LarMar vs "Rich mod" PPIMV

[Ondrej]

I've been recently doing some research on these two master-volume mods, and apparently, they seem to be the exact same schematics, differing just in the pot/parallel resistor values, which results in different tapper and behavior of the input resistance.

I've been using LarMar in my AC30 build, implemented as a 12-way rotary switch+resistor ladder, but I was thinking of finally getting a good dual pot and having a nice continuous master volume.

George Metropoulos has been advertising Rich mod in his Plexi build and I am sure there were some discussions on this topic at his forum, but unfortunately, the forum is (temporarily) down now, and so is his wiki.

The question is, do I want to go LarMar or "Rich mod" way?

Here's my observation:
* Lar-Mar would use lower pot value (slightly higher than the power tube bias value) + high parallel tapper resistor to protect the circuit in case the tapper is in the air. E.g. instead of using 220k fixed bias resistor, you'd use 250k pot + 2.2meg resistor.

* Rich mod would use higher pot value + close-to-bias parallel resistor. This, however, changes the law of the pot to being much closer to linear AND the resulting input resistance of the pot increases with turning the pot left (lower the volume) = changing the RC time constant = allowing more bass with lower volumes.


Does anyone have any experience with AB-ing these two, esp in cathode-biased amps?

Thanks

[rooster]

Where are you seeing the Rich schematic? I thought George was shut down?

[pdf64]

Rich mod would use higher pot value + close-to-bias parallel resistor. This, however, changes the law of the pot to being much closer to linear

I can't see that it would work like that; as I understand it, it may result in a linear taper pot giving more of an audio taper response, and an audio taper being exaggerated further.
See http://www.geofex.com/Article_Folders/p ... tscret.htm

Rich mod would use higher pot value + close-to-bias parallel resistor... the resulting input resistance of the pot increases with turning the pot left (lower the volume) = changing the RC time constant = allowing more bass with lower volumes

With appropriate sized coupling cap values, I think this may not give an audibly noticeable effect. It would also have a benefit of reduced bias excursion at the power tube control grids when overdriven, so mitigate for blocking distortion, which would be more likely at higher settings.
See http://www.ampbooks.com/home/amplifier- ... excursion/
Pete