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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    Still a bit disgruntled by the lack of dirt.
    I tried my only known good 12AX7 in every position and there was a slight improvement every time.
    Maybe all these valves are a bit tired and worn out.
    I’ve ordered a full set from a guy Ive used a few times in the past, Karltone in North Wales. Tune-Sol for V1, Electro Harmonix for V2, JJ for reverb, Shuguang for the PI.
    Lets see if that breathes a bit of life into it.


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  2. Badside

    Badside Tele-Meister

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    The classic Plexi circuit doesn't generate much distortion on its own. It needs to be cranked to get some overdrive, and an overdrive pedal in front always help.

    If you want something dirtier you can cascade the channels (or convert to 2203/2204 specs, aka single channel JCM800, it's just a cascaded plexi with some small changes)
     
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  3. Bitsleftover

    Bitsleftover Tele-Meister

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    I know this is a 1974 Metal face rather than a plexi, but it’s still the 1959 model before the 2203 cascade gain stages.
    Mine maxed out has about the same amount of distortion as this guy from 6:05 onwards. But then he turns his guitar volume pot up at 7:15ish and gets twice the dirt I do.





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  4. Bitsleftover

    Bitsleftover Tele-Meister

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  5. engineer

    engineer Tele-Meister

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    I really like these amp building threads, so thank you for sharing!
    They give me hope that one day I might do some soldering.

    But how about some video or audio clips?
    Maybe a video where you showcase all this features and we can listen in? Please?


    (Also, sorry to read about your job. These truly are crazy times. In Sweden, the same happened with SAS airline but hospitals took on many that worked in the cabin and re-trained them since they already have excellent interpersonal skills and highly adept in safety work. Also several that worked loading on the luggage bands went to work for on an online pharmacy that temporarily needs more hands packing. So there are ways to manage if we are open to the idea.)
     
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  6. King Fan

    King Fan Poster Extraordinaire

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    Wow, I’d lost track of this. What an amazing build! Your skills and care deserve huge props. Thanks for sharing.


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  7. Paul-T

    Paul-T Tele-Meister

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    this is looking fantastic . Your build is beautiful.

    and thanks for tipping us to Karl-tone. We probabyl need a dedicated source thread for Europe here at some point (although maybe it's too late for us Brits now).

    The tube-town board looks handy, too, I'm planning a micro deluxe and bought the board you drill... that one looks much much handier.
     
  8. Bitsleftover

    Bitsleftover Tele-Meister

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    Thanks Engineer, I’ll see what I can do with regards to a video. I live in a flat (apartment) so it’s hard to bring any noise during the lockdown.
    Thank you for the kind words also.


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  9. Bitsleftover

    Bitsleftover Tele-Meister

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    Thank you King Fan!


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  10. Bitsleftover

    Bitsleftover Tele-Meister

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    Cheers Paul
    The board, posts and staking tool are great. Really easy and convenient.
    I used to use ampmaker for virtually everything but since his site has been down I’ve been using Tube-Town for most things. Modulus amplification are great too. Their faceplate service is amazing


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  11. Dannebrog53

    Dannebrog53 TDPRI Member

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    That seems to be the Scandinavian way
     
  12. Paul-T

    Paul-T Tele-Meister

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    I know you're trying to crank up the gain still... what is the verdict on the one-tube reverb? I can't suss out your schematic without more thought, but is that on your main board?

    (asking because my next project is a micro deluxe, to which I plan to add two valves, reverb and tremonator once I've got the main board working)
     
  13. Bitsleftover

    Bitsleftover Tele-Meister

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    At the moment Ive removed the reverb while I tackle the gain and NFB issues I’m having.
    But, It’s not going great.
    I have installed it really successfully into my 5F2A so I’m confident Ill get it working. At the moment though I’m holding fire. I may end up using that tube to add another gain stage (cold clipper) and maybe go with the Tube-Town LND Verb kit.
    I’ll dig out the stuff I had on the one tube reverb for the Princeton. I fitted a vibro-champ tube tremolo as well. Really happy with both in that build.

    [​IMG]

    That’s the separate reverb board there at the back right in front of its tube socket.


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  14. Bitsleftover

    Bitsleftover Tele-Meister

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    So, the negative feedback doesn’t work. I thought it did when I initially tested it but its definitely not working now.
    Ive taken a lead from the orange 8ohm speaker tip, run it through a switch with a 15k resistor, then on to the junction of the 6.8k PI tail resistor which then runs up to the 25k presence Pot.
    There is no change at all when I switch the NFB in or out. In fact, there is no change even when I short the resistor so have a direct connection between the speaker tip and the PI tail.
    The presence control does nothing at all.
    It all tests good for continuity from tip to pot.
    What have I done wrong? I’m starting to doubt my sanity!!!
    Help!!!
    [​IMG]
    [​IMG]


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  15. DanceCanse

    DanceCanse TDPRI Member

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    I had the exact same issues with my NFB and presence control on my Blackvibe. I’m still uncertain if I’ve got it fixed or not. Rob gave some great troubleshooting advice in my thread if you’d like to reference it, but it sounds like you’re already on top of it (you’re WAY more knowledgeable than me). I’ll be anxiously watching you’re progress . I hope you can get it figured out!

    https://www.tdpri.com/threads/blackvibe-micro-my-first-non-kit-build.1003655/page-2
     
  16. Dannebrog53

    Dannebrog53 TDPRI Member

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    @Pick_n_Strum here in the States you can get the predrilled .125" thick turret boards from Mojotone, you'll need the taller turrets designed for the .125". Monotone also sells the tool to install the turrets.
     
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  17. Bitsleftover

    Bitsleftover Tele-Meister

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    Thank you.
    I can’t help thinking my lack of gain and NFB problems must be linked.
    I went through and checked every component value today.
    I’ll reflow every joint tomorrow.
    Hopefully the new tubes will be here early in the week.
    Very confused


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  18. Bitsleftover

    Bitsleftover Tele-Meister

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    Ok 2 things....
    1st, Question for Robrob really....
    With regards to the 4.7k resistor.
    Since the function of that resistor is reduce the effective resistance of the 25k presence pot to around 5k, should it not be connected directly across the pot before the output travels through the presence cap to ground?
    The way it is wired now is always allowing full frequency range to bypass the presence pot and cap straight to ground reducing the efficiency of the presence control?

    I have just wired mine that way to test it, but unfortunately when I plugged it in....no output!?!?!?!?

    So 2nd, I tried to swap my speaker cable from the 16 ohm jack to the 8ohm speaker jack, but as I did there was an loud buzz and I got a bit of a zap from the metal speaker jack plug. Ive reflowed the speaker connections but still no output at all. No more shocks.
    Hopefully I haven’t fried the OT.
    Any ideas?


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  19. MatchlessMan

    MatchlessMan Tele-Holic

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    I’m a bit late to the party but subscribed!
     
    Last edited: Mar 29, 2020
  20. DanceCanse

    DanceCanse TDPRI Member

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    What about wiring it like this: 24618EB9-7907-4345-8122-4F71C67EE5DD.jpeg
    I stole that image from Robrob’s page on amplifier voicing. I’m still at the “paint by numbers” level of understanding when it comes to the presence control. So I’m still trying to wrap my head around the original way we had it vs the change you made vs this way.
     
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