Fixed Bias w/ PPIMV Bias Affect?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by deltaboy, Nov 3, 2017.


  1. deltaboy

    deltaboy Member

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    does the PPIMV affect bias voltage as you turn it up and down since it ties into the bias voltage input part of the circuit?

    I have a 250k+250k PEC and I’m trying to integrate it into a jtm45 style amp.

    I was thinking of making the grid to bias input resisters 1k and then tie that into the 250k PPIMV which would be similar to the static 220k resistors normally in that position.
     
  2. HotBluePlates

    HotBluePlates Member

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    Short answer: No, unless something malfunctions.

    Under normal conditions, there is no direct current flow from the output tube grid, so no bias voltage change happens at the output tube when the master volume is rotated.

    If the output tube has grid current (for whatever reason), then yes the master volume will alter the tube's bias. This is an abnormal condition.

    If the master volume pot loses contact between wiper & resistance track, the tube loses all bias and will redplate. A large-ish resistor (say 2.2MΩ) from raw bias to master volume wiper can help prevent this failure mode.

    Better PPIMV circuits have a coupling cap before & after the master volume so the master volume never impacts bias, even in the event of a master volume pot failure/open.
     
  3. deltaboy

    deltaboy Member

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    How does the variable resistance of the PPIVMV stacked pot that also takes the negative bias voltage bias input and brings it to the grid not effect voltage and thus bias?

    i.e. PPIMV all the way up you are in a stock configuration and all the bias voltage is making it to the power tube grids, turn the PPIMV way down lowering resistance, bleeding signal off to ground and it doesn’t take any of the bias voltage with it?
     
  4. Dan40

    Dan40 Supporting Member

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    with no current flow there is no voltage drop across the resistance.
     
  5. HotBluePlates

    HotBluePlates Member

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    What @Dan40 said (and what I said earlier).

    You're making a logical analogy between how a bias pot appears to change bias voltage, but did not give us a schematic to consider. @Dan40 & I have to assume you're adding a master volume correctly to a circuit, in which case it won't do what you fear. But let's look at the issue, and maybe consider what would happen if you do it the "wrong way".

    Ohm's Law
    Voltage = Current * Resistance

    For there to be a drop in the voltage, there must be a current through the resistance. Look at the schematic of a Deluxe Reverb. Down towards the bottom-right of the schematic is the bias supply. From a tap on the power transformer, there is a 470Ω resistor, a diode, a cap-to-ground. Those parts form the raw bias voltage, which will be somewhere around -50vdc.

    From the bias cap, there is a 10kΩ pot and a 10kΩ resistor to ground. The -50vdc is dropped across 20kΩ total resistance, through which there is a current of 50v/20kΩ = 2.5mA. If we set the bias pot halfway (5kΩ) there will be 15kΩ between the bias pot wiper & ground. 2.5mA * 15kΩ = 37.5v, so the output of the bias pot is -37.5v.

    But look at those 220kΩ resistors from the bias pot wiper to the tube grids. What will the voltage be at the tube grids? Still -37.5v.

    Why? The tube grid is just a bit of metal sitting in a vacuum, not connected to anything. Assuming the tube is functioning properly, when the tube has bias there is no current flowing in/out of the grid. Now use Ohm's Law: 0mA * 220kΩ = 0v, so no bias voltage is dropped across the 220kΩ resistors and all the -37.5v appears at the tube grids.
    _____________________________

    "Wait... What about guitar signal and those 220kΩ resistors?"

    There is an a.c. voltage output from the phase inverter. This a.c. voltage passes through the 0.1µF coupling cap to the top of the 220kΩ resistor & 6V6 tube grid. You could say a capacitor looks like a piece of wire to alternating current (as a first approximation). So the bias cap's junction with the bias diode & bias pot "looks like ground" to a.c., because the other end of that cap is connected to actual ground.

    If the a.c. voltage output of the phase inverter is 22v, then there is 0.1mA of current through 220kΩ resistors (slightly less because of the resistance of the bias pot, etc). When you see the Master Volume control vary voltage output to the tube grids, it's because of the a.c. voltage drop across the Master Volume pot's resistance due to alternating current resulting from the voltage output of the phase inverter.
    _____________________________

    You didn't provide us with a schematic to discuss. If we're talking a Lar-Mar Master Volume where the bias is applied at the non-tube end of the master volume pot, there's no path for direct current (bias) to get from the bias supply, through all the resistances, to the tube grid, and back to the bias supply. An electrical circuit is a circle (which is why the verbiage "circuit" is used). With no circular return path to the bias supply, there is no current and no voltage drop; all point have the same bias voltage present.

    But let's say you didn't mean the Lar-Mar PPIMV; let's look at a different PPIMV (below). If the dual-500kΩ pot was added to an existing amp without also adding the 1µF caps after, that would screw up the bias the way you're thinking. Why? Because now there would be a path for direct current to flow, from the bias supply, through the 220kΩ resistors, back through the master volume pot and to ground. In this case (with no 1µF caps) a lower master volume setting lessens the resistance to ground, allows more direct current and causes the bias voltage to fall somewhat.

    But that would only happen when a tech/builder/etc is adding a master volume and doesn't know much about electronics. When you asked your question, I gave you the benefit of the doubt cause I know you'll do it the right way (or at least "copy from the best"), so you'd have those 1µF caps.

    The "100n" caps are the same as the ".1" caps in the Deluxe Reverb schematic, and the 220kΩ resistors shown are the same as the 220kΩ resistor in the Deluxe Reverb schematic. The stuff inside the box below are all the parts added to add a master volume to that Deluxe Reverb circuit. So the master volume would screw up the bias voltage as you guessed, but only if you incorporated the master volume in the wrong way. The 1µF caps are open-circuit to d.c., and thereby prevent the bias from being altered when the master volume is manipulated.


    [​IMG]
     
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  6. deltaboy

    deltaboy Member

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    Fantastic response, I would be employing the rich-mod variety which is very simple and led me to some of my assumptions, but I assumed it was all good if so many people used it bias wise but just wanted the technical reasoning as to why.
     
  7. HotBluePlates

    HotBluePlates Member

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    I'll admit being curmudgeonly about "mods" and names for circuit changes like "the rich mod". That's because the name is commonly understood only for people that frequent whatever site has been discussing the mod, and doesn't tell anyone anything about what it does or how it does it.

    So I went looking...

    Rich Mod Instructions (See schematic fragment below)

    The 220kΩ resistors are connected to the bias circuit output, as is the "ground end" of the Master Volume ganged pot. So we're still left with the open-circuit situation as far as d.c. (bias voltage) is concerned, but we get voltage division of the phase inverter output (to fulfill the volume control function).

    [​IMG]

    Rob Robinette posted the Frondelli Master Volume on his site, which may be a better approach than the Rich Mod Master Volume (I have not listened to both, so I can't say for sure).

    The key difference in the Frondelli Master Volume is the input & output lugs of the volume pot are swapped. Bias doesn't require a safety resistor from raw bias to wiper to act when/if the wiper lifts off of the resistive track.
     
  8. Steppin' Wolfe

    Steppin' Wolfe Member

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    thanks for the analysis, Dan and HBP. And....I want to thank Kevin O'Connor for using 'Post-PI'....as I am sure he also would use 'Pre-PI' when talking about that type of MV. PPIMV is meaningless to me because I don't know what the first 'P' intends to mean. That is the first time I have seen anyone use what I consider to be a correct method of shortening those two phrases...pre-phase inverter MV and post-phase inverter MV.
     
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