Understanding the VibroChamp's Tremolo Circuit And How It Might Be Implemented

Discussion in 'Amp Tech Center' started by separateness, Dec 12, 2019.

  1. separateness

    separateness TDPRI Member

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    Greetings. This is my first post here although this website has been for me a great resource for information for over a decade. By way of introduction I am a guitarist, tinkerer and I have a modicum of education in electronics. I come to you today seeking some understanding of the Virbro Champ's tremolo circuit and also how I might implement something like it.

    I will start at the beginning. I own a Twin Reverb. It was master volume twin, 100 watts. I paid to have it blackfaced. Some time later and several years ago it was completely submerged in a flood. I pulled everything out of it and rebuilt it. When I rebuilt it I did so to basically ab763ish specs with some caveats. I only have one input, and one channel. The bright switch is an input kill. The other Bright switch enables the tremolo. There are probably some other things.

    But the tremolo. When I rebuilt it I thought I would get a little adventurous and so I implemented the tremolo as an output tube bias vary type made with solid state components. I even had some tiny PCBs made for this purpose. If I did it correctly the schematic I went by should be attached to this post.. we shall see.

    This is very cool in a lot of ways. It sounds great, I have a cool blinking LED on the front of my amp that tells me how fast the trem is working in case I go deaf. In some ways, however, it isn't so cool. In order for this tremolo to be effective, I have to set the bias of my amp kind of cold. It is usable and it sounds good but it isn't where I would set it if it weren't for the trem. Me being me and never being satisfied I looked around a bit and lo and behold I find the Vibro Champ's trem circuit.
    [​IMG]
    Now, I understand circuits to some extent and I understand tremolos and phase shift oscillators (sort of), but there are a few things here I do not understand and this is a big part of why I am here today.

    1. Why isn't there a coupling cap between the cathode follower part of the trem and the cathode of the modulated gain stage? Why doesn't this, even without LF oscillation muck up the bias of that tube? If there is 175VDC at the cathode of that cathode follower then my rudimentary understanding tells me that it is putting 3.6 volts at the cathode of that 2nd half of the V1. I know I am wrong but I don't know why I am wrong and it is killing me.

    2. If one were to modulate the cathode of a gain stage for tremolo effect, as is done here in the VibroChamp, what output voltage swing would you be looking for? 4Vpp?

    3. Is there any good reason why my solid state trem circuit (attached below, God willing) cannot easily be utilized in a vibrochamp style trem in my twin?

    Thank you
     

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  2. elpico

    elpico Tele-Holic

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    1. the follower's cathode might have 175Vdc on it, but that cathode isn't the point that's connected to the other tube right? The other end of the cathode load resistor is, the one that's almost at ground. The only thing between that end and ground is a 25k pot in parallel with the 1547ohm tail of the other stage, which works out to about 1450 ohms. 1450 ohms is 2% of the total resistance of the cathode follower load resistance, so the voltage that can appear at that point is only 2% of the 175V you were imagining, or about 3.7V

    2. for a 200v plate voltage yeah 4V should be enough to cut it off

    3. at a glance, yeah I can't see any reason that wouldn't be useable with a few tweaks
     
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  3. separateness

    separateness TDPRI Member

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    Yes, however the schematic has the modulated tube's cathode at 1.5V. This is the discrepancy. Shouldn't it be the bias setting (~1.5) + 3.6V?

    What is the method used to determine what is enough voltage. Plot the load line and see how much bias change is necessary to reach saturation or cutoff?

    Good to know.

    Thank you very much for your response.

    I also would like to be able to keep the LED indicator. If you or any one has a good idea for this, that would be most helpful. I have seen schematics where LEDs were used to bias the cathodes of tubes. Is there any good reason I couldn't swap the 470Ω resistor for an LED?
     
    Last edited: Dec 12, 2019
  4. Ten Over

    Ten Over Tele-Meister

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    Yes. An LED will bias the LND150 very cold and it won't oscillate.
     
  5. elpico

    elpico Tele-Holic

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    1. The control settings at the moment the measurements were taken are not given, but from that 1.5V reading you'd have to assume the tremolo was turned to zero right? At zero the pot is eliminated from the cathode follower altogether so no voltage is coming from the follower to the other tube. At that setting the 1.5Vdc from the other tube is all you get and everything should function normally. I don't own one of these but the drawing suggests turning the intensity pot to max can raise the bias to 3.7V, which would put the tube in cutoff. From that I'd assume the tremolo on this amp is deep enough to fade completely out between pulses when you max the control.

    2. You have to check the data sheet for 12ax7 tubes . Page 3

    3. you could probably keep the LED in the tail of your mosfet follower, but if you're reworking that tail to the vibrochamp arrangement you need to put the LED above the intensity pot not between that and ground. This LED just changes brightness, it never turns off right?
     
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  6. separateness

    separateness TDPRI Member

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    Ahhhh! Thank you, this pieces this little puzzle together for me. I didn't take into account what setting the nobs would be at when the measurements were taken.

    Excellent, will do.

    The one I currently have goes dark or very close to it. I can see this working well. Would it not deform the sine wave, though?
     
  7. Ten Over

    Ten Over Tele-Meister

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    Once you factor in the current from the gain stage, the cathode should be at 4.0V. 4.0V doesn't put the gain tube in cut-off. It does make it into a cold-clipper, however. This may be one of the reasons you don't see this kind of tremolo in bigger, more serious amplifiers.
     
  8. Ten Over

    Ten Over Tele-Meister

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    If the bias is at -4V, the plate voltage will be over 300V. Even at their highest Eb line of 300V the tube isn't in cut-off.
     
  9. elpico

    elpico Tele-Holic

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    that line shows only 0.03mA left at -4v but okay, yes technically that's not 100% cutoff. I don't think the distinction makes any functional difference here. Are you suggesting I needed to tell him 4.2V would be better? Anything around there should be plenty.
     
  10. separateness

    separateness TDPRI Member

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    I should think that 4.0 cathode bias is close enough to have an audible and interesting effect. Why exactly isn't the tube at 1.5 + 3.7 =5.2V.
    I've attached a copy of the load line of the tube I wish to modulate. It appears to me that the ~3.6 V makes the gain stage something quite different, as mentioned above. Perhaps this could be cool, perhaps not at all. But, why not add a coupling cap between CF and tube cathode? All this would do is increase the bandwidth of the stage, no?
     

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  11. Ten Over

    Ten Over Tele-Meister

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    There are two voltage sources: 175V and 320V. You need to use the superposition theorem when there are two voltage sources. You also need to know the resistance of the tube at that operating point and the charts are very crude. Luckily I have some notes on a Vibro-Champ and that tube was idling at .07mA making the tube resistance 4.5M (or so).
     
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  12. elpico

    elpico Tele-Holic

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    Hmm, there's more moving parts here than you can work out with a line on a chart and I'm not sure the best way to explain it. When you turn the intensity pot up the bias of that stage isn't going to sit at 3.7Vdc. It's going to be constantly changing. 3.7 was just a ballpark number for it's center value based on the few voltages given on the drawing, but that bias voltage will really be swinging a few volts above and below that number as the trem oscillator does it's thing. When the bias voltage swings high the idle current through the tube is reduced (if not perfectly cutoff ;)), and when idle current goes down into the curved part of the line on that data sheet the gain goes down. Bias voltage swinging high = low gain, bias voltage swinging low = normal gain. That changing gain is what produces the volume changes of the tremolo effect.

    From the static loadline perspective you're still going to design the stage the normal way, assuming the intensity pot is at zero so the the bias voltage is 1.5 or 1.75 or whatever the particular tube you happen to plug in gives. The idea of the tremolo circuit is just to make the bias swing down to that bias value (max gain) then up to some much higher value (min gain) and back down etc.
     
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  13. elpico

    elpico Tele-Holic

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    Odd side effect I didn't think of before pushing submit: in this circuit if you were to turn off the tremolo via the footswitch instead of the panel control then the intensity knob acts as a variable bias knob for the second gain stage, so in theory you really can bias it at a steady 3.6v. If someone has one at home I'd be curious to hear whether there's a noticeable change in sound as you sweep the intensity control with footswitch off.
     
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  14. separateness

    separateness TDPRI Member

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    Oh dear. Mesh/Nodal/Superposition were not my favorite parts of my time as a student.

    This is something I anticipated and did not care for and why I think this circuit could do with a large coupling capacitor. Attached is a mockup of an implementation of this circuit in my amp which seems to function in a crude circuit simulator. I cannot find anything glaringly obviously wrong with it.

    It will be a while before I will get around to implementation as I'm already building a bass amp at the moment (and painting a room, and repairing plaster, and building a chicken coop...) but when I do eventually implement this circuit I will post the results here. I think this could be a relatively noninvasive and interesting tremolo for fellow amp tinkers to have up their sleeve, especially given that the PCB I had made for them is so small.
     

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  15. elpico

    elpico Tele-Holic

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    Leo didn't leave a journal explaining the reasoning behind all these little details (although his patent applications are sometimes full of explanations), but one thing we know is he didn't like distortion. He designed his amps to make clear, clean sounds. If you were setting out to design this particular bias wiggler trem and you didn't arrange things so the bias is colder than normal when the trem is enabled then the amp may go into distortion on the louder parts of the swing even though it was clean with intensity on zero. The way he's arranged it here the loud parts of the tremolo swing will be about the same volume, headroom, and tone as normal. Without the dc coupling that's not true, the loud parts of the swing will have less bias voltage than normal and clip easier.

    There are other types of trem besides this if the cold bias bothers you. Often with this stuff though, it's the many ways the circuit is not ideal that gives it the character it has. If you really want to get weirded out, I suspect this thing does some wobbly unintended stuff to the stage after it as well because the voltage across the output coupling cap is swinging by like a hundred volts.
     
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  16. Ten Over

    Ten Over Tele-Meister

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    The DC version (Vibro-Champ) only adds to the gain stage bias. With the tremolo on full, the bias will vary from around the unmolested voltage to over 5V. The AC version (same circuit with a large coupling capacitor) will add to and subtract from the gain stage bias because the coupling capacitor converts fluctuating DC into AC. It will not add anywhere near as much voltage to the bias as the DC version and I predict that the guitar signal won't have anywhere near the amplitude variation that the DC version has.
     
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  17. separateness

    separateness TDPRI Member

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    For some reason I imagined that the DC coupling would be more of a problem than it apparently is. The idea of having a 'Triode Bias/Intensity' knob is actually kind of neat in some ways. According to this simulator, I am able to get a fair amount of usable range on the pot without completely shutting off the tube if I go with DC coupling. I think I might go with this arrangement.
     

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  18. Ten Over

    Ten Over Tele-Meister

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    I think Leo or somebody at his company designed an amplifier that distorts. They may not have been overly concerned about this because it was just a cheap little practice amp and, besides, it will clean right up if you turn the Intensity control down.

    I've heard plenty of stories about big-name artists recording with a Champ, but my non-tremolo Champ sure didn't sound like that. You don't suppose they used a Vibro-Champ with the tremolo foot-switched off and the Intensity control up, do you?
     
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