom Freeman likes this.
  3. Tom Freeman

    Tom Freeman TDPRI Member

    Posts:
    17
    Joined:
    May 11, 2016
    Location:
    Toronto
    King Fan, thanks for answering! Yeah, there's definitely a lot of guidance on Billm's site for a number of the smaller mods I eventually want to try. I've also found some information that will help me with a few of the basic mods. For example, I found one guy on another forum who listed the values Bill suggests for the tone stack mod, and another post that seems to describe the power stiffening mod (though I'll have to double check that before I can be sure it's right). I guess I just want to see if I can get some help with the bias mod, as I'm getting all sorts of alternative ways of doing it. Since that seems to be the riskiest of the basic mods, I'd really appreciate it if anyone who's done them could give me a bit of advice on that!

    As for the Blues Jr being too small a platform, I'm not worried about that too much. The cabinet size has never bothered me, and if it ever does, I'm not against building a larger one (or even converting the amp into a head lol)
     
  4. King Fan

    King Fan Friend of Leo's

    Posts:
    4,784
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    Good on ya, mate. BJ pros, feel free to jump in here -- I may steer this nice gentleman wrong at any moment. o_O

    You're right adjustable bias would be nice. You probably saw this little thread on the Fender forum from 2008 mentioning a single-pot method (Billm comment at end).

    http://www.fenderforum.com/forum.html?db=&topic_number=658174

    Also, though I know nothing about BJs or modding on the PCB, this general guide from Doug Hoffman about adding variable bias to Fenders might be useful.

    http://el34world.com/charts/Biascircuits.htm
     
  5. 5F6Animal

    5F6Animal Tele-Meister

    Posts:
    172
    Joined:
    Jan 4, 2016
    Location:
    Beaumont CA
    I did the mods with the presence mod too. Along with a speaker change and tubes, the amp was a totally different thing.

    The ability to set the bias is a big thing on that amp. Fender really cooked the tubes on the BJ.

    I highly recommend doing the twin tone stack mod too.

    The instructions are very clear and easy to follow too. Go for it, you'll be happy with the results.
     
    Tom Freeman likes this.
  6. Tom Freeman

    Tom Freeman TDPRI Member

    Posts:
    17
    Joined:
    May 11, 2016
    Location:
    Toronto
    5F6Animal, would you happen to still have the instructions for doing the bias exactly as Bill describes? I've been trying to find them online, but ever since his site closed down the online, I've been getting different answers from different places, and I want to make sure I minimize risk to my amp (and avoid electrocution ha ha).

    Yeah, I'm hoping to eventually do all those mods. How versatile is the presence control? Was it at all awkward putting it in?
     
  7. 5F6Animal

    5F6Animal Tele-Meister

    Posts:
    172
    Joined:
    Jan 4, 2016
    Location:
    Beaumont CA
    I sold that amp years ago(Totally stupid move!). After the mods it was basically a mini bassman. I loved every part of it after the mods and speaker swap. The presence control worked great. Just as good as it would have from the factory. You do need to drill a hole in the chassis for the pot though. Not at all hard to install the wiring, I believe it was 2 wires and removing a resistor or something.

    Billm went by negative grid voltage for the bias, that's not the way to do it right but it is the easiest(and safest) way to do it. I prefer the plate to center tap shunt method with your DMM on the milliamperes setting. Having 2 meters is great for that method. One to check the ma and the other to check the plate DCV.

    I'm headed off to work now, I'll check back later or send me a pm. RobRob and countless other guys can help you with basing the amp. You do need to have installed the bias trim pot to do it though. Another easy mod.
     
  8. chas.wahl

    chas.wahl Tele-Meister

    Posts:
    116
    Joined:
    Dec 8, 2012
    Location:
    NYC
    Just a casual observer of the BillM site, but it looks like the info's all here:
    http://billmaudio.com/wp/?page_id=90 (The Billm Mods in Detail)
    Am I missing something?
     
  9. ICTRock

    ICTRock Tele-Afflicted

    Posts:
    1,493
    Joined:
    Jul 31, 2014
    Location:
    Tampa,FL
    I'll cut/paste something I posted over at madbean after a similar request

    "if you can bear with me I'll run them all down for you(at work at the moment). I just came across this thread and I've done mods to both green and cream boards. The ones I don't care about (like drilling the chassis to add stuff) aren't included but I've done the tone, twin stack, reverb, standby, 4/8/16 Hammond transformer, etc. And you can save a lot of money because most of billm's mod parts are available over at antique electronic supply if you know where to look for them.

    obviously some of his pages give you a straight rundown and instructions with values of what to do like the following
    http://billmaudio.com/wp/?page_id=58 - twinlike mid knob
    http://billmaudio.com/wp/?page_id=34 - fixing green reverb
    http://billmaudio.com/wp/?page_id=141 - cathode follower
    Original Fender® Part Number 00373230000 - better reverb taper pot, better master volume taper as well both 50KB stock, changed to 50KA
    Tube compliment for maximum clean headroom with cathode follower mod v1 12AY7, V2 12DW7, v3 12AT7
    Hammond 1760F 4/8/16 output transformer for Blues Jr
    Granted I'm not home to check my amp but I believe he swaps the 22n bass cap for 100n like the blackface twin. he also swaps the 22n mid cap for a 15n as well. If you wanted a true blackface twin tone section you'd need to change the taper of the bass pot to an audio taper and change the mid pot from 25K to 10KA and the mid cap to 47n.

    Sparkle mod changes c35 on a green board (c9 on cream) from 1.5n to a variable from zero to 3n. zero would be series III value, 1.5n is stock series 1 or 2, 3n is darker than useable(imo). I waited on this mod to last and voiced it accordingly. I found stock on the series one and two to be darker than "the fender sound" I was looking for. Also consider removing and jumpering the 33n capacitor at C19 (c10 on cream board)

    presence control changes negative feedback resistor r26 from 7K5 into a 10K pot.

    Mod brand reverb tank https://www.tubesandmore.com/products/P-RMOD-8EB2C1B
    btw the long decay version doesn't play well with the green board even after the mod

    power stiffening mod - adds a second 47uf cap in parallel with the 47uf in the power supply
    fixed bias mod - increase r31(r52 cream board) to 27K
    off standby on switch - https://www.tubesandmore.com/products/P-H520
    green board instructions
    4 poles left to right, top to bottom numbered 1-4 (pole 1 oriented middle of switch body, left hand side when looking at switch)
    pole one to black lead from S2A, pole two to CP12(remove existing lead at CP12), pole three to black power cord lead, pole four to lead that was at CP12

    cream board instructions
    same switch orientation and labeling as before
    pole one to black power cord lead, pole two to CP11 (remove existing lead at CP11), pole three to black lead from S2A, pole four to lead that was at CP11"
     
  10. Tom Freeman

    Tom Freeman TDPRI Member

    Posts:
    17
    Joined:
    May 11, 2016
    Location:
    Toronto
    Thanks everyone who chimed in here - I've been waiting a while for parts to arrive, and finally got the chance to install the mods the other day. Based on a lot more research than what I ever cared to do for any of my courses at university, I have compiled this information on the mods:

    Tone Stack:


    C5 Treble - 250pF, Silver mica

    C7 Mid - 0.015uF, Orange Drop

    C6 Bass - 0.1uF, Orange Drop


    C2 0.022uF Input Coupling

    C8 0.022uF Output Coupling



    Twin Stack:


    Jumper across mid pot terminals (see Billm’s site)


    Oscillation Prevention:

    Solder 100 pf ceramic cap across r30?


    Bias


    50K Bourns 3299 trimpot

    http://www.fenderforum.com/forum.html?db=&topic_number=658174



    Power Stiffening


    F&T 100 uf 500v Electrolytic Capacitor

    Essentially, for all of the basic mods except the bias control, all that has to be done is swap out components for the values listed above. I've included suggestions for what type of component that is often suggested on most pages (ie. orange drops for most of the cap swaps). As for the bias control, I found a guy on another site who kindly sent me a pdf of the bias trimpot mod from when he did the mod. I'll include it here for anyone wanting it - the instructions for installing and setting the bias are pretty straightforward if you take your time. As people always warn, be careful and don't electrocute yourself when doing the mods.

    The mods were definitely worth it, and the amp sounds much bigger - almost overwhelming, in a good way. The bass performance is so much stronger that I'm considering building a new cab to do it justice and let it resonate properly.
     

    Attached Files:

    King Fan likes this.
  11. randomhitz

    randomhitz Tele-Meister

    Posts:
    366
    Joined:
    Jan 29, 2008
    Location:
    Missoula, MT
    Tom--thank you for all your effort. A couple of quick questions. What should the voltage rating be for the tone stack and coupling caps be? Also, what kind of material should the coupling caps be made from?
     
  12. mpassell

    mpassell TDPRI Member

    Posts:
    64
    Joined:
    Sep 26, 2011
    Location:
    San Francisco CA
    I would use 600V caps for those. Probably a bit high but it's insurance against a short off the power rail.
     
  13. Tom Freeman

    Tom Freeman TDPRI Member

    Posts:
    17
    Joined:
    May 11, 2016
    Location:
    Toronto
    Yeah, I believe I used 600V for all the orange drops I used. All the caps I swapped were orange drops except the silver mica caps I used for the treble control and the over R30 (although ceramic should be fine for that if you want to save money - it doesn't affect the sound, it just prevents oscillations).

    Also, when you go to put everything back, take the time to properly dress the and braid the wires as explained on Billm's site. It really helped make the amp much quieter, especially when the bias was correctly set.
     
  14. jonmichael02302

    jonmichael02302 TDPRI Member

    Age:
    51
    Posts:
    56
    Joined:
    Aug 3, 2016
    Location:
    Brockton, MA
    I was curious, on C2 & C8 I see you put in .022 uf caps... & I was told to use. 0022 uf caps & before I install these mods, I want to be sure I get it right. Can anyone confirm which ones to use. I find it more than strange our values are identical with the exception of a few .0's. Any help would be greatly appreciated.




     
  15. dankilling

    dankilling Tele-Afflicted

    Posts:
    1,437
    Joined:
    Oct 28, 2014
    Location:
    Lehigh Valley, PA
    .0022uF coupling caps would really take bass out of the signal. .022uF are much more common there.
     
  16. jonmichael02302

    jonmichael02302 TDPRI Member

    Age:
    51
    Posts:
    56
    Joined:
    Aug 3, 2016
    Location:
    Brockton, MA
    Only reason I ask is that according to the schematics C2 and C8 call for .0022 but hey, when has fender ever got it right when building these mass produced amplifiers....
     
  17. dankilling

    dankilling Tele-Afflicted

    Posts:
    1,437
    Joined:
    Oct 28, 2014
    Location:
    Lehigh Valley, PA
    There is a formula to calculate the actual frequency roll-off based on input and output impedance. Here's the tool I like to use to play with different values before breaking out the soldering iron.

    https://www.ampbooks.com/mobile/amplifier-calculators/coupling-capacitor/calculator/

    Remember, the frequency range of a guitar begins at 82hz-1200khz, but that is without harmonic frequencies included so I would rather err high than low. Too low and you waste energy and the dreaded 'farting out' muddiness that people like to take out of other circuits like the 5e3. Their coupling capacitors are big enough that the low-end roll off is actually lower than a bass guitar.
     
  18. dankilling

    dankilling Tele-Afflicted

    Posts:
    1,437
    Joined:
    Oct 28, 2014
    Location:
    Lehigh Valley, PA
    I quickly plugged in some values based on C2 at 50% volume and came out with this-
    Low E gain with .0022 = -8.71db
    Low E gain with .022 = -6.38db

    So basically, there is a 2.33db difference in passed 82hz frequency between the two. Your ear would hear that for sure, but all things considered you would need to then pass that signal through the remaining caps to figure out what the actual end result would sound like. And that's without tone stack shaping tossed in there (I'm trying to see this as simple as possible to illustrate the point)
     
  19. jonmichael02302

    jonmichael02302 TDPRI Member

    Age:
    51
    Posts:
    56
    Joined:
    Aug 3, 2016
    Location:
    Brockton, MA
    Wow, I really appreciate your taking the time to do that test. Guys like you, with such knowledge, fascinate the hell out of me & inspire me to learn all i can about amplification electronics. I can't seem to get enough of it.
    If I may ask, now that you've run that test, which route would you go? Stick with specs at .0022uf or go with the .022uf?
     
  20. dankilling

    dankilling Tele-Afflicted

    Posts:
    1,437
    Joined:
    Oct 28, 2014
    Location:
    Lehigh Valley, PA
    Don't get too excited- I know just enough to be dangerous :p

    In any case, for a Blues Jr I'd like to get a bit more thump, so I'd probably put in the .022's and see where that takes me, knowing I would need to fine tune as I went.
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.


  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.