Plexi Micro Build

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

  1. Badside

    Badside Tele-Meister

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    You have the full signal going across the 4.7k resistor, that sets your negative feedback (this and the resistor from the speaker output form a voltage divider)

    The capacitor then lets only higher frequency go straight to ground (which means no NFB at those frequency, aka presence boost). The potentiometer then adjusts how much signal is bypassing the 4.7k resistor via that capacitor (higher resistance = less shunted signal = less presence boost)

    The 4.7k resistor and the 25k potentiometer don't do the same job, they can't be paralleled. What goes through the pot also goes through the cap, while everything goes through the resistor.
     
  2. Bitsleftover

    Bitsleftover Tele-Meister

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    [​IMG]
    Thank you for the explanation.
    I totally agree with your description of how rob has it laid out.
    Please could you clarify though... if you compare his circuit with the Marshall or even the Fender schematic, the presence cap is not bypassed by a 4k7 or anything else. Everything passes through the .1 cap. That’s the (probably stupid) point I’m trying to make.
    The way I see it, on the original schematics, the tail passes through a 5k variable resistor shunting signal though a .1 cap to ground.
    Adding a 4k7 resistor across a 25k pot comes very close to 5K. But only if wired straight across the pot.
    Wiring the two in parallel (to ground) as shown allows a path of least resistance to ground for 4/5ths of the pot movement.
    Please educate me. I read tons of books on this subject, but the most understandable and valuable info I gain is always from you guys. If I’m wrong......and I usually am........ tell me where I’m going wrong.
    Please and thank you


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  3. Badside

    Badside Tele-Meister

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    This schematic is the "original" way of wiring presence circuits in the 5F6 Bassman and early Marshalls. The result is the same, except here the "tail" of the NFB loop is obtained by the 5k potentiometer itself (5k vs 4k7... close enough). So whatever isn't shunted via the cap, the potentiometer effectively acts as a 5k resistor. Then the wiper of the pot varies how much signal gets bypassed and shunted to ground via the capacitor. The result is the same, but this places some DC on the potentiometer which is why it was modified (it's no big deal but can make the presence control scratchy)

    Basically, in all cases you want a 5k-ish "tail" on which the NFB voltage is developed. Then you want to partially allow some high frequencies to bypass that to obtain a presence boost.
    What doesn't go through the capacitor, should always see roughly a 4k7-5k resistance. Then presence boost is obtained by bypassing this with a capacitor, and using a potentiometer to control how much bypasses it is how you can control how much presence boost you get.

    These are two ways you can achieve that. This original version allows completely removing the presence boost (when rolled down, no signal gets to bypass), but places DC on the pot which can be scratchy. But practically speaking, save for a slightly different taper, you get the same thing.

    The thing to remember is, only the signal going through the cap gets to bypass the 4k7 resistor in the modern version. So paralleling the resistor with the pot would do a totally different thing.
     
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  4. Bitsleftover

    Bitsleftover Tele-Meister

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    Ahhhhhhh I seeeeeeeee!
    Thank you for that brilliant explanation.
    I was thinking of the old style 5k pot as a variable resistor and missing the fact that the full 5k path across it to ground is always present.
    Your explanation made everything clear.
    Thank you for taking the time to set me straight.
    I really appreciate it.


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  5. Bitsleftover

    Bitsleftover Tele-Meister

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    OK Next. Why are we making no sound?
    I’m thinking Ive fried my OT.
    There is 250VDC from the sleeve of the insulated speaker jack to chassis ground. (Hence my shock)
    Powered off I measure 0.5ohms between +ve and -ve speaker jack terminals on both 8ohm and 16 ohm jacks and 0.9ohms between both +ves.
    I’d say I”ve fried that pretty convincingly.
    Is my power tube likely to be fried also do you think?


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  6. Bitsleftover

    Bitsleftover Tele-Meister

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    [​IMG]

    I thought I’d have a look at the 1 tube reverb I fitted to my Princeton since it worked really well in that application.
    This is how I did it on that occasion.
    It differs from the one I’ve used this time slightly, mainly in the way the mix pot is wired.
    I’ll adjust this build to match then see how it sounds when the new OT arrives.


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  7. Bitsleftover

    Bitsleftover Tele-Meister

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    So back to the whole making no sound at all thing....
    I could really use some help with this fault.
    To recap, Whilst troubleshooting the presence control not working I stupidly unplugged the speaker from the 16ohm jack and straight into the 8 ohm jack.
    There was a buzz (like open circuit buzz), then no sound.
    I quickly switched off and put it back in the 16ohm socket and powered back up.
    Still no sound.
    With power still on, I waggled the speaker cable to check for a bad contact, but I got a noticeable shock from the metal speaker cable plug. The shock caused me to partially remove the plug again, and sound briefly returned, then went again a second or so later.
    Power down. Plugged back into 16 ohm jack fully. Nothing.
    Full confession time. I later realised I had an 8 ohm L-pad between my 16 ohm output and my 16 ohm speaker. I forgot I’d swapped speakers.
    I assumed I’d fried my output transformer so I’ve taken some measurements.
    OT Primary-
    Red-Brown 175.2ohms
    Red-Blue 126.8ohms
    Brown-Blue 302ohms
    OT secondary-
    8ohm tap to common is only 0.5ohms
    16ohm tap to common is only 0.5ohms
    16ohm tap to 8 ohm tap 0.9ohms
    Also, with power on I measured the isolated speaker jack common to ground and got 250VDC (hence my shock)
    I figured the only way I’m getting DC on my OT secondary must be an internal short.
    I ordered a new OT and a new 12BH7
    They arrived and I measured the OT but got almost exactly the same results from the new one.
    I don’t know what to do.
    I thought it just be that I cooked the 12AU7? Could that put 250VDC on my speaker cable? Anyway, I pulled the tube and the 250VDC is still there???
    Apart from being reluctant to throw another OT and power tube at it hoping for the best, my solder station just gave up the ghost and I’m waiting on a new temp sensor for it so I cant do anything anyway.
    Anyone got any ideas in the mean time?


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  8. Bitsleftover

    Bitsleftover Tele-Meister

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    Ah! I should have measured between primary and secondary shouldn’t I ?! That’s where the short will be. Duh!
    Tomorrow’s job. My new puppy’s just settled and I’m not waking him up banging and clattering now!


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  9. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Your OT measurements look normal to me so it's probably OK.

    If your chassis and speaker jack are properly grounded I don't see how you could have received a shock from the speaker plug. Verify continuity from your power cord's ground lug to the chassis, ground bus, input jack sleeves and speaker jack sleeve.
     
  10. Bitsleftover

    Bitsleftover Tele-Meister

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    Thank you Robrob. Original OT refitted. All grounds remade.
    New 12BH7 fitted.
    Back with lights and sound.
    Ive not tried the original 12AU7 yet. Will do that next time I power up.
    How very strange!


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  11. Bitsleftover

    Bitsleftover Tele-Meister

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    So following on from another recent thread...
    Ive changed the power tube cathode bias resistor to 680ohm.
    With a B+ of 305VDC and a cathode drop of 13.9 Robs bias calculator comes out at around 85% if I’m using it correctly.
    Does that sound right?
    Thanks[​IMG]


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  12. Bitsleftover

    Bitsleftover Tele-Meister

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    Installed the rest of the tubes I bought. Sounds really nice but definitely way down on gain. The JTM Normal Channel is almost clean on 10. The SLP channel is brighter and more aggressive for sure with the cathode split. But still only about what I’d expect on 5.
    Better with both channels jumped. But still not anywhere near what I’d expect.
    I’ve been through and checked everything again and can’t fault it.
    I can’t open it up because I don’t want to annoy the lockdown neighbours. But I’ve got everything on 10 through a DrZ Airbrake clone set to bedroom levels.


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  13. BigDaddy23

    BigDaddy23 Tele-Holic

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    Your bias calculation looks right. Make sure you get in the habit of measuring the actual resistance of the cathode bias resistor for accuracy. In your case, any low reading isn't likely to push you >100% MPD.
     
  14. Bitsleftover

    Bitsleftover Tele-Meister

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    Thank you BigDaddy


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  15. Bitsleftover

    Bitsleftover Tele-Meister

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    Am I
    A- Going completely stir crazy?
    B- Lowering my expectations daily?
    Or
    C- Is this amp starting to sound better and develop more gain every day?
    Can that happen? Do things settle in?


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  16. Paul-T

    Paul-T Tele-Meister

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    A is always possible for all of us. But things usually sound better when we're more relaxed, don't they?

    Personally I can't 'audition' stuff in the way some people do, I'm thinking too hard. I have to come at it sideways before I can decide what I like. We're all so subject to confirmation bias.

    But in any case, if it's sounding better that's great - and with social distancing, hey, there's no one around to contradict you!
     
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  17. Bitsleftover

    Bitsleftover Tele-Meister

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    That’s a really good point. The amount of times I’ve done a mod or fitted a new part and immediately declared how good it is. Only to decide it wasn’t for me some time later.


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  18. Bitsleftover

    Bitsleftover Tele-Meister

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    Made a little more progress on the 1 tube reverb.
    I followed Mart the Hat’s advice in another thread and took the reverb send straight from the output of the cathode follower before the treble cap.
    Then I’ve put a 100k mix resistor between the treble pot and the input to the PI and made the reverb return on the PI side of that resistor.
    I’ve made it all switchable via a 4PDT switch to take it totally out of the signal chain.
    There appears to be much more reverb on tap now. (More testing required at a more socially acceptable hour) But early indications seem good.
    There does seem to be quite a large drop in output when I switch the reverb in though.
    I wouldn’t say it really reduces the amount of dirt though (just volume) which makes me wonder if the PI and power tube are breaking up at all.
    Is there a way to recover a bit of the signal level?
    Can the PI plate resistors be increased a bit to increase the gain?
    Or maybe drop the 470k grid stoppers to the power tube?


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  19. Bitsleftover

    Bitsleftover Tele-Meister

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    I wired a DPDT switch up with 2x 150k resistors across the 2 470k grid resistors of the power tube (bringing it down to effectively around 100k) and there was no difference at all that I could hear.
    I think that is a dead avenue.


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  20. Bitsleftover

    Bitsleftover Tele-Meister

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    I think my next route will be to try VVR on the power tube.
    I know from other low watt builds it’s been surprising how far it can reduce the headroom before it starts to compress. It should add a bit of power tube compression/distortion which might be just what I’m missing.


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