Good info, thanks!pdf64 wrote: ↑Mon Jul 10, 2017 1:09 pmAs your 2nd stage plate only seems to have the gain of the power amp after it, I think that the ratio of likely signal level to potential CC noise is high, and so a CC would be fine there, as per RGs guidance.FourT6and2 wrote: ↑Sat Jul 08, 2017 10:25 pm...I wouldn't use CCs on the input plate stage (V1a) or the second stage in a high-gain amp (V2a) for noise reasons. The stage just before the PI in the amp I'm building is a cathode follower and just has the single 100K resistor on the tube socket as seen in typical Marshalls. The article says the PI Plates would be a fine place to use CCs. While a global NFB affects how much a CC will distort there, it still does distort a little, no?..
As the gain of a CF stage is close on unity and pretty much independent of the cathode resistor value, the voltage coefficient of characteristic of a CC cathode resistor won't be able to have any effect on the stage output, ie there's no potential a CC cathode resistor on a CF stage to add harmonics.
The degree to which a global NFB loop is able to reduce distortion is dependant on the open loop gain and the negative feedback ratio.
Generally, the loop reduces gain by about 6dB, so (within the linear range) I think that the distortion generated within the loop will be reduced by a similar amount.
Be aware that although NFB reduces distortion overall, whilst reducing low order harmonics, new higher order harmonics are introduced, so the tonality may be less pleasant.
Also, with NFB, because the large signal performance within the linear range is more accurate, with slighter higher signal levels the onset of clipping will be more pronounced / extreme, with a potentially harsher tonality.
With amps that have a presence control in the NFB loop, at higher frequencies the degree of NFB will be dependant on the presence control setting.
I think that even order harmonics that are introduced equally in both sides of a balanced system (eg CC LTP plate resistors) will result in odd order harmonics when the balanced signals are combined back to single ended
I'll probably just stick with MF everywhere, then. Seems that will be more beneficial in the grand scheme of things than trying to futz about with CC mojo