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Filament buss question

Discussion in 'Shock Brother's DIY Amps' started by moosie, Nov 23, 2017.

  1. clintj

    clintj Friend of Leo's

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    If you follow the schematic from V5a, the oscillator output goes through several rounds of attenuation via large resistors and divider networks, so that the signal at V4b is only a couple of volts. A couple of volts of bias swing on V4 is right where we need to be, as a small triode only runs in the 0-4V range.

    Remember, V5b is actually looking to the grid of V4b to get its signal. That's roughly where it taps in and starts its own voltage division process. Its ultimate goal, by design, is to make V4a see the opposite of what V4b sees. Same amplitude, same speed, opposite phase. And it looks like it's actually doing a decent job of it, it was just drowned out when trying to meter it vs scope it.
     
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  2. clintj

    clintj Friend of Leo's

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    Further thoughts: if you probe each successive junction from the V5a plate to the V4b grid, you can track the attenuation of the signal. 30V is way too big a signal for a usable tremolo circuit using a preamp type tube; but that would be useful for a power tube bias vary type.
     
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  3. King Fan

    King Fan Poster Extraordinaire

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    Cool, you two. Part technical tutorial, part suspense novel. Thanks for both.
     
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  4. moosie

    moosie Doctor of Teleocity Silver Supporter

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    OK, back with more fine-looking results, darn it.

    • Tremolo turned completely off, via footswitch
    • Input: tone generator on Android phone
    • Output: Guitar amp, just to make sure what the tone sounds like. After I hear the tone, I put the amp on standby while I'm taking measurements, so it's not annoying.

    No Signal
    • V4a grid: 30 mvpp
    • V4b grid: 60 mvpp
    • V4a plate: 274 V
    • V4b plate: 264 V
    • Output: 35 mvpp

    100 Hz
    • V4a grid: 70 mvpp
    • V4b grid: 290 mvpp
    • V4a plate: 256 to 272 V
    • V4b plate: 256 to 276 V
    • Output: 72 mvpp

    5000 Hz
    • V4a grid: 240 mvpp
    • V4b grid: 60 mvpp
    • V4a plate: 268 to 280 V
    • V4b plate: 260 to 268 V
    • Output: 65 mvpp

    Seems like the hi pass (V4a) and lo pass (V4b) filters are working.


    So, the oscillators are working. The signals are getting through the proper channels. What next, somehow combine the two?
     
  5. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Tremolo Speed and Intensity: middle of sweep

    V4 grids (A = yellow, B = purple)

    Tremolo off, no signal:

    4A, 4B grids - tremolo off, no signal.png


    Tremolo on, no signal:

    4A, 4B grids - tremolo on, no signal.png


    Tremolo on, 100 Hz signal:

    4A, 4B grids - tremolo on, 100 Hz signal.png


    Tremolo on, 5000 Hz signal:

    4A, 4B grids - tremolo on, 5000 Hz signal.png
     
  6. moosie

    moosie Doctor of Teleocity Silver Supporter

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    For completeness sake, mostly. Nothing to see here, except what we already know: the oscillation is not making it to the output. Tremolo intensity is still in the middle.


    4A grid (top) vs output: no signal

    4A grid vs output, no signal.png


    4B grid (top) vs output: no signal (similar to above, really, if you ignore my scaling inconsistency)

    4B grid vs output, no signal.png


    4A grid (top) vs output, 100 Hz:

    4A grid vs output, 100 Hz.png


    4B grid (top) vs output, 100 Hz:

    4B grid vs output, 100 Hz.png


    4A grid (top) vs output, 5000 Hz:

    4A grid vs output, 5000 Hz.png


    4B grid (top) vs output, 5000 Hz:

    4B grid vs output, 5000 Hz.png
     
  7. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Top waveform is the output jack. Bottom waveform is the output of both V4 triodes, where they first join together, just upstream of C33.

    100 Hz, intensity 100%:

    V4 plates, before C33, 100 Hz, depth full.png


    5000 Hz, intensity 100%:

    V4 plates, before C33, 5000 Hz, depth full.png


    100 Hz, intensity 50%:

    V4 plates, before C33, 100 Hz, depth half.png


    Note how quickly both waveforms flatten out (too much so) at 50%.

    Also note the odd waveform shape. Just visible at 100%, but I think it's there (more subtle) at lower intensity settings.
     
  8. clintj

    clintj Friend of Leo's

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    Should be quiet at work today, I'll dig through all those scope traces today and see what comes to me.

    Nice work there!
     
  9. clintj

    clintj Friend of Leo's

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    Oh, my. I see something after watching Keith's video. He's getting about 4Vp-p in his amp, and you're getting far less than that. We need to figure that out.

    It looks like there's not enough LFO signal on either V4 grid to actually drive them into cutoff, or near cutoff for that matter. That leads to no "squish" of the signal, and no tremolo effect - especially the harmonic part. The signal out of one triode, at the output jack, should look like the attached pic.

    Ok, I apparently can't attach a pic today. Do a Google image search for "AM waveform" to see what I'm referencing.

    Backtrack from the grid of V4b to the V5a plate, and note the pk-pk voltage at each junction.
     
  10. clintj

    clintj Friend of Leo's

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    Oh, there we go. Now it's playing nice with pics. main-qimg-8dd3f6fd92c41f0cdec628d7316fe5df-c.jpeg
     
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  11. moosie

    moosie Doctor of Teleocity Silver Supporter

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    OK, working on this now.

    I see what you mean about Keith's. 1-2V each way, depending on speed setting. I'm only getting about 10% of that oscillation.
     
  12. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Backtracking from V4b Grid, to V5a Plate
    No signal
    Speed 50%
    Intensity 50%

    Probe 1X

    V4b Grid - 400 mvpp

    V4b Grid - 50,50.png



    Intensity wiper - 600 mvpp

    R39 - 50,50.png



    Intensity input - 5.7 vpp

    Intensity Input - 50.png



    Probe 10X

    Upstream of C26 - 102 vpp

    C26 - 50.png



    V5a Plate - 180 vpp

    V5a Plate - 50.png
     
  13. clintj

    clintj Friend of Leo's

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    Ok. I need the hard copy I have at home of the schematic, and some time to crunch numbers.
     
  14. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Hang on. OK, at 50% depth, I'm getting 400 mvpp on V4a grid, as shown above. But at 100%, I get 2.6V. Actually, that's with a 10X probe. On 1X, it's more like 2.1V.
     
  15. moosie

    moosie Doctor of Teleocity Silver Supporter

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    It's OK, you can take the schematic with you to your big NYE gig. I'm sure that'll go over well. :lol:
     
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  16. clintj

    clintj Friend of Leo's

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    I can't remember, and I can't find it in this thread. Did we check C26 and C27 for correct value? It seems like there is a tremendous amount of attenuation around that area to me, even without getting into higher math.

    And did we verify the 10M pot is actually a 10M?
     
    Last edited: Dec 31, 2017
  17. moosie

    moosie Doctor of Teleocity Silver Supporter

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    I verified C26 and R37 this evening. .0046 uf, and 4M9. A bit high, but within spec.

    The pot is verified. I tested it and wrote it on the side, early on, and then twice more during the build. 11.1M RA.
     
  18. moosie

    moosie Doctor of Teleocity Silver Supporter

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    C27 is right on, 101 nf.
     
  19. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Tell me about reverse audio pot specs. This intensity pot is full resistance between input and wiper, when pot is on zero. That aspect is not "reversed". But the taper is reversed. Normally an audio pot on 50% of sweep, will exhibit much higher than 50% of it's resistance between input and wiper. This one is at approximately 3M, or less than 30% of max resistance.

    Is this what we should expect from a 10M RA pot?
     
  20. moosie

    moosie Doctor of Teleocity Silver Supporter

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    It's probably correct. I'm seeing the same behavior on the Speed pot, 3MRA. Guess I just never knew that. Add it to the list.
     
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