Whoever it was it was not me.Gaz wrote:I remember someone swearing somewhere that this MV only sounds good with cathode bias, and NFB. Don't remember if there was even an explanation, but I liked hearing that cause it sure sounded like crap in my fixed bias NFB amplifier!
The bias method could have a minor influence because in the fixed bias amp, the series resistance to the power tube grids is a bit lower with the PPIMV dialled down which means that there may be a bit more bias voltage to the tubes making them run a bit colder than at the full up setting. This will not happen in the cathode biased amp where the voltage to the power tube grids is always zero. It works on amps with both bias methods but probably more transparent in the cathode biased amp.
As regards the NFB the PPIMV clearly works better (i.e. more transparent) in amps without global NFB and this is why: in amps with global NFB some portion of the signal at the loudspeaker output is fed back to the PI. If you turn down the PPIMV you will get less signal at the speaker output and hence less signal in the NFB loop. At a certain dialled down setting the NFB loop will collapse because there is no more feedback signal available. Whether you like the resulting tone is purely a personal matter.
The Spitfire is cathode biased and does not have global NFB so these two prerequisites are fulfilled. The reason that the PPIMV still will not be transparent in this amp is because it takes away the signal strength needed to overdrive the power tubes - the primary source of overdrive in this amp. The same PPIMV in a DC-30, Clubman, Chieftain or Lightning will work much more transparent because these designs do not rely on power tube overdrive as much as the Spitfire because they have some preamp and PI overdrive available.
Cheers Stephan