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JMP rework

Discussion in 'Amp Tech Center' started by wanderin kind, Jun 23, 2018.

  1. wanderin kind

    wanderin kind Tele-Meister

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    threw this together, sounds alright, Marshall 50 Watt master Mk 2

    weird 10K for first stage cathode, tone stack moved up front, 820 pf instead of 250 pf in tone stack for more juice and less ice pick, .047 and .1 instead of .022, 500 K vol pots, reduced cathode bypass cap on stage 2 for gain reduction and NFB, no bypass on stage 3 for the same reasons, 10K presence pot, 150K pwr grid R's, 470K/470K freq compensated divider ahead of stage 3, 5.6 K stoppers, 1.75K screen R's, and i only got shocked three times which is a new record,
    dude says "it finally sounds like a Marshall!" which is weird as it is more like a Fender with EL34's than a Marshall,
    Marshall JMP 50 Watt Mk 2.png if he is happy, i am happy,
     
    Last edited: Jun 23, 2018
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  2. Wally

    Wally Telefied Ad Free Member

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    I can’t think of that as any Marshall circuit. If it has big preamp overdrive as in the 2+1 Gain/tone stack topography of a 5F6A Bassman(Marshall) then one might think that the first stage is acting as a cold clipper????
    I hav a feeling that it doesn’t really act like a 5F6A/Marshall circuit does...especially as to how the tone controls interact with the preamp gain/volume adjustment. That is, if one pushes gain in the first stage the tone controls have to be brought down as in the AB763 Fender circuits in order to avoid over-distortion of the signal in those first two stages.
    I agree that if the user is happy, then all is good. I have heard a few amps that have a silimilar sound to a 5F6A/Marshall circuit while being quite different in their topography. The most notable of those was a Dr. Z Rte. 66 with an EF86 I put gain stage.
     
  3. printer2

    printer2 Poster Extraordinaire

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    The first stage would be lower gain and clean, not enough signal to act as a cold clipper. More like a Trainwreck.
     
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  4. bftfender

    bftfender Poster Extraordinaire

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    love mine, has trainwreck 3 MV, been in studio with 4 dif Marshall amps this past 2 months and the depth and clarity of the amp is stand out
     
  5. wanderin kind

    wanderin kind Tele-Meister

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    Ampeg used 5.6K cathode resistors on stage 1 of some of their amps like the B15N, but in conjunction with a 470K plate resistor, so you end up with a different load line.

    that thing i ended up with looks similar to the A7 wreck on page 10, just swap cathode resistors on V1 and V3, >

    http://www.blueguitar.org/new/schem/trainwreck/wreckxpr.pdf
     
  6. Wally

    Wally Telefied Ad Free Member

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    IMHO, the circuit shown in the OP is nothing at all like any Trainwreck Express. The Express...all of them in that link....use a gain stage/tone stack/gain stage topography that is akin to the AB763. In the Express, the output of the second gain stage feeds a cold clipper in the third gain stage.
    Your circuit does not look like that to my eye and understanding.
     
  7. robrob

    robrob Poster Extraordinaire Ad Free Member

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    [​IMG]
    That fully bypassed 10k cathode in the first stage would set a cold bias, reduce gain but add asymmetric distortion on the cutoff side of the signal. Not clipping but harmonic and intermodulation distortion from operating down low in the curvy part of the plate characteristics chart which would fatten up the clean tone.

    [​IMG]
    The operating point is at the intersection of the green lines, down low in the curvy part of the graph.

    The 120k slope resistor slightly reduces the tone stack's load on the previous stage which boosts gain and tips the bass/treble balance slightly toward the treble.

    The second stage with its tiny .47uF bypass cap would thin the bass to keep downstream overdrive in control by reducing the possibility of blocking distortion and reduce overdrive low end mush.

    The two 470k resistors that follow form a voltage divider that dumps half the guitar signal to ground to keep from over overdring the phase inverter. The bypass cap on the lower leg of the divider dumps even more high freqs to ground to help reduce fizz.

    The presence circuit design allows the amp to function if the pot fails.

    The 150k power tube grid leaks are a nice compromise between Fender's 220k and Marshall's 100k.

    The larger 1.75k screen resistors will add power tube overdrive compression and sustain. Screen voltage fluctuation changes not only the operating point along the transfer curve but also changes the shape and slope of the curve. This dynamic transfer curve morphing adds tons of nonlinear, harmonic and intermodulation distortion. This is the main reason most people prefer the sound of power tube distortion over preamp (triode) distortion.

    I looks like a good design on paper.
     
    Last edited: Jun 25, 2018
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  8. wanderin kind

    wanderin kind Tele-Meister

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    Thank you very much for that generous analysis!

    Here is the Drake OPT used in this amp, we did a take-a-part to check for arc over from coil to core as the amp was acting funny> Marshall OPT.png
     
  9. wanderin kind

    wanderin kind Tele-Meister

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    Power supply choke: Marshall Drake T-100 choke.png
     
  10. Wally

    Wally Telefied Ad Free Member

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    Rob, I see that same graph in your site’s explanation of a cold clipper. I appreciate your analysis of wanderin’s circuit.
    What exactly is the effect of placing that particular stage at the input rather than in the third stage as in the Trainwreck Express. The two preamp circuits are in a manner of speaking inverted, as I see them.
     
  11. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The plate characteristics graph will be the same with or without a cathode bypass cap so I used the cold clipper graph. The bypass cap just gives you more signal swing along the AC load line.

    With a 10k fully bypassed cathode in the first stage we'll get much more gain from the bypass compared to the unbypassed cold clipper but with around 1.5v of swing to the cold side available the signal straight from the guitar should not clip. It will thicken up the clean tone nicely with added nonlinear, harmonic and intermodulation distortion.

    With no cold clipper downstream the preamp will stay much cleaner with less preamp clipping. Most likely the power tubes will be the first stage to clip like in the 5F6A Bassman. The clipping will tend to be more symmetric without the cold clipper.

    The Marshall JCM800 with it's cold clipper in the second stage will generate lots of preamp clipping which makes its pre-phase inverter master volume more functional. The master volume reduces the signal into the phase inverter and power tubes reducing their overdrive distortion but since the preamp is clipping we can get low master volume overdrive tone.

    Placing the cold clipper in the third stage like in the Trainwreck Express the hotter signal from the second stage generates even earlier and more gnarly clipping in the cold clipper. We end up with a much more aggressive preamp with a lot less headroom.

    JCM800 Preamp With Cold Clipper In The Second Gain Stage
    [​IMG]
     
    Last edited: Jun 25, 2018
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  12. Wally

    Wally Telefied Ad Free Member

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    Thanks for the comparisons, Rob. I had never seen such a bypass on the input stage....learning is for me. The TW Express intrigues me although I have never heard one or a clone of one. In today’s world of demand for something other than extremely loud amplifiers, earlier onset distortion with what looks like a punchy low end might be interesting..even when cleans are wanted. I like amps with which one can set up at the edge of balanced clean and good distortion and control things with guitar output. How loud that has to be is another question...different in every amp, ime.
    Wanderin’, your approach seems to point at a cleaner signal with later breakup. Rob’s mentioning that the power tubes might be the first to distort is interesting. From what I understand, Ken Fischer’s started design goal was to achieve breakup that was simultaneous in all stages.
    So many options...so many paths, right? I’ll look forward to your results.
     
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  13. printer2

    printer2 Poster Extraordinaire

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    Sorry Wally, I should have mentioned the addition of gain in the third stage.

    [​IMG]
    The 10k resistor is dropped to 2.2k, not quite center biased. As far as the difference in clipping ability, the difference between the two values changes the dynamic range somewhat, but when you think we can easily give that stage an eyeball figure of at least 50V then they do not seem too different. Building a Trainweck type of circuit is on my to do list but many things are above it, one day.
     
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  14. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Adding a switch to parallel a resistor with the 10k cold clipper cathode resistor will let you go from cold clipper to a normal unbypassed gain stage. You won't gain any headroom though because lowering the cathode resistance to around 2k will boost gain and cause grid clipping in the following stage, in the TrainWreck Express it's the phase inverter. You basically get a choice between softer, sweeter cut off clipping in the cold clipper or harder grid clipping at the phase inverter.

    [​IMG]
     
    Last edited: Jun 26, 2018
  15. printer2

    printer2 Poster Extraordinaire

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    Switch? Did someone mention a switch? Might have posted this before, can't remember. From a Bassman/Marshall second gain stage to cathode follower, into a Trainwreck (converting the cathode follower to a cold clipper). Hopefully when winter comes I'll work more on electronics.

    [​IMG]
     
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  16. Nickfl

    Nickfl Friend of Leo's

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    A lot of very clever switching going on there! I especially like the grid/cathode bias switch on the first triode. I've had the idea kicking around in my head to do a 6bm8 powered mini trainwreck along the lines of the mini tweed overdrive special I built last year and there is quite a bit going on in your schematic that could apply to something like that...
     
  17. printer2

    printer2 Poster Extraordinaire

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    Which is funny, I am not into high gain amps, just more curious then anything. I might make a octal amp with the grid leak first stage. The tone control I am putting in a tube front end with Class D amp behind it. As soon as I finish the current guitar.
     
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