PPIMV with fixed bias output and bias vary trem

[EETStudent]

I want to make sure I properly install a post phase inverter master volume on a new 5G9 build Im completing. The circuit features a fixed bias output, and a bias vary tremolo. What would be the recommend PPIMV setup to accomodate this circuit?

[hasserl]

I would use the Bruce Collins style dual pot isolated by 4 caps. http://www.blueguitar.org/new/schem/...d/pi-m-vol.pdf

[EETStudent]

I looked at schematic and considered a good option, thanks for that. Quick question as for reading those parts, what is the significance of those black/gray dashed lines between the pot wipers. What does that indicate?

[tboy]

The dashed lines indicate that the part is a dual (ganged) pot rather than two separate pots.

[Chuck H]

Here's a slight mod on the typical "Lar Mar" or "KF" PPIMV that works with bias vary trems. This is my modification of someone elses pic. But the difference in wiring should be obvious enough. In the original thread I posted this on it's reported to work just dandy.

[Wombaticus]

Here's a slight mod on the typical "Lar Mar" or "KF" PPIMV that works with bias vary trems. This is my modification of someone elses pic. But the difference in wiring should be obvious enough. In the original thread I posted this on it's reported to work just dandy.

The theoretical objection to this arrangement is that as the volume is reduced, the loading on the LTP varies, becoming quite extreme at low volumes. That's not to say that the result might not be sonically satisfactory...but if I had to guess, I'd predict slightly weird behaviour at low volumes...how does that jibe with your experience?

[Chuck H]

I've never done it. I just changed the typical PPIMV for a guy that wanted bias vary trem and also didn't want his bias voltage dependant on the pot wipers. What kind of notable affact would loading the PI outputs cause?

[Wombaticus]

I've never done it. I just changed the typical PPIMV for a guy that wanted bias vary trem and also didn't want his bias voltage dependant on the pot wipers. What kind of notable affact would loading the PI outputs cause?

Well, I'm not sure how it will sound, exactly, but one can guess. If you consider the load line for the LTP, it is possible, naturally, to draw the load line at DC, and another at AC that also reflects the post-coupling cap loading. So what will happen is that as the volume is decreased, the AC load line will start to rotate clockwise around the operating point of the tube. Theoretically as the volume is turned to zero, it will have infinite slope - run straight up and down. But what happens before that should be the same as what always happens when you rotate a loadline: decreased gain; increased harmonic distortion; decreased ouput signal swing; increased input sensitivity. These aren't (all) necessarily undesirable effects for a volume control, but mind that these are effects intrinsic to the PI, and changes attributable to the divider itself will be on top of these changes. So I'm not sure how it would all play out. It might be fine, but on the other hand, it could make the taper seem weird, or even result in oddities like increasing breakup as the volume is decreased.

[Chuck H]

Well, I must admit that I don't entirely get how to read the charted specs. But I thought that when the tube is setup to make primarily voltage gain that the loading was less critical to linear operation. With the tube operating mostly reletive to it's DC characteristics. Then the plate load becomes the load and the AC is more incidental. There are a lot of circuits that load the post coupling cap output of voltage gain stages without the consequences your predicting. Tone controls and such that simply load the signal at frequency or the volume controls in a 5E3 for example.

[Wombaticus]

Well, I must admit that I don't entirely get how to read the charted specs. But I thought that when the tube is setup to make primarily voltage gain that the loading was less critical to linear operation. With the tube operating mostly reletive to it's DC characteristics. Then the plate load becomes the load and the AC is more incidental.

Well, I'm reasonably certain that what I describe is what is actually happening with respect to the AC signal, although I'm willing to stand corrected if someone can propose a more nuanced theory. But from where my understanding sits now, any following load is parallel to the plate load, and must certainly influence the behaviour of the stage.

There are a lot of circuits that load the post coupling cap output of voltage gain stages without the consequences your predicting. Tone controls and such that simply load the signal at frequency or the volume controls in a 5E3 for example.

I had forgotten about this, but good example - and yes, well, the 5E3 volume controls are funky as all get-out, especially as regards their "all-or-nothing" behaviour, and the tonal variety available through their range of adjustment. Which can be either a good or bad thing, depending on your viewpoint.

[EETStudent]

In an attempt to steer the thread, does anyone have any experience installing a ppimv after a paraphase inverter?

[Chuck H]

In an attempt to steer the thread, does anyone have any experience installing a ppimv after a paraphase inverter?

I haven't. But it should be easy enough. Same as the LarMar (power tube grids on the wiper) but you would add the dropping resistor for the divider that feeds the inverting stage to the grounded end of BOTH pots.