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Proofread my 5F2a layout, please?

Discussion in 'Shock Brother's DIY Amps' started by King Fan, Mar 20, 2020.

  1. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Anybody need something to do besides bleach the counters and watch sports from last year? :)

    Give this a look -- deep debt of gratitude to @robrob , who turns out beautiful DIYLCs and then freely shares them on his website. This was derived from his 5F1 layout.

    Warning: this isn't some 'idealized' 5F2a -- it's supposed to represent the actual one I built some years ago. That was my first build, and I was working outwards from an oddball 5F1 kit, and you'll notice some items I might do differently now. But it sounds good.

    I'm not sure I've translated reality to paper correctly, so let me know if you have questions or see problems.

    If you own a 5F2a, a couple details: the NFB cut is very subtle but just gives a bit earlier onset of what I call 'creamy' tweed breakup. I'm not actually sure I have log pots in both slots -- they're not labeled.

    My 5F2a.png
     
    Last edited: Mar 20, 2020
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  2. D'tar

    D'tar Friend of Leo's

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    Possibly some unnecessary grounding happening.... You may want to remove the shunt to ground connection on input 2. Look over your bus as well. Keep em coming, looking forward to seeing a lot of action in shock brothers. Hoping for some good debugging threads!!! Lol

    Stay healthy!!!
     
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  3. Junior Little

    Junior Little Tele-Meister

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    If both input jacks are isolated I thought both would have to be grounded...
     
  4. D'tar

    D'tar Friend of Leo's

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    They can be isolated and grounded but you dont want both shunting the tip to ground. Makes for a quiet amp plugged into input 1
     
  5. Paul-T

    Paul-T Tele-Holic

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    Looks good to me.

    What theoretical effect would having a single ground bus have vs Rob's separate pre-amp and power amp ground?
     
  6. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Heh, good eye as usual, my friend. I'll bet that isn't the way it's wired! I've been lazily working from photos, and photos are prone to parallax and overlaps -- plus my ground wiring on this first build wasn't pretty. Rather than open it up I'll put your note on my 'change list.'

    Thanks, great question. Merlin (link available here) shows us a typical and an idealized single bus.

    upload_2020-3-20_12-43-52.jpeg

    Mine fails, first, by being grounded at the 'noisy' power amp end, and also because it gathers the grounds in random (as laid out by Fender) order. He points out the features that 'improve' his version b:

    "...the bus now runs all the way from the reservoir capacitor to the input jack, and is connected to the chassis at the input jack itself. All power and ground connections are now made much closer to their relevant smoothing capacitors, thereby minimising the interaction of audio and power currents. The speaker ground has been moved to the quieter side of the ground currents flowing from the power valve, and all grid leaks are now connected to their respective cathode resistors, rather than directly to the bus."
    In partial answer to your question, he notes the simple version may work fine in small simple amps, and that the optimized version actually has many advantages (if you can design your own layout).

    "This method of grounding is highly recommended as it offers near-ideal performance while being straightforward and intuitive."
     
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  7. chas.wahl

    chas.wahl Tele-Meister

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    I'm interested in understanding more about this approach and reasoning for it.

    I'm planning to build a 5F2-A with two inputs, but forgo the usual connection from input 2 shunt to input 1 tip, to defeat the conditional voltage divider formed by the two 68k resistors when only input 1 is used (resulting in 34k grid stopper). Instead, I plan to wire the two inputs independently, their only connection being at the junction of the two grid stoppers on pin 2, and at ground (at the termination of the circuit ground for the entire amp, per Merlin Blencowe's model shown by @King Fan above.

    The question I have it this: without the connection between input 2 shunt and input 1 tip, is it then proper to shunt both jacks' tips to ground (when each jack is unused)? It seemed to me that both should be shunted to ground, but I'm a newb.
     
  8. Paul-T

    Paul-T Tele-Holic

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    Back when I knew about this stuff (did electronics a long time ago) I did a lot of star earthing and designed a lot of circuit boards around it. I remember some folks, analysing Quad designs etc, theorised that rigorous earthing was integral to the sound of the amps. Even transistor amps that used valve-style earthing sounded better.
     
  9. D'tar

    D'tar Friend of Leo's

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    Yes, You can do that, however.... whatever value you then use for the grid stopper on each input then becomes the other inputs grid leak value. When you study it you will see the elegance of the original configuration. Two complete individual inputs may require two separate triodes.
     
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  10. chas.wahl

    chas.wahl Tele-Meister

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    Hmm. I don't want to hijack this thread's original request, so I'll follow this up independently -- thank you.
     
  11. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Not sure from my view from way over here, but it looks like the 3 filter caps on the left are grounded on the buss and with a wire to the star. Should be one or the other.

    The NFB to the 22k resistor is attached to ground?
     
  12. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The 6.3 heater CT can terminate at 6v6 pin 8. (Shorter wire).

    No grid stopper or screen resistor on the 6v6?

    Why have two 16uF in parallel when one 32uF can be used?
     
  13. FenderLover

    FenderLover Poster Extraordinaire

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    Thanks for posting that. Just curious why you grounded at the P/S end anyway, instead of the input as illustrated?
     
  14. Paul-T

    Paul-T Tele-Holic

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    I used 3 x 22uF for mine. Hackworth and others have mentioned that Fender used four caps on the original because higher values were disproportionately expensive. Today there's no reason you can't simply use three filter caps in a Princeton.
     
  15. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Good catch on the duplicate ground. Thanks.

    Way too easy to copy one layout (the 5F1) while also trying to modify it. Or let's say way too easy to draw without thinking.

    As for the NFB, you mean via the 1500? Or did I draw it grounded elsewhere? I seem to be loving up on grounds...


    upload_2020-3-20_17-11-46.jpeg

    Um, it's vintage? Makes it look like a Princeton? My 5F2a board (and most 5F2a boards) came with turrets for two? Or just maybe... Did I mention I built this up from a 5F1 kit? so I already had a 16.

    Great question. Do you know, I don't recall. (Facepalm). But I *think* as a noob I read every article I could about grounds, all way over my head, and saw one that said it was not good (or necessary) to use the chassis as a signal ground path. Weirdly, I think the amp is about as quiet as SE can be.

    At some point, I do recall reading all those articles about grounding and realizing I was just trying to improve on a big brass plate with grounds dumped on it seemingly at random. :D
     
  16. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Some revisions; thanks to you all. @D'tar -- see if this makes more sense -- terminations on that ground shunt wire were hidden in the photos. But again, drawing without thinking. :)

    If I built this now I would do some things differently, but as noted, in this layout I'm trying to record what I actually did. Y'all may inspire me next to draw my 'optimized' version; split the grounds like Rob does, run the CT straight to the reservoir cap ground like Rob does, switch both hot and neutral....

    My 5F2a.png
     
    Last edited: Mar 20, 2020
  17. D'tar

    D'tar Friend of Leo's

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    Well, I suppose I am unsure what your intentions are with the inputs. I like the first version, minus the shunt to ground jumper on input2.

    In the second version without the input1 tip to input2 shunt jumper, I believe you now have made a voltage divider giving you a low and lower input? Perhaps I am missing something there?

    I think LLC was seeing the nfb cut connected to the ground of the footswitch jack which would be chassis gnd if not isolated or use a trs jack.
     
    Last edited: Mar 20, 2020
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  18. FenderLover

    FenderLover Poster Extraordinaire

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    And there's the rub. There is only one circuit 'ground', and whether you reference the chassis to that ground at the P/S end or the input is the difference which may cause a loop when connected to something else (at the input). Neither place uses the chassis as a ground path, unless you go with the popular split-ground method (an intentional ground loop). The rest of Merlin's text that goes with the illustration explains that part.

    In the end I believe you will still have a quiet amp unless you get out the high dollar analyzer and try to measure something that we can't hear. But... connecting the chassis to ground at the input (if it really doesn't make a difference) prevents you from having to isolate the jacks, which is one more PITA.
     
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  19. D'tar

    D'tar Friend of Leo's

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    I look forward to V3;)
     
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  20. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    That is it. The NFB wire starts from ground at the footswitch.
     
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