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Filament buss question

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

  1. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    Noting my post about the reverse nature of the pots... If I put a resistor in parallel across the input and ground lugs, unless it was huge value, wouldn't it reduce total resistance?

    If more resistance deepens trem I would have thought to add a resistor in series, on the lead coming into the input lug. Just shift the whole thing a bit higher, and retain the sweep. No?
     

  2. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    That pot will have some amount of signal voltage on the input, and ground on the other. The wiper, as it travels, sweeps from effectively grounded to the full pot input signal, with the signal sent on depending on where in that range it is.

    Adding resistance from what is now the grounded lug to ground shifts that range. An analogy would be your speedometer. It's normally 0 to 100, and the needle lies in that range. Adding resistance changes the range to 20-120, basically.

    Paralleling a resistor from input to output would drastically lower the end-to-end resistance of the pot, dumping a good bit of tremolo signal to ground.

    As far as taper, the 6G12 actually used a linear 10M FWIW.
     

  3. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Adding resistance before the pot input would deepen the tremolo signal upstream of the added resistor, but might actually weaken the max amount of usable tremolo by attenuating the signal across the resistor, before the wiper.
     

  4. D'tar

    D'tar Tele-Afflicted

    Jan 11, 2013
    WNY

    I remember reading that now. I should have presented that more as a question then. how would a 25k or 50k pot act then I wonder. I'll step back from the spectator line now.

    Great job by the way.
     
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  5. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    Thanks. I was greatly surprised by learning that. I was going nuts, trying other guitar pots on hand - standard 250KA. Thought no, that can't be, I must have them facing the wrong way when I'm thinking about this. But no. Apparently "reverse" doesn't mean what I always thought.
     

  6. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    I just realized something about those pot specs.

    RA coding means 'audio taper, with the taper reversed' (not the rotation).

    A means 'audio taper'.
    B means 'linear taper'.
    C means 'anti-log taper' (audio, but with rotation reversed).

    I was conflating RA and C. These speed and depth pots are RA, not C. Highest resistance from input to output is at the max setting. With a C pot, it would be on the minimum setting.
     
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  7. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    Your speedometer analogy is exactly what I was thinking of. I just didn't / don't know how to do it.

    Trying now.
     

  8. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    OKAY! I placed a 1meg resistor between the wiper and ground, on the intensity pot, as @clintj suggested. All I had was a carbon comp, and it seems to have added noise. It's temporary. You can see it sticking up on long leads.

    I think it did the trick, but tell me, do you hear some grit, as if I'm overdriving the tremolo circuit? Amp is still the 71 Super, settings pretty much the same, except a bit more bass, perhaps. The guitar for these clips is the same as yesterday, a Collings City Limits, with Throbak humbuckers.

    20180103_013047.jpg


    Apologies in advance for the aimless noodling.

    We'll start with the 50% depth...


    Now some 100% depth noodling:


    Here I've attempted to hear the separate triodes at work, with one clear bass note and one clear treble note, not the same pitch. In person, I could definitely hear it working. So cool! Overall volume stays mostly the same, which is what Leo was shooting for, and is different (better) than any trems that followed, IMO. But in the details, I could the highs and lows swapping places. Can you?


    Here's speed on zero:


    Here's speed on about 30-35%, which is the fastest example. I didn't look, but the bulk of the clips today were probably around 20%. It gets too fast, in my opinion, but it doesn't matter, because it gets slow enough.



    I have a 2k trim pot arriving Friday, so we can play with that cathode if we still want to. Any other ideas at this point, or are we about done here? I probably won't be able to resist trying a 1.5M resistor on that pot (again, Friday), but if I'm overdriving things, it's probably not the best idea.
     

  9. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Move that resistor as follows:

    One lug of that pot should already be grounded (the bottom one on the schematic). Break that connection, and install the resistor between that lug and ground. That's where I was aiming to have you install it. From wiper to ground like that just radically changed the sweep range of the pot instead of moving the whole range higher.
     

  10. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    Moosie, that's a very useful note. (Your trem is starting to sound awesome, BTW). I'm guessing *most* people think reverse pots have 'reverse rotation' -- you see a ton of threads saying "do lefty guitars need reverse pots?" The name is unfortunate, really, but "right hand logarithmic" isn't likely to replace it in this universe.

    I find it confusing even when I know what it 'should' mean. Following the TDPRI motto 'pics or it didn't happen' I'll paste in this nice graph from R.G. Keen. His whole article about pots is superb, and he notes about reverse taper: "This is used in some bias circuits and in controlling the speed of certain RC oscillators, which is where the audio tinkerer runs into it most."

    RGKeen_pot_taper copy.png

    Mr. Keen's notes about this graph are priceless:

    Curve 1 is linear taper. If we clip one lead of our Ohmmeter (Hey! There he is again!) onto the leftmost lug, and the other lead on the center lug, then the resistance we read as we rotate the pot clockwise will fall on the curve that goes diagonally upwards. The proportion of the total pot resistance we traverse as we turn the pot is linearly proportional to the amount of rotational travel we turn.

    Curve 2 shows what happens with an audio or logarithmic taper. As we turn the shaft, the proportion of resistance we traverse increases slowly at first, more slowly than the percentage of rotation. As we get past half the available rotation, the rate of resistance traversed speeds up as we get closer to the furthest rotation. This compensates for the human ear by increasing sound levels very slowly at first, then faster as the ear's sensitivity falls off at higher sound levels.

    When we buy "audio taper" pots, we usually get something like Curve 3. For less expensive pots, manufacturers use a two or three-segment approximation to Curve 2. It's not perfect, but it usually works OK. Curve 4 is the typical resistance versus rotation curve for reverse log pots. In real life - that is, if you ever found one of these in real life - it is usually a two or three segment approximation, too.

    If you have an unknown pot, you can figure out what taper it is. You measure the resistance from end to end, then turn the pot exactly to half its rotation and measure the resistance from the counterclockwise lug. The crosses on curves 1, 2 , and 4 show the most probable values. If the resistance is 50% of the total resistance, then the pot is linear. If you measure only 10% to 20% of the total resistance, the pot is an audio taper. If you measure 80%-90% of the total resistance, the pot is a reverse log taper.

    At this point I should probably explain what a counterclockwise lug is. Of the three contacts on the pot, the wiper is easiest to pick out. If you turn the shaft fully counterclockwise, the wiper lug will show very small resistance to one of the other contact lugs. This is the counterclockwise or "cold" terminal. Turning the shaft fully clockwise, the wiper will show very small resistance to the most clockwise lug, also called the "hot" lug.
     
    Last edited: Jan 3, 2018
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  11. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    Oh.




    :oops:




    So, I mostly I heard the difference I expected to hear. Interesting.

    I'll get on that in a couple hours. Thanks!
     
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  12. Snfoilhat

    Snfoilhat Tele-Meister

    Age:
    36
    366
    Apr 8, 2016
    Oakland, CA
    If you read (and read into) the record of changes on the schematic, I interpret some of the Version 1.3 and Version 1.4 stuff to be about changing the balance of input signal and output signal. Version 1.4 reduced input signal by changing a usual 100K plate load resistor at V1B to a split 68K/33K plate load (sending 33% of signal to B+/ground) and upping the gain at V4 by moving from 100K plate load resistors to 220K. But that change must have altered the bias of the V4 triodes, as y'all have been discussing. Moosie's revibe as it sits seems to have the lower gain (Version 1.3) split plate load at V1B and the lower gain (Version 1.4) 100K plate loads at V4A and V4B. Maybe Gehring was trying to avoid running out of headroom, or maybe it was about getting unity gain (since the Revibe is a sort of effects pedal and that's expected/desirable). Sounds good!
     
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  13. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Not a problem. It's sometimes hard to accurately describe a change using words, when you don't have a good drawing tool at hand. The pot itself needs to be unaltered as far as the low/mid/hi range. We need to insert some resistance to ground, though.
     

  14. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    Interesting. A point of clarification, please...

    Your comments are ALL about the ver 1.4 change. According to the schematic notes, 1.3 was the Tore T mod, changing V1b from cathode follower to common gain stage.

    Ver 1.4 involved three changes. First, change V1b to split load, and second, change V4 plate loads to 220k. Third, remove R16 (which since I don't have the earlier schematics, I can only assume was somewhere around the tank area.)

    Sound right? You're not saying something about the Tore T change, are you? If so, I don't understand.

    Your 'headroom' comment... I wonder if going to the 100k resistors is when I really started to notice some clipping. It actually sounds good, but I might not always feel that way. (switchable, hehe...)



    I'm at the bench now, will try to have some new clips up in an hour or so.
     

  15. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    The 1M resistor didn't do a whole lot. Here are some clips from 2 resistors in series that total about 1.7M. Sorry about the clipping. I finally realized it's my Zoom mic. I probably hit the dial while setting it up. I don't hear it in the room.

    Depth starts out at 50%, and increases later to 75%. Before, I'd only rarely have used 50%. Now it seems pretty usable.



    Depth 100% on this clip. Various speeds. Speed still never goes above 35%. I like that this depth would be too much in many situations. Nice to have the extra for when it is wanted.

    Note the beginning I cycle through a couple of starts / stops of the vibrato. After hearing the footswitch click, it takes about four seconds for the wobble to get up to full. That's pretty cool.

     

  16. Snfoilhat

    Snfoilhat Tele-Meister

    Age:
    36
    366
    Apr 8, 2016
    Oakland, CA
    My mistake--you are reading it right despite my having written it wrong. I should have said split load is lower signal and part of version 1.4, and 100K is lower gain and part of versions 1.3 and earlier.
     

  17. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    I'll give those a listen tomorrow when I get some time to unwind. Today was my Friday at work.

    The #1 question until then: how do you like where it's at now? Do you want to continue to tweak a little more, or are you satisfied as is?
     

  18. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    Tweak a little more, I think.

    It sounds pretty good now, but I'd like to hear how V4 bias changes affect the sound, with the trimpot that should get here by Saturday. I plan to give my ears a break for a few days, to get some objectivity back.

    Here's what's milling around my poor brain...

    We know the LFO, hi/lo filters, and phase shift are working.

    Looking at Leo's sample waveforms, the oscillator creates the large scale sine wave shown in waveform #1.

    The clever phase shifter creates the mirror image sine wave shown in waveform #2.

    The filters split the audio signal, which halves are then overlaid on waves 1 & 2, as shown (treble on 1, bass on 2).

    But notice that the audio waveforms are each louder in the upper portion of their respective cycles.

    Finally, the two are mixed after V4, and the opposing oscillations negate themselves out of existence, shown as a straight line in wave #3. This leaves only the phase-split, amplitude-varying audio signal. The beauty is not just the cool warble, but the relatively constant overall volume level as seen in wave #3.


    Since the oscillator sine wave is removed before output, it's only function, as I understand it, is to cause the bias to vary in the V4 triodes, which in turn performs the amplitude modulation on the audio signal.

    Increasing the depth pot value increases the amplitude of the LFO, which is then used to vary the bias.

    To get the warble, we need enough LFO amplitude, and optimal V4 bias to be able to use the LFO most effectively. We got the LFO with the change to the depth pot. All that may be left is the bias tweak.


    Also, I'm still not clear what difference the plate load resistor values make. 100k vs 220k. And whether I should consider putting the 220s back in before messing with the cathode resistor.


    And finally, with all the messing I've been doing to the wire dress (I guess), the unit is no longer quiet. Ugh. But I can work on that later.

    Leo's diagrams:

    Screen Shot 2018-01-04 at 12.03.23 AM.png
     

  19. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    My head's spinning a bit. I see how the V1 change could impact the V4 / V5 tremolo, but I don't understand how.

    Let's see, splitting the load resistor reduces the input audio signal. Which means what? Like you mentioned, Gehring was just tweaking unity gain? He found he was inadvertently amplifying the audio, and didn't mean to be?

    But I could use a little help with how that interacts with the V4 plate loads changing at the same time.
     

  20. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Changing V1 from a cathode follower to a gain stage boosts the signal to the tremolo circuit, which would boost the strength of that part of the total output. That may have been a decision to get a more tremolo heavy final sound - who doesn't want more tremolo?

    Splitting the load resistor is a good way to adjust the over all gain of that signal path (the tremolo circuit). Maybe he found that the tremolo half was too strong, and wanted to trim it back a little? The question then is what was the deleted resistor? The number puts it around the reverb recovery stage, and maybe it was part of a voltage divider or something. I'd need an earlier schematic to be able to make a judgment there.

    Raising the overall plate load slightly raises gain, but it makes the tube less distorted and shifts the bias point. The distortion in this case is the asymmetry that is needed to cause the "squish" of the audio shown in the sample waveforms. I'm still curious about that. One thought is it may have been done to get more reaction to the LFO signal, to balance the stronger guitar signal.

    When that trimpot gets here, it's going to be used to tweak the value of the 4k7 resistor on the V4 cathode circuit by having it in series with it. We want that total resistance variable from 4k7 to 6k7, so we can make subtle little tweaks to dial in the bias to the sweet spot. Once that's dialed in, we measure and replace the 4k7 with a single resistor equal to the determined value.
     
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