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Preamp signal through my RR800V

Discussion in 'Shock Brother's DIY Amps' started by NSB_Chris, Jun 21, 2020.

  1. NSB_Chris

    NSB_Chris Tele-Meister

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    This is relevant to several of my threads/posts but couldn't decide which so just started a new thread.

    I have built two amps which are a variation of Rob Robinette's RR800V. I used the amp enough that I know where I like it. I recently purchased my first oscilloscope and made an attempt to trace a signal as it propagated through the amp. I got some unexpected results and wanted to see if others could help me understand what is at work here.

    The preamp stages in the first channel are pretty straight forward Marshall JCM800. I put in a sign wave signal and tested at 4 points. See the schematic below.
    IMG_3821.jpg

    The first two test points after the fist stage were not too surprising. The 200mV peak-to-peak signal went to about 4V peak-to-peak and after the volume pot set at 3'oclock the signal went down to 2V peak-to-peak. Both of these signals did not have obvious signs of significant distortion.

    After the second stage, just after the coupling cap, the signal was distorted on voltage, but still had a sine wave trace pattern. See picture below.

    The surprising part was when I tested after the voltage divider. This trace shows significant waveform distortion. Why such a huge difference in the character of the signal after the voltage divider??? Does the bright cap have that big of an affect on the waveform???

    Not looking to alter the amp. I love it just the way it is. Just trying to understand how things work!

    At point 3:
    IMG_3819.jpg

    At point 4:

    IMG_3820.jpg
     
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  2. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Does it sound right? I always tell people, after the first stage there's no such thing as a sine wave in a Marshall, so this seems like what I would expect to see.
     
  3. Ten Over

    Ten Over Tele-Holic

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    Grid current from V2 pin 7 is clamping the positive voltage at point 4 sort of like a diode. Point 3 doesn't clip because the 470K/500pF combination has an impedance of 432K at 292Hz so that the diode action isn't felt there.
     
  4. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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  5. NSB_Chris

    NSB_Chris Tele-Meister

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    Not understanding yet but getting there...

    So are you saying that the clipped looking waveform is from V2 and not V1B? The signal coming off the plate of V1B is what I am measuring right after the C7 coupling cap, but when the signal hits the warm biased first stage of the cathode follower, it distorts and I see that reflected on V2 pin 7?

    If I did it correctly, the operating point of V1B is shown below. It is distorted but not showing a rounded or clipped trace appearance on one lobe like I would have expected.

    Screen Shot 2020-06-22 at 8.55.43 AM.png
     
  6. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Yeah, that's basically it.

    I need to look into the cold clipper. At first sight I would agree with you, but it's clearly not distorting that much according to your scope.
     
  7. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    This is what I would expect (for a B+ of 245V):

    coldClipper.png

    Reality is clearly wrong.
     
  8. NSB_Chris

    NSB_Chris Tele-Meister

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    Fascinating! I thought the lion share was happening at the V1B section. I would have expected the top lobe to be really rounded looking after V1B and the cathode follower section would be just along for the ride giving a buffered signal to the tone stack. The distortion has to be there after V1B just not obvious visually on the wave form but it is setting up the cathode follower for the more dramatic distortion.

    Edit: I was writing at the same time you were posting. Your plot is what I would have expected after V1B. Very interesting!
     
  9. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Few remarks:
    - V2A is not part of the cathode follower. It is just a warm biased gain stage. It is however influenced by the cathode follower.
    - The cathode follower is said to have quite a contribution to the overdriven sound.

    See here for more info:
    http://www.valvewizard.co.uk/dccf.html
     
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  10. NSB_Chris

    NSB_Chris Tele-Meister

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    That explains it thanks!!!! I learned a bit about cathode followers working with my effects loop send circuit and read a lot about them, but missed the valvewizard section you linked. I didn't know what to call the gain stage that is "married" to the cathode follower. I learned a lot on this test/exercise. Thanks!!!
     
  11. sds1

    sds1 Tele-Afflicted

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    Cool thread, nice scope. You love it?
     
  12. NSB_Chris

    NSB_Chris Tele-Meister

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    Yes, I really do like it, but I have a lot to learn on its use and features.
     
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  13. sds1

    sds1 Tele-Afflicted

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    I noticed in the pics you have at least one channel in DC coupling mode. Unless you care about your DC offsets, that should be in AC coupling mode.

    If you can figure out FFT mode (in the MATH menu), that will be fun to look at. Visualize the harmonic distortion.
     
  14. NSB_Chris

    NSB_Chris Tele-Meister

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    Thanks! I played around with AC and DC coupling, but must have ended up leaving it there. It was on my list to look at the FFT mode, but needed an easy win! :)
    I spent the most time figuring out how to get a sine wave from a signal generator app out my phone so I could send it into the amp. I looked at the signals coming out of the guitar I used primarily with this amp to get the input signal in the right range. Would be interesting to use an ebow or something to get a consistent guitar string vibration signal going out into the amp. However, I am not going to buy one for that purpose only.
     
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  15. Ten Over

    Ten Over Tele-Holic

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    The plate characteristics graphs that you guys are using are for fully bypassed cathodes. Neither the cold clipper nor the warm biased gain stages have bypassed cathodes. The plate characteristics graphs for unbypassed stages are radically different. The curves are straight parallel lines that are much more evenly spaced than the bypassed curves and the slope is much lower. Each value of cathode resistor has its own graph.
     
    Last edited: Jun 22, 2020
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  16. Ten Over

    Ten Over Tele-Holic

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    You are not giving the cold clipper stage anywhere near enough signal to cause clipping. Unbypassed stages require more input to achieve cut-off clipping than bypassed stages and the larger the cathode resistor gets the larger the input signal needs to be for cut-off clipping.
     
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  17. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Good point. Will try to incorporate that in my scripts.
     
  18. NSB_Chris

    NSB_Chris Tele-Meister

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    Another subtlety that I was not aware of. The JJ tube data sheet just gives you one and makes no mention of assumptions for the graph. I was using a convenient website for plotting to be lazy and don't remember there being anything regarding bypassed on unbypassed. Time to step back and pay more attention to information on plotting load lines!!!
     
  19. NSB_Chris

    NSB_Chris Tele-Meister

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    OK, I have been reading: http://www.valvewizard.co.uk/Common_Gain_Stage.pdf
    It is starting to sink in and I can better see what is going on... I have a couple of other books that arrived today which may help.
    This continues to be informative. Thanks for indulging me!
     
  20. tubegeek

    tubegeek Friend of Leo's

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    One of the best articles on tube guitar amp tech I've ever read.
     
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