Mark, my thinking about VTF is somewhat similar to my experiences with SRA: that it's difficult to find any two of the same model cartridge, that are identical in either of those characteristics.
You can physically see zero SRA, if you have the patience and tools, so no problem there. But with VTF, there is no practical way to "see" the coil's position with respect to the magnetic flux field. However, its (the coil's) optimum position can be inferred by measuring the cartridge's output for a given test tone at slightly different VTF settings.
I do it using the VU meter on my tape deck, which is not refined enough a tool to find the actual highest output point. So what I do is look for the VTF settings just above and just below the (assumed) optimum that yields the same meter setting.
So for the Tranny for instance, we assume the optimum VTF would be (theoretically) 1.9 gm, halfway between 1.8 and 2.0 gm, according to the manufacturer. So let's say you take a meter reading of a 1 kHz tone (Cardas record) at VTF 1.8 gm, and you adjust the meter to give a readable number like 0 VU. Then you increase the VTF ABOVE the optimum to where you again get 0 VU. Let's say that turns out to be 2.1 gm. The average (between 1.8 and 2.1) would be 1.95 gm. And at 1.95 gm, the meter should read slightly higher than 0 VU.
Even using this crude method, I determined my Tranny W had the most output at around 2.0 gm (with slightly less output just above or below) That was just after break-in, and I'm going to do it again just to see if anything's changed -- it certainly wouldn't surprise me to find it now requires a little less VTF.
Of course VTF also affects tracking, but I've always held that if a cartridge isn't tracking properly anywhere within its specified VTF range, something's not right with the setup or with the cartridge itself.
.
You can physically see zero SRA, if you have the patience and tools, so no problem there. But with VTF, there is no practical way to "see" the coil's position with respect to the magnetic flux field. However, its (the coil's) optimum position can be inferred by measuring the cartridge's output for a given test tone at slightly different VTF settings.
I do it using the VU meter on my tape deck, which is not refined enough a tool to find the actual highest output point. So what I do is look for the VTF settings just above and just below the (assumed) optimum that yields the same meter setting.
So for the Tranny for instance, we assume the optimum VTF would be (theoretically) 1.9 gm, halfway between 1.8 and 2.0 gm, according to the manufacturer. So let's say you take a meter reading of a 1 kHz tone (Cardas record) at VTF 1.8 gm, and you adjust the meter to give a readable number like 0 VU. Then you increase the VTF ABOVE the optimum to where you again get 0 VU. Let's say that turns out to be 2.1 gm. The average (between 1.8 and 2.1) would be 1.95 gm. And at 1.95 gm, the meter should read slightly higher than 0 VU.
Even using this crude method, I determined my Tranny W had the most output at around 2.0 gm (with slightly less output just above or below) That was just after break-in, and I'm going to do it again just to see if anything's changed -- it certainly wouldn't surprise me to find it now requires a little less VTF.
Of course VTF also affects tracking, but I've always held that if a cartridge isn't tracking properly anywhere within its specified VTF range, something's not right with the setup or with the cartridge itself.
.