2003 Blues Jr mods for your consideration ...

Discussion in 'Amp Central Station' started by toddm7, May 25, 2021.

Tags:
  1. toddm7

    toddm7 TDPRI Member

    Age:
    70
    Posts:
    9
    Joined:
    May 25, 2021
    Location:
    Florida
    My first post of this kind ... anywhere. I modded my Blues Jr, replaced many signal resistors with Kiwames, and did a lot of the typical things like replacing the main power cap C25 (Cream bd MIM)
    with F&T 100uf.

    What I would appreciate is advice about where I went wrong with bass energy. I will list the changes so you can read the schematic from left to right, and tell me what would be better and/or where I went wrong. I did quite a lot of research before making the mods ... and maybe got some bad advice here & there. The .0068uf cap for C22 I never read anywhere ... I just was experimenting with different value caps to see if reverb could become more useable.

    https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Blues-Junior-Schematic.pdf

    C1 4.7uf C2 & 8 .0047uf C3 47pf C4 4.7uf C5 500pf C6 & 7 .022uf (Sozo blue for bass)

    R13 (bass pot) to 1meg C22 (reverb return) to .0068uf C9 2200pf R19 Master to 1meg

    R27 NFB to 120k and R35 & 36 screen to 1K (bias resister R52 to 27k)

    There is a bit of hum ... could be because the back cover is not yet on? I need to track that
    down. I have used a scope in the past ... but do not have one now. Some state to pull tubes,
    starting with V1 ... but I'm sure there are other ways to go about it without a scope.

    The reverb tank is the stock small one ... but with .0068uf in C22, the reverb is quite a bit better,
    even when cranked 100%

    Thank you in advance for all your expertise. Please overlook my newbie status.
    I know just enough to be dangerous
     
  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Posts:
    3,672
    Joined:
    Dec 30, 2019
    Location:
    california
    Welcome to TDPRI.
    What were you trying to accomplish with *bass energy*? More bass or less?

    The bit of hum might be the back cover...

    Good to hear the reverb is better.

    Ime the Blues Jr should not be modded for more bass because the power transformer only rated for 70mA. Bass takes power and this PT does not have enough power to get there. In addition the output transformer is not built to provide very low frequencies. You have to work with what you have. The mods you have implemented both add bass and remove bass in different parts of the circuit.
     
    secretsoundz likes this.
  3. toddm7

    toddm7 TDPRI Member

    Age:
    70
    Posts:
    9
    Joined:
    May 25, 2021
    Location:
    Florida
    Thank you so much for welcoming me here. I definitely was going for less bass. I have to turn the bass control almost all the way down. I'm thinking maybe the 1meg pot that replaced the 250k pot for bass? Would that have caused the bass to be more prominent? I read somewhere that changing the slope R to something like 130k or more will shift the bass and/or cause it to be less. Changing the cathode bypass cap on V1 from 47uf to 4.7uf should have helped reduce bass. Coupling caps C2 & 8 were at .02uf from stock .002, so going to .0047, would that cause bass to be that much higher?

    So yes ... if you could let me know what changes to make to reduce bass energy, I'd very much appreciate it. Thank you for your help

    By the way, I installed a C Rex in the amp ... that helped it quite a bit
     
    Lowerleftcoast likes this.
  4. toddm7

    toddm7 TDPRI Member

    Age:
    70
    Posts:
    9
    Joined:
    May 25, 2021
    Location:
    Florida
    I ordered OD225p .0022 from Mouser for C2 & 8 .... see if that helps.
    If that doesn't do it, I will also change C10-C16:

    C10 from .033 to .01 // C11 from .01 to .0068 // C12 from .1 to .068 //

    C13 & 14 should be fine as is // C15 & 16 from .022 to .01

    What I do not understand is how other guys write about wanting more bass from
    the Blues Jr .... like BillM's mods had C2 & 8 at .02. I checked out Marshall
    circuits, and many or all of them are .02 for couplers. So ... I'm mystified why I have
    an insane amount of bass ... Hopefully some of you who are far more knowledgeable
    than I am can give me a clue ..
     
    Lowerleftcoast likes this.
  5. Dacious

    Dacious Poster Extraordinaire

    Posts:
    9,754
    Joined:
    Mar 16, 2003
    Location:
    Godzone
    Get a 5751 for V1. A lot of the post-CBS Fender designs like the Hotrod and Blues series has a ton of gain which makes them touchy.

    Reducing it makes the preamp more manageable.

    in the typical Fender/Marshall/Vox tonestack treble globally controls mids and bass.

    If the amp sounds harsh and nasally dropping treble slightly often smoothes that out.
     
  6. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Posts:
    3,672
    Joined:
    Dec 30, 2019
    Location:
    california
    The internet is a handy place to figure out some of your questions. There are tone stack calculators, preamp tube calculators, high and low pass filter calculators. A lot of great resources.

    Here is a link to a graphic display of a preamp tube.
    https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/calculator/

    Here is a link to a high/low pass RC calculator:
    http://www.muzique.com/schem/filter.htm

    Let's discuss the Blues Jr coupling caps. C2 forms a High Pass RC filter with the 1M volume pot. 1M is the R (resistance) and .0022uF is the C (capacitance). Plug those numbers into the calculator. The result shows a corner frequency of 72Hz. Frequencies above ~72Hz are not affected. The *corner frequency* is about 3dB less than the flat signal at 72Hz. The signal looses 6dB per octave below 72Hz. For reference the low E string is at 82Hz.
    Let's try it with a .02uF cap. The corner frequency is 8Hz. To my ear, no attenuation of bass can be detected at 8Hz.
    The .0047 cap has a corner freq. of 34Hz.

    (I may be corrected by more knowledgeable shock brothers regarding C8.) C8 is more complex because of the reverb circuit. C8 also forms a RC filter with several resistors... R18, R46, the 1M master volume pot, and a portion of the reverb pot. R46 and a portion of the reverb pot are in series. Let's say they equal about 125K. That 125K is parallel to the 1M master volume pot equals about 50K. 50K in series with R18 equals about 480K.
    R=480K C= .0022uF. This RC filter has a corner frequency of about 150Hz. This cutoff frequency is about one octave above the 82Hz E string.
    R+480K C=.0047uF. This RC filter has a corner frequency of about 71Hz.
    -------------------------------------------------------------------

    C9 scrubs off highs. The 2200pF is dumping more highs to ground than the original 1500pF cap. With the C-Rex speaker you may want to remove that cap and keep the highs.
    -------------------------------------------------------------------

    If you want to cut some more lows the cathode caps can be reduced to 2.2uF. (I think 2.2uF will work with the C-Rex.) Depending on how much you want to reduce the bass... Marshall used .68uf... ime reducing the cathode cap to .68uF works with a high gain amp but not so much for a clean amp. OMMV.
    -------------------------------------------------------------------

    I have not had much luck with changing the tone stack in the Blues Jr. Ime changing the bass pot doesn't do much. Changing to 1M will allow you to get more bass but you can turn the pot to get rid of it. Most of what I have tried I put back to stock.
    -------------------------------------------------------------------

    If you like clean... try halving the resistance of the dropping resistors R47, R48, R49. You can just clip in resistors in parallel to have a listen before soldering in the lower values. This doesn't make it a clean amp but it lightly pushes it in that direction.
    Leave C15 and C16 as is. Generally bass is reduced at the beginning of the circuit or the end but not both. I would leave C11 in place. Changing C10 to .0047 would be the equivalent of a .0032uF because it is in series with C11.
    I don't think C12 is important for your quest.
    Those Marshalls most likely had .68 cathode caps in the preamp.

    The C-Rex is a bassy speaker. Some of what you are hearing is the speaker doing what it does.
     
  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Posts:
    3,672
    Joined:
    Dec 30, 2019
    Location:
    california
    The Blues Jr doesn't hum as much when the ribbon wires are touching the rear panel shielding. You may need to coax them a bit to get them there. Hopefully that will take care of the hum.
     
    TJStone likes this.
  8. toddm7

    toddm7 TDPRI Member

    Age:
    70
    Posts:
    9
    Joined:
    May 25, 2021
    Location:
    Florida
    Thank you both so much for your great advice and helping me understand this bass situation better. I'm going back to .0022 for C2 & 8. Someone wrote that if the bass pot is upped to 1meg, the slope resistor will have to be a lot higher ... like from 180k to 250k. I ordered a 220k Vishay to try for slope. The 2200pf cap ... I'll try removing that. Also, I have a 5751 in V1, a 12AX7 for V2, and a 12AT7 for V3. The amp sounds amazing ... I was just wanting to tame the excessive bass ... plus get rid of the hum. Thank you for letting me know about the ribbon sets. Someone had stated the blue wire ... if it is anywhere near the V5 ribbon can cause problems. So I had pushed that ribbon down ... and had blue riding high enough above it. But ... after what you stated, I'll reverse that. I'm not a fan of noise ... one of the reasons I bought Kinman pickups for my Strat (69 Woodstock). Thanks again!
     
  9. toddm7

    toddm7 TDPRI Member

    Age:
    70
    Posts:
    9
    Joined:
    May 25, 2021
    Location:
    Florida
    Just ordered a 2.2uf for V1 cathode bypass ... per your advice

    By the way, when I crank Master wide open, amp noise level is too prominent. (I installed a 1meg pot there that had been 50k) I was thinking of installing a resistor from wiper to ground like R6 (130k) on Vol pot. 220k is what I was going to try, but 100k might be better. That way I can crank Master up 100% and not be in the noise zone. Let me know if this is a good or bad idea.
     
    Last edited: May 27, 2021
  10. toddm7

    toddm7 TDPRI Member

    Age:
    70
    Posts:
    9
    Joined:
    May 25, 2021
    Location:
    Florida
    Here is what BillM posted in 2011 regarding 2200pf in C9: There's a 1500pF capacitor (C9)
    that rolls off high frequencies. You could replace it with a larger cap, say .003uF
    (3000pF). That would darken it considerably. .002uF might be a good compromise.

    This is one of those mods that I can't recall where I got it from .... just did it.
    Since you brought it up, I wondered if that should be something a lot smaller, like
    100pf. Then I played my amp again, and the high end sounds so agreeable to my
    ear. Of course ... that is with my Strat. Humbuckers ... might not sound so great.
    Is that the "sparkle" mod?
     
  11. toddm7

    toddm7 TDPRI Member

    Age:
    70
    Posts:
    9
    Joined:
    May 25, 2021
    Location:
    Florida
    Anyone?
     
  12. SomeGuyNamedRob

    SomeGuyNamedRob Tele-Meister

    Age:
    44
    Posts:
    288
    Joined:
    Sep 3, 2020
    Location:
    Ottawa, Canada
    I think if you're modding your amp 2003 times you probably bought the wrong one. :lol:
     
  13. Teleguy61

    Teleguy61 Friend of Leo's

    Posts:
    3,545
    Joined:
    May 17, 2010
    Location:
    Eastern Massachusetts
    It is too bad that the Billm site is down, and the info is apparently gone.
    I have a Billm modded B Jr, and it is much more manageable.
     
  14. toddm7

    toddm7 TDPRI Member

    Age:
    70
    Posts:
    9
    Joined:
    May 25, 2021
    Location:
    Florida
    Amp sounds a lot better with the various mods ... but yeah. I bought the wrong amp.
    Hind sight is 20/20, right? However, it's been a learning experience. Glad I did not do
    all this to my Vox AC15 ....

    My intention is to buy a Deluxe down the line ... would've been a far better choice.

    The bass is now definitely tamed. Sweet sounds from this amp, but midrange control does just about nothing. Someone far more knowledgeable than I am could tell me why. It has the original 25k pot and treble 250k pot, but due to some advice, I changed the bass pot to 1meg.
    Caps are .022 for bass & mid with 500pf for treble. Slope is now 220k. C2 & C8 are back to stock .0022uf. C1 is 2.2uf for cathode bypass. All of that tamed the bass, but like I said, midrange (and I did do the Twin mod), does next to nothing.

    I'm wondering about changing the bass pot back to 250k ... and the slope back to 100k.
    Would that improve control on midrange?

    Thanks again for the help ...
     
  15. toddm7

    toddm7 TDPRI Member

    Age:
    70
    Posts:
    9
    Joined:
    May 25, 2021
    Location:
    Florida
    In case someone does research about modding a Blues Jr, I thought I would add a few more things here:

    Even with the C Rex speaker, which seems to be one of the best speakers for this amp, the problem of the too small cabinet made of garbage wood is still an issue. With a Deluxe size cabinet made of pine or birch, you'd have a better sounding amp. However, if you do not want to go that route, something I looked into was applying sound deadening material inside the enclosure. If you do research on that, this is something many have done with amp enclosures to eliminate unpleasant enclosure freqs coming across. This is the material many purchase:
    Sonic Barrier 1" Acoustic Sound Damping Foam with PSA 18" x 24"

    I've never read anywhere of anyone suggesting this on a Blues Jr. It might be out there ... I just never came across it.

    The other thing I've not come across is the SAG mod for the Blues Jr. I'm going with a 120 ohm / 25 watt RH wire-wound, plus one 1N4007 diode (helps R run a bit cooler).

    I plan to build a deluxe tweed 5e3 amp kit, possibly from trinity, since I've read good things
    about their kits. That will most likely replace this Blues Jr. But this has been a great
    learning experience ...
     
  16. gorillone

    gorillone TDPRI Member

    Posts:
    6
    Joined:
    Jul 23, 2012
    Location:
    Omegna (VB) - Italy
  17. Teleguy61

    Teleguy61 Friend of Leo's

    Posts:
    3,545
    Joined:
    May 17, 2010
    Location:
    Eastern Massachusetts
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.