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jcm800 preamp changes

Discussion in 'Shock Brother's DIY Amps' started by Tom Kamphuys, Aug 8, 2020.

  1. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    I've recently build a JCM800 6V6 (RobRob):
    https://www.tdpri.com/threads/jcm800-6v6-variant.1027817/

    The amp is quite bright (at the lower volumes I tried).
    I've been analyzing the preamp signal:
    https://www.tdpri.com/threads/signal-tracing-through-a-jcm800-6v6.1040391/

    In the last signal tracing thread one can see that I found two strange effect in my preamp (and I haven't found a reason why other jcm800's shouldn't have it).

    A) With the gain around 3 a 'sharp bends' occurred. It was found to be draining of the larger capacitors in the tone stack through the cathode resistor. The cathode follower was unable to sink the current.
    IMG_20200801_232033400.jpg


    B) Once that problem was solved, it became clear that another strange signal showed up at high gain settings. It was found to be caused by the pre-warm-biased-stage treble bleeder.
    IMG_20200803_160514599.jpg


    Some robrob tweaks will be reverted. The 330k in series with the gain pot will be replace with stock 470k as I don't need the gain. The 470pF treble bleed will be replaced with the 1000pF as this will actually let more mids through instead of shrill highs. This comparison will be with the stock jcm800 preamp as a starting point.

    My solution seems to be a win-win-win situation and shares quite a bit with the advice of @LetItGrowTone.

    1) Remove the 470pF treble bleed
    2) Lower the cathode follower cathode resistor to 47k (Merlin's favorite)
    3) Increase the slope resistor of the tone stack to 56k
    4) Lower the smaller tone stack capacitor to 250pF

    1) will get rid of B), but quite a lot of highs will be lost.
    2) and 3) will get rid of A) and will drop some bass/mid.
    4) is to conform to the 56k standard TMB tone stack.

    The overall drop in preamp signal can be made up by a different MV setting.

    Here is the comparison in the time domain (stock: green, new: blue):

    MV 10%:
    Ten_Time.png


    MV 100%
    Full_time.png

    Capacitor draining A) is solved and so is B), although it was hidden by A).


    And in the frequency domain:
    Ten_Frequency.png

    Quite some highs are lost, but this is somewhat evened out by a bass/mid loss. Overall the amp will be less bright, but this is still adjustable by the tone stack pots.

    Of course, actually trying and listening will give the final verdict. I don't have a stock of resistors and capacitors, so this theoretical exercise is to find out which ones I absolutely need to order along a bunch of others to justify the shipping cost.

    Any comments?
     
    Last edited: Aug 8, 2020
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  2. EmWeAh

    EmWeAh Tele-Meister

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    Sorry, nothing to add currently but I am very curious about the outcome of your alterations. Please keep us updated...
     
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  3. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Tom, I just discovered that I made a mistake in calculating the NFB resistor value for the JCM800 6V6. I spec'd a 56K when 100k is what is needed. We get less NFB signal from the 6V6 power amp but we get more by going from the JCM800 4 ohm secondary to the 6V6's 8 ohm secondary so 100k is what we need. The JCM800 6V6 webpage has been updated.
     
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  4. andrewRneumann

    andrewRneumann Tele-Holic

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    I hope you like what you get, because you might be standing on the edge of a fiddle abyss.

    Removing a treble cap somewhere seems like a simple and advisable solution to having too much high frequency content. I would do that first and then spend a bit of time just playing the amp to see if you like it just the way it is.

    As for modifying the CF and tone stack... why do you feel the need to do this? Is it just to see if you like it better? I can tell you from experience that unless I am targeting a clear and distinguished aspect of the tone, if I don’t hear them side by side, I doubt my ability to determine which was “better”.

    Good Luck Tom! Thanks for sharing all the learning and including the traces.
     
  5. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Yes, I know...
     
  6. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    My ears are also not that good. But my engineering background just can't cope with what seems to be an error, audible or not.
     
  7. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Thanks for the heads up. I'm going to check for myself as it makes for a nice exercise.
     
  8. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    @robrob: looks like you missed the 100k NFB update in the 6V6 schematic.
     
  9. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Nice work!
    Now you are at the point where the amp frequencies will be adjusted to the speaker. I am pretty sure the hotrod deluxe speaker would not be the speaker for this application. OMMV.
     
  10. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    I did my homework. Basically is boils down to the fact that 50W x 4 Ohm (14V signal) is roughly equal to 22W x 8 Ohm (13V signal), so the NFB resistor has to stay stock 100k.
     
    Last edited: Aug 9, 2020
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  11. NSB_Chris

    NSB_Chris Tele-Meister

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    I am confused. I built several amps based on the RR800V and all mine taken directly from the posted schematics which I think were there at the start had 100k for the NFB resistor taken from the 8K secondary.
     

    Attached Files:

  12. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    It was changed:

    Screenshot_20200810-062723.png
     
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  13. robrob

    robrob Poster Extraordinaire Ad Free Member

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    As far as I remember the NFB resistor has always been 56k for the JCM800 6V6 but it has been revised to 100k. The JCM800 Micro's 10k NFB resistor is correct and has not been revised.

    I also decided to remove all my "tweaks" and just use the factory JCM800 preamp and let builders decide what tweaks they want to make. Version 3.1 is the current version of the layout and schematics.
     
  14. robrob

    robrob Poster Extraordinaire Ad Free Member

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    I did Tom, thanks, it's fixed now.
     
  15. NSB_Chris

    NSB_Chris Tele-Meister

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  16. robrob

    robrob Poster Extraordinaire Ad Free Member

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    NSB, that is the current V3.1 schematic and layout. The post links to the graphics on my website so it's up to date.
     
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