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Plexi Micro Build

Discussion in 'Shock Brother's DIY Amps' started by Bitsleftover, Feb 27, 2020.

  1. Bitsleftover

    Bitsleftover Tele-Meister

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    The 12BH7 provides a little more volume than the 12AU7 so volume shouldn’t be a limiting factor.


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    Last edited: Apr 16, 2020
  2. Bitsleftover

    Bitsleftover Tele-Meister

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    Got the -ve feedback and presence control working. I wasn’t getting any squealing associated with +ve feedback. Just no effect when switching in or out. I switched the OT primaries just to check and it’s now working.
    The presence control is very subtle. Although that might be a problem with my hearing rather than an amp issue though.


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  3. Bitsleftover

    Bitsleftover Tele-Meister

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    Right! I need help!!!
    I think I might have found my missing kahunas!
    I’ve been over everything with a fine tooth comb for the 100th time tonight and Ive finally noticed I have 379k path from both channels plate resistor/coupling cap junction to ground.
    I traced it back past the 100k plate resistors and the 10k dropping resistor on the power rail to the B+2 node where it measures 269k to ground, through the LND mosfet in the tremolo circuit.
    (While Ive been trouble shooting this Ive had the signal part of the tremolo disconnected, but I’d forgotten about the power supply side)
    It’s way too late to plug in and wake the neighbours just to prove my findings, but the 370k-GND went open circuit as soon as I snipped the tremolo power supply cable.

    Here’s a pic of the tremolo circuit I added-

    [​IMG]

    The red is where I connected up to the B+2. The yellow shows the 268K path to ground.

    This is where I hooked up the tremolo power to the existing Bassman Micro circuit.
    [​IMG]


    So my long winded question is.....
    Since I’m only using the DC B+2 to power the LND mosfet and the Roach, how do I prevent the AC signal passing through the circuit to ground? Do I need to use an inductor? Or will a suitable capacitor block all audible frequencies? If so, what values am I looking at?
    Or... as an afterthought (while typing).... If I hooked it up on the other side of the choke, would that do the trick?

    As always...thank you in advance.
     
  4. Bitsleftover

    Bitsleftover Tele-Meister

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    Tried it before the choke, after the choke and totally disconnected. Made no difference at all. Hmm


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  5. Paul-T

    Paul-T Tele-Holic

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    I wish I knew enough to offer informed comment. Is it worth trying Sluckey/ the el34 forum?
     
  6. Bitsleftover

    Bitsleftover Tele-Meister

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    Hi Paul, I think I’ve ruled the trem circuit out.
    I don’t really understand how it can be different from ditching almost half of my signal via my vol pot... but it obviously is because snipping the wire made no difference at all.


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  7. rocksmoot

    rocksmoot Tele-Meister

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    [Removed unnecessary advice]

    I'm gathering parts for a build of Rob's Micro Bassman which I hope to start soon. Good luck!
     
    Last edited: Apr 24, 2020
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  8. Bitsleftover

    Bitsleftover Tele-Meister

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    Made a bit more progress with the one tube reverb.
    I’ve been looking at how some manufacturers integrate reverb (other than the standards fender circuit) into their amps.
    Matchless use two 12ax7s with an otherwise fairly similar circuit to the one tube reverb.
    The clever bit is in the integration though.
    Instead of placing a mix resistor in the signal path, then taking the send before it and the return after, they tap the send from right before the PI, then inject the return in at the input to the other triode of the PI.
    I rigged it up on my amp but it was very weak. Then I realised most of my reverb signal is being lost through the presence circuit.
    As an experiment, I disconnected the wire from the PI to the presence pot and the reverb worked a charm. Really nice in fact. A very rich reverb with no loss at all. Almost worth deleting the presence control entirely. If I was building a push pull amp with no NFB I’d definitely run the one tube reverb with it. My next project was going to be an AC4. But now I’m considering a sort of AC30 micro. Or even a Matchless DC30 or Nighthawk Micro with reverb.
    Anyway, I wired it back up to the normal side the tried shorting the mix resistor between the treble pot and PI and it works much better with much less loss. Almost non at all. Not as good as the matchless method, but much better than I had it before. I’m very happy with the reverb now.
    This lack of dirt problem is really starting to bug me now so I’m going to reflow every joint in the amp and see if it’s just a dry joint somewhere.


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    Last edited: Apr 26, 2020
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  9. Bitsleftover

    Bitsleftover Tele-Meister

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    More progress but more problems.
    Reflowed every joint.
    Loads of gain now.
    Tons of it!
    Too much in fact.
    I’m getting what I think is blocking distortion (sounds like a nasty fuzz pedal with a run down battery falling in on its self) whenever I play it with the PPMV anywhere above 2 on the dial.
    It also starts squealing at idle with either the Channel Vol or PPMV set higher than about half.
    I figure its a V1 problem since the squeal changes when I turn the treble pot.
    I also temporarily fitted a 1 meg MV pot (as a variable resistor) at the input to the PI and the problem reduced in volume in proportion with guitar signal.
    Ive tried 4n7 caps across V1 plate resistors. Slight improvement. But not much.
    Ive disconnected the NFB loop so that’s not it.
    Ive also removed the tremolo and reverb to rule them out.
    Ill try again tomorrow. Ill adjust V1 cathode resistors and bypass caps.
    Ill get to the bottom of it eventually.
    I’m all ears if anyone has any words of wisdom for me though.
    Cheers
    Mick


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  10. Bitsleftover

    Bitsleftover Tele-Meister

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    [​IMG]
    I finally got round to drawing up a schematic.


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  11. Bitsleftover

    Bitsleftover Tele-Meister

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    PDF Copy
     

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  12. Bitsleftover

    Bitsleftover Tele-Meister

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    All done!
    I corrected some lead dress issues and removed about 4 feet of excess cable due to poor routing.
    I still had a strange high pitch oscillation which I traced to the heater supply. I think I had what Merlin refers to as "rectifier hash" which was manifesting as high frequency noise in the heater supply from the transformer.
    I've added a voltage divider off my B+ at the reservoir cap to double as a bleed resistor and elevated my heater center tap to about 40v. (As per robs site)
    That cured the high pitch oscillation but still left me with a little bit of 50hz hum, so I binned the transformer's center tap and replaced it with a 200 ohm humdinger pot. which got rid of that beautifully. (again thanks to Robs site and Merlins book)
    I drew up a Plexi style Faceplate and Rear Panel in Inkscape and exported that to PDF which Tube-Town used to make up a really convincing reverse cut Acrylic Faceplate.

    I'm delighted with it so far.
    The Bright channel is lovely. Bright, aggressive and brash just as it should be.

    I wanted to reduce the number of switches a bit to clean up the appearance and usability.
    The cathode split switch stayed. It's key to the difference between JTM/JMP (To me anyway)
    The tonestack and V2 cathode bypass switches were combined into a 4PDT switch which now gives 'even more!' Engaging the switch brings in the 'modern' Marshall tonestack and a .68uf cap across V2a cathode resistor for a bright push.
    Both switches pop quite dramatically when flipped. I might look into fixing that, but neither will be used on the fly so I might just live with it.

    The Normal channel is very full and fat. Ideal for cleaner settings.
    Jumped they work nicely together. The normal channel can be used to really fatten up the bright channel.
    Un-jumped, there's no real interaction between channels.

    The tremolo on the normal channel sounds great. I increased the value of the intensity pot from 25K to 250k to prevent the tremolo dragging the normal channel signal down too much. That allowed me to get rid of another switch. Its always connected now. However, the Intensity control doesn't do much between 0-4 then its fairly smooth from 5-10. That's not a big problem for me though. I'll live with that. I've made up a footswitch for the tremolo from an old light fitting which I don't really like but it'll do for now till I find something more suitable.

    The reverb sounds great. Much better than I'd hoped. I've dialed the internal Dwell trim pot back to about half and that seems to be about the Goldilocks setting for me. I found that hooking the send up from the cathode follower before the signal enters the tone stack and injecting the wet back in after the tone stack, allowed me to delete the mix resistor and that has really reduced how much the reverb loads the dry signal down. That allowed me to delete the reverb bypass switch as well.

    I don't hear much difference between Tube and Solid State Rectifier. Maybe I will when I've had more time to play it in. Still, for the sake of a switch and 2 diodes I already had in my spares box I'm glad |I included it.

    I think the 10" Greenback suits this amp better than the Celestion Gold 10" I'd initially intended to use. I guess I'll have to build another amp to use that with!

    Huge thanks to Robrob for the Bassman Micro LTP design. And huge thanks for all the other brother's help, encouragement, patience and support over the past few months.

    Pics and vids to follow.
    Thank you fellas.
     
  13. Bitsleftover

    Bitsleftover Tele-Meister

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    My attempt at a DIYLC layout
     

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  14. Bitsleftover

    Bitsleftover Tele-Meister

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    [​IMG]


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  15. Bitsleftover

    Bitsleftover Tele-Meister

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    [​IMG]


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  16. Bitsleftover

    Bitsleftover Tele-Meister

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    Up to date schematic
     

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  17. Dannebrog53

    Dannebrog53 TDPRI Member

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    Great news @Bitsleftover! Wow, 4 feet of excess wire! Congratulations getting it sorted now it's time to enjoy it. I hope you post some audio clips of the different channels and settings.
     
  18. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Very nicely done @Bitsleftover. This was a very ambitious design and build. I nominate ^post as post of the month.
     
  19. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Nice to here you have it sorted. Great thread.

    You might add this to your toolbox...
    A small cap (~100pF or so) from V1 plate to cathode can reduce oscillation.
     
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  20. willholt92

    willholt92 Tele-Meister

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    Absolutely STUNNING build mate, if mine ends up half as good as this I'll be over the moon.
    Layout looks great too, top work.

    Must admit there is something really great about a 10" Greenback.

    If you don't mind me asking how much did Tubetown charge you for the face/rear plates? I'll be getting ready to get some done for my JTM micro in the near future.

    Well done again mate, great inspiration for me and many others I would imagine.
     
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