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Seems like I’m doing this backwards...

Discussion in 'Shock Brother's DIY Amps' started by RollingBender, Mar 29, 2020.

  1. RollingBender

    RollingBender Tele-Afflicted

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    Here’s what happens when good plans go off into the weeds... I’m running out of parts to swap and have resorted to using some odd bits from the bottom of the bin.
    0468EC29-0338-4858-BB01-7A3A16C92404.jpeg
    Ive got the bias sorted out and have spent the whole weekend trying to get this conglomeration of circuits to get along together.

    The trem sounds really good. I swapped a cap out to slow it down. The reverb problem was acoustical so I wrapped it with some vinyl and that seemed to fix the squeal. There’s some hum in the reverb that follows the level but it’s not terrible. The 3 switch pots are for high/low input, bright cap, and lead voicing bypass cap on the first gain stage. They all do what I wanted but the bypass cap switch “pops” when you switch back and forth between stock and tight.

    V1 is very microphonic. I’ve tried about 10 different 12AX7’s in that position...some are worse than others but they all are microphonic to some degree. I also tried a few 12AT7’s and 12AU7’s in V1 to investigate this problem and the gain problem. They helped but I want to be able to keep the 12AX7 in that socket.

    The main problem I have been chasing is the over abundance of gain and hiss. I started messing with adding another mixing resistor, like what RobRob did on his Blackvibe to simulate the load of the missing channel but I ran out of time to be able to test it before the warden called for lights out.
     
    Last edited: Apr 27, 2020
  2. BigDaddy23

    BigDaddy23 Tele-Holic

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    Try strapping a 1meg resistor across the DPDT bypass switch. 1 leg to the cap pole and the other to the center pole that goes to ground. That will sort out the pop when switching as the cap can slowly charge. Have a look at the V1 bypass on this schematic. I built a similar amp and it works a treat on the switch with inaudible switching.

    Two_Stroke_TAN_Schematic.jpg.jpeg
     
  3. RollingBender

    RollingBender Tele-Afflicted

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    I got to throttle it up at lunch time today. Getting closer but still over overdriving. Actually, if I keep the volume knob on the guitar at 6 or 7, it’s just how I would expect the amp to behave. I bet with a less efficient speaker, the hiss would be completely at an acceptable level too.
     
  4. RollingBender

    RollingBender Tele-Afflicted

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    Wired up the footswitch tonight. I gutted this modern Fender footswitch, installed a new 1/4” stereo jack and made a stereo cable for it with some microphone cord. Works great!
    A2CBE33E-527A-48F0-877B-A2B1873188CF.jpeg
    Tip is trem, ring is reverb, and the shield is the common ground.

    It’s gotta help the AB763 mojo with the footswitch actually having “Fender” printed on it.
     
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  5. RollingBender

    RollingBender Tele-Afflicted

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    Here’s a cell phone video demonstrating some tones.

     
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  6. BigDaddy23

    BigDaddy23 Tele-Holic

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    Sounds great! Nice, diverse range there with those switched pots. Other than differing bias options in the preamp, what were the others doing? Bypassing the pots?
     
  7. RollingBender

    RollingBender Tele-Afflicted

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    The Gain pot has a switch for high and low inputs. The Treble pot has the bright cap switchable. The Bass pot is for the first gain stage bypass. I added the two 1M resistors to the bypass switch but I’m still getting the Pop. My circuit differs from the one you posted above in that both the cap and the resistor are different values so I’m switching between both rather than just the capacitor.
     
  8. BigDaddy23

    BigDaddy23 Tele-Holic

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    The Reverb and tremolo sound excellent. Is it possible to re-wire your bypass cap switch so that ‘ground’ is off the center lug? That should help deal with the pop.
     
  9. RollingBender

    RollingBender Tele-Afflicted

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    Here’s my voltages...

    B+=303.2
    B+1=302
    B+2=231.8
    B+3=217.9

    V1a-1st gain stage 12AX7
    P1=231.8
    P2=0
    P3=1.846

    V1b-tone stage
    P6=231.8
    P7=0
    P8=1.827

    V2a-reverb driver 12AX7
    P1=231.8
    P2=.002
    P3=1.694

    V2b-reverb recovery
    P6=151.3
    P7=0
    P8=1.197

    V3a-final preamp gain stage 12AX7
    P1=151.7
    P2=.001
    P3=1.181

    V3b-tremolo
    P6=184.7
    P7=.001
    P8=1.760

    V4-phase inverter 12AT7
    P1=173.3
    P2=44.94
    P3/8=65.23
    P6=135.6
    P7=43.53

    V5-power tube 6BQ5
    P2=-11.14
    P3=.030
    P7=302.3
    P9=303.0

    V6-power tube 6BQ5
    P2=-11.13
    P3=.030
    P7=302.6
    P9=302.9

    9.1 watts/tube
    ~76%

    I now have clean all the way up the dial! I took out the wiring on the high/low switch and wired it direct and while I was in there I discovered that the 1M resistor was basically a wire (near zero resistance). Actually would like to have a bit of that dirt back though...

    Now I have a buzz to chase in the reverb circuit. It increases with the reverb knob and does not disappear when the reverb is turned off with the footswitch.
     
  10. RollingBender

    RollingBender Tele-Afflicted

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    I finally got it sorted out.

    I was missing 1/2 of two different voltage dividers. The first one was the R2 of the divider coming off the reverb pot. The second one (not on the schematic but I put it in) was the R1 coming off the 3rd gain stage coupling cap and going into the phase inverter.

    I messed around with those 2 plus the voltage divider of the NFB circuit and thusly tamed the overdrive, reverb and noise. Got me a wonderful sounding amp now.

    There is some tweaking to be done now but nothing too drastic...just voicing to get back to that blackface mojo.

    Here's the back panel for the chassis i came up with.
    2E69BAEA-ACBB-4090-89C8-A7709B6B69D8.jpeg

    I have some fake tolex arriving tomorrow. It was made by Argentinian cows.
     
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  11. RollingBender

    RollingBender Tele-Afflicted

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    Fake tolex arrived! These are some big, oddly shaped sheets. The device that makes this stuff must be big and oddly shaped.

    I made up some “patterns” out of some 1/8” Masonite which comes free in each box of vinyl floor planking and a few more “patterns” out of cardboard and finished this puzzle optimizing my cuts. I’ll get 6 amplifiers this size from the tireless work of one lowly blue cow from Argentina.
    32F673D2-2CF4-46BC-B50D-71C16CDBDC9E.jpeg
    Should be able to figure out some sort of matching handle out of the scraps left over.

    for size reference, each of the Masonite “patterns” is 11-3/4” x 60”
     
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  12. BigDaddy23

    BigDaddy23 Tele-Holic

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    Wow! How thick is it?

    Is it genuine blue cow or were a number of vinyls harmed in the process of making said blue cow?
     
  13. David Barnett

    David Barnett Doctor of Teleocity

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    ...and smelly.
     
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  14. RollingBender

    RollingBender Tele-Afflicted

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    No vinyls, wild or domesticated were harmed in the covering of this amp...just 1 blue cow.

    First seam was the best.
    E0788944-5B53-4019-9344-95F2899D7731.jpeg

    I’m planning on making some brown leather corner protectors on also. I’ll have to check my leather stash for something that will work.
    2BCE8E8E-9670-400D-A431-8B7905734574.jpeg
    I may also swap out my brass screws for chrome. Since the chassis is the color it is and the handle hardware cannot be found in brass, it would probably look better if everything was from the same corner of the color wheel.
    5D8BAC8C-523D-43B6-AB75-E6182BC2D3B3.jpeg
     
  15. RollingBender

    RollingBender Tele-Afflicted

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    Glamour shots!!!

    684E7102-F0BE-4520-A992-A2768557DF58.jpeg 9AC10CBD-EA5F-4BD8-A167-2250232DF26F.jpeg 6A7563AF-9CEF-40AC-A6FE-4114241761E4.jpeg E2789916-4D56-4943-B5B0-51E4031189E2.jpeg
     
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  16. tubegeek

    tubegeek Friend of Leo's

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    Beware: The factory cabling has one of the "hot" wires (the tremolo connection I think) OUTSIDE the shield and the other (reverb I think) connection INSIDE. Because the trem on/off doesn't need to be shielded and that setup reduces any tendency to get tremolo oscillations bleeding into the reverb signal. I'm not at all sure where to get that particular cable - and if you don't get that problem don't worry about it.

    On the other hand, you have the proper logo on the switch so that's gotta help!

    Handsome! Sounds great, too!

    PIPELINE!!!!!!!!!!!!!!!!!
     
    Last edited: May 22, 2020
  17. RollingBender

    RollingBender Tele-Afflicted

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    I made my footswitch cable out of an old mic cable and a couple TRS 1/4” jacks. I read about the special cable that Fender used but I’m experiencing no ill effects from how I wired it up.
     
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  18. tubegeek

    tubegeek Friend of Leo's

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    Fair enough, that's good. I basically only mentioned it to save you some insanity if you did end up having this problem, it can be tricky to figure out. Rock on!
     
  19. RollingBender

    RollingBender Tele-Afflicted

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    I have now played 3 gigs with this amp and I am really loving it. I have a small pedal board that I bring along with an OD, flanger, and a chorus... I use the OD on about 3 tunes, the flanger on 2 Waylon songs, and have yet to step on the chorus (I'd use it if we needed to play a “purdy” song). It’s just a great sounding amp.

    The first gig I played with it, I used a Tele through most of the show (as I usually do) but pulled out a Strat for a couple songs. The bass player from another band I’m in came to watch, he said, “it sounds really good with the Tele but with the Strat, WOW!”

    Here’s my issue with the amp though. Although it sounds fantastic clean and up to the point where it starts breaking up, when it gets pushed over the edge, it turns to mush. I’m strongly suspecting the OT. When I ordered parts for this build, I was intending to use 6AQ5 power tubes and picked an OT for a Blues Junior because the specs lined up with what I was after. But I ended up building with EL84’s because they are a bit more “normal” and more readily available. The PT is nearing the limit of its capacity but should still be within specs for what I’m asking of it with a small bit to spare but I’m guessing that I’m overloading the OT when a full 18 watts gets pushed into that little Blues Junior OT.

    What say the knowledgeable folks here???
     
  20. Pick_n_Strum

    Pick_n_Strum Tele-Meister

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    I really enjoyed the thread! I'm curious how / where you inserted the gain pot? Is it based on one of the master volume mods (thinking specifically about the RobRob mods)?
     
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