Hi SY,
I actually wasn't thinking of the pimm ccs when I wrote that, but realized in any case that all the cathode resistor does with the ccs is raise the plate resistance - not particularly useful, and actually worsens the effect of Cmiller on the frequency response. I'm not sure with a ccs load whether or not it would worsen distortion or not, but it might so why risk it..
I hardly ever use cathode feedback any longer, it seemed useful back when I was still wed to more commonly available higher mu types, and I used fairly large plate resistors and cathode followers. Linearity seemed quite decent, but I'll admit that the instrumentation at my disposal in those days was pretty ancient with little capability to measure anything under 0.05% - 0.1% and all of my circuits were either in that vicinity or below it. Unfortunately I also did not have ready means to look at the spectrum of the distortion. Happily of course all of this has changed. I have 2496 FFT based analysis available on my pc, and an analyzer with a residual of around 0.0008% when it's really working properly.
I actually wasn't thinking of the pimm ccs when I wrote that, but realized in any case that all the cathode resistor does with the ccs is raise the plate resistance - not particularly useful, and actually worsens the effect of Cmiller on the frequency response. I'm not sure with a ccs load whether or not it would worsen distortion or not, but it might so why risk it..
I hardly ever use cathode feedback any longer, it seemed useful back when I was still wed to more commonly available higher mu types, and I used fairly large plate resistors and cathode followers. Linearity seemed quite decent, but I'll admit that the instrumentation at my disposal in those days was pretty ancient with little capability to measure anything under 0.05% - 0.1% and all of my circuits were either in that vicinity or below it. Unfortunately I also did not have ready means to look at the spectrum of the distortion. Happily of course all of this has changed. I have 2496 FFT based analysis available on my pc, and an analyzer with a residual of around 0.0008% when it's really working properly.
Also it seemed to pick up some weird artefacts from the portable CD player I was using as a source on quiet bits of tracks...... 
Regarding CCS and cathode degeneration, Pimm says somewhere on his website a cathode bypass cap isn't required because because the operating point will always stay the same with a CCS, I'm not sure about the ramifications of that but I don't have a bypass cap.
Regarding CCS and cathode degeneration, Pimm says somewhere on his website a cathode bypass cap isn't required because because the operating point will always stay the same with a CCS, I'm not sure about the ramifications of that but I don't have a bypass cap.
bigwill said:Also it seemed to pick up some weird artefacts from the portable CD player I was using as a source on quiet bits of tracks......
Regarding CCS and cathode degeneration, Pimm says somewhere on his website a cathode bypass cap isn't required because because the operating point will always stay the same with a CCS, I'm not sure about the ramifications of that but I don't have a bypass cap.
It's probably a good idea to keep the bypass cap or use about 3 red leds in series for bias as this has the effect of keeping rp down around 8K and is good for HF extension and noise performance. The unbypassed resistor is also a noise generator which gets gained up by about 27dB (21 X).
You need to build this thing into a metal box or a partially shielded wooden baseplate/metal top plate - sitting out in the open it will pick up everything and I mean everything. Not to belabor the point but the higher rp caused by the unbypassed cathode resistor does increase the susceptibility to external noise sources by virtue of the higher rp - other mechanisms of course include the presumably high impedance grid circuit - this though is unavoidable if you want to present a reasonable load to your source components.
kevinkr said:
I will try this
I can understand how noise from the resistor would be amplified - the grid to cathode voltage difference changing according to the noise signal. However, I don't quite understand how a parallel cap or diode string would effect the signal performance - doesn't the same current pass through the tube constantly - regardless of signal, so there's nothing to bypass?
I don't fully understand why it does, but it does. And dramatically. When I tried this with a bipolar cascode CCS load on an ECC81, the distortion was so gross that I didn't take a spectrum, it was easily visible on a scope. The drop across the cathode resistor was 1.7V, and subbing in a red LED to give the same bias dropped the distortion to nearly zero.
Yes, I was surprised. Repeated it several times, same result.
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