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5881 in a 5f2a

Discussion in 'Shock Brother's DIY Amps' started by jsnwhite619, Jul 11, 2020.

  1. cobaltu

    cobaltu Tele-Meister

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    Apology accepted. I should have mentioned when I suggested the ~2K/~80K setup that I was trying to achieve a voltage gain that guitar players generally find more familiar and appealing. It's true that your 47K plate load line does get you closer to the 'golden ratio' but it sacrifices a lot of voltage gain that is often more useful in guitar amps - where the goal is to produce some distortion.

    Your load line would probably be great for a driver stage, like the first gain stage of an amp - which in many Fenders - drives both tone stack and volume control.
     
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  2. tubegeek

    tubegeek Friend of Leo's

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    Surprisingly, the gain is almost the same, 25.75 vs. 27.5.

    If we use the same approximation and swing symmetrically from the bias point to the 0V grid line, we get 5.2V pk-pk, and output goes 60 to 203. 143/5.2= 27.5. Again ignoring loading effects - for either operating point the loading effects will be manageable.

    The conditions you selected offer more headroom and output swing, but not actually very much more gain. For an input tube, you'd have to drive the amp with a pedal to make use of all the headroom available either way.

    A quick stab at the distortion, the 47K plate load gives about 11% and the 80K about 8%, so that comes out as an advantage for the 80K choice, assuming we're looking for clean input amplification.

    When I get a chance I'll look at something like 180K plate load and see what @elpico 's preference will yield.

    The 12AY7 is inherently a good driver tube, that's due to its lower rp. This is what led me to want to look at alternative operating points to begin with. I'm only a little surprised that an almost 2:1 difference in plate load has no influence on the gain with this tube. And again, I'm exploring this based on my own leaning towards lots of current in a small signal tube.

    A phono preamp I designed idles st something like 15 or 20mA in its input stage. (Using 6C45P.) It's basically a small power power amp, and I used that to drive a phono eq network with about 1/5 the impedance of a typical design. That was an experiment that worked really well, and it's one reason I like to pursue higher current even in small signal stages. VERY lively and responsive sound!
     
    Last edited: Jul 31, 2020
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  3. tubegeek

    tubegeek Friend of Leo's

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    Oh what the hell:
    In purple:
    180K plate load
    -3Vbias @ .67 mA, cathode R 4K5, voltage on the pate at idle 130.

    Output swing 62 to 211, 149 v pk-pk / 6V pk-pk = gain of 24.8.

    About 46K rp, plus 4K5=50K5. 50K5 || 180K = 39K4.

    @elpico here's what it looks like - not to put words in your mouth, so does 180K suit your preferences? (I guess 220K might be worth looking at too, and that's a value that gets used for small signal tubes often enough. Maybe tomorrow.)


    12AY7 180k45.jpg
     
  4. elpico

    elpico Tele-Afflicted

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    I knew I wasn't communicating well there. I don't have a preference for a higher plate load. I was only trying to illustrate that what's "better" depends completely on the goal.

    What is this stage driving? If it's anything other than a 1meg resistor/pot then these lines are pretty imaginary.
     
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  5. jsnwhite619

    jsnwhite619 Friend of Leo's

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    For the sake of this thread and amp build, let's say it is a single ended 12a_7 preamp Champ style amp.
     
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  6. elpico

    elpico Tele-Afflicted

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    In the tweed champ you can almost forget about the volume pot and focus on the plate resistor, but black face champ is a totally different story. None of these lines would be representative of reality there because the tone stack load is so significant. It's reactive too so a simple line can't represent it.

    FWIW I think tubes that are "feeling" a reactive load like that are a significant ingredient in why tube guitar amps sound cool. I've always found the fender/hiwatt style of hooking the tone stack to the plate of a stage that, at least on paper, isn't an ideal driver for it sounds more colorful and interesting than the Marshall/vox cathode driven version.
     
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  7. Mark1406

    Mark1406 TDPRI Member

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    A 6V6 can handle 400VDC rail voltage in class A/B, it can't handle 400VDC in class A, you will be blowing 6V6's every few minutes.

    Regards

    Mark
     
  8. jsnwhite619

    jsnwhite619 Friend of Leo's

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    So, @tubegeek and @cobaltu , I swapped the first 1.5k bias resistor on V1 with a 2.7k. It really fattened up the tone with the 12ay7. Lower clean headroom, but fatter & thicker and warmer sounding. With that done, I went back and swapped the 8k primary back to the 5k on the OT, and that sounds better now as well while also gaining back some headroom. I tried both a while back, and at the time the 5k was too thin sounding. Now, with the preamp bias cooled down, it sounds better than it did before. The 8k sounds incredible too, but it lost nearly all clean headroom with the 2.7k in there.

    What would be expected if I change the 100k plate load resistor for a different value? I have some 82k resistors on hand.

    Playing with a dB meter and plugging in some numbers with an online calculator today, I'm going to give a best estimate at approx. 10 watts from this amp on 10.
     
  9. tubegeek

    tubegeek Friend of Leo's

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    With a 12AY7:
    For shhhhts and giggles, parallel the 100 with one of the 82Ks and parallel the 1K5 you just took out with the 2K7 you put in its place. Just tack solder them to the R's that are already there. That'll be awfully close to the operating point I was curious about, and it'd be a reversible 5 minute mod.

    Also - would you measure the voltage at V1a plate before & after, and the voltage at the supply point for the plate load before & after?

    Nothing I'm suggesting can do any damage, it's all within that amp's capacities.
     
  10. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Here is a quick clip with my Les Paul and the 12ay7/2.7k bias resistor

     
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  11. cobaltu

    cobaltu Tele-Meister

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    That's SICK!!!
     
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  12. HBamps

    HBamps Tele-Meister

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    I am curious about this. How does this change the character the amp vs the standard 22k to the top of the cathode of V1b?
     
  13. HBamps

    HBamps Tele-Meister

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    And why is that font so small? :p
     
  14. jsnwhite619

    jsnwhite619 Friend of Leo's

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    It allows for NFB and an extra gain stage.

    After all this testing it out with my Tele, I honestly jumped a little bit when I hit it with my Les Paul through that 12" Celestion. :D That was the volume on 6 I think.

    The NFB switch doesn't make a huge bit of difference, so I'm thinking of picking one value and sticking with it. What I think will make a much bigger difference in having two distinct tones would be to place the 2nd bypass cap on a switch instead. Any suggestions if it's better to switch the +/- on a bypass cap? I'm thinking there may be a better or worse way regarding any pops when it's switched?
     
  15. elpico

    elpico Tele-Afflicted

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    Switch the + but add a high value resistor between the two so the unused cap has enough trickle current to stay charged up. 100k should be reasonable.
     
  16. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Cannabis Rex arrived yesterday. This thing is a beast with a Les Paul!

     
  17. James Knox

    James Knox Tele-Holic Silver Supporter

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    Rock Solid ...
     
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  18. D'tar

    D'tar Friend of Leo's

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  19. jsnwhite619

    jsnwhite619 Friend of Leo's

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    This thing can get mean with a Tele!

     
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  20. mjcyates

    mjcyates Tele-Meister

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    Amp sounds great!
     
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