TB tonestack myths?

[lion]

There was a myth many moons ago that the Vox TB tonestack was copied from an old Gibson design (GA-70 IIRC). The evidence for the theory was that a supposed error in the Gibson diagram (actual amp wiring differing from factory diagram) was carried over to the TB tonestack. The claimed "error" was the grounding of the bottom leg of the 1M bass control pot. It was also said that the "error" was the main reason for the interaction between the bass/treble controls of that AC30 TB tonestack - ie the last 10% rotation of the bass control causing a heavy dip in the mid range etc.

I think the error myth has since been buried. The grounding of the bass pot was indeed part of the intended design. However, for a couple of reason, I've been led back on the subject and I wonder about a couple of things:
1) would the tonestack/bass control work at all (to any useability) if the bottom leg of the bass 1M was not held to ground (there's still the 10k resistor)?
2) is the well known treble/bass interaction due to the bass pot grounding at all or is that just another myth?

Unfortunately Duncan's TSC doesn't have the ability to simulate the 1M bass pot disconnected from GND.



Any thoughts from people more knowledgable than me would be appreciated. Thanks.

[Bill Mountain]

You could raise the bass pot to something like 10M and only use the first 10% of the rotation.

Or go here to create simple sims: falstad.com/afilter

[lion]

Thanks Bill
I guess raising the bass pot to 10M would simulate close to a ground disconnect 

I'm trying to understand the theory of how the complete TB tonestacks works, but I have a hard time wrapping my head around exactly what does what in the circuit and what causes the t-b interaction. Time for more study and read up.

Erik

[lion]

You could raise the bass pot to something like 10M and only use the first 10% of the rotation.

Spent a bit of time Duncan's Tonestack Calculator setting the bass pot value as 10 megaohms, and only using the first 10% of the available pot rotation.

Below are the resultant plots from Duncan's TSC for comparison. I've chosen 5 settings of the treble/bass pot - identical in both plots - giving 5 response curves in each plot. To the left for the standard/correct tonestack with the lower leg of the bass pot at GND, and to the right for the "disconnected ground" circuit version.

The 5 combinations of bass/treble settings are:
A: bass MAX - treble MAX
B: bass MIN - treble MIN
C: bass MAX - treble MIN
D: bass MIN - treble MAX
E: bass 50% - treble 50%


It's clear that the plots for the B+D+E settings seems pretty close in both circuit versions (bass pot grounded or not), but A+C shows substantial differences.
To answer my own questions: The stack will work with the bass pot not grounded, but the "mid sucking quirk" only happens to the full degree with the bass pot grounded.

EDIT:
I also notice that with the bass pot disconnected from GND the HF from about 5kHz keeps flat even with the treble at MIN.

Erik