Tubes Asylum

RE: Yup, but the 5751 can be used in place of a 12AX7 so long as

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Jim McShane

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In Reply to: RE: Yup, but the 5751 can be used in place of a 12AX7 so long as posted by Tre' on December 20, 2015 at 20:20:40

My point was that the mu does not equal the stage gain. Your examples clearly show that, thanks for the homework. Also, based on the relatively high circuit gain I'm assuming the circuit model had the cathode cap fully bypassed. Without the bypass the gain would be significantly lower.

And of course the grid resistor of the next stage is in parallel with the plate load of this stage so the effective load would be lower too, which impacts gain as well.

Follow Ups:

RE: Yup, but the 5751 can be used in place of a 12AX7 so long as - Tre' 09:22:46 12/21/15 (1)
"Without the bypass the gain would be significantly lower."

Published plate curves always assume there is no feedback applied.

"And of course the grid resistor of the next stage is in parallel with the plate load..."

The grid resistor of the next stage, in parallel with the plate resistor, IS the plate load. That is to say, the plate resistor alone is never the total plate load.

"My point was that the mu does not equal the stage gain."

True but consider a 6sn7 with one of Gary Pimm's CCS in the plate circuit and the grid resistor of the following stage at 1 meg ohm.

At 1kHz Pimm's Self Biased CCS has an impedance of about 3 gig ohms, that in parallel with a 1 meg ohm grid resistor of the following stage gives a total plate load of about 999667 ohms.

The published plate curves of the 6sn7 in TubeCad shows a stage gain of 21.

Now that's impossible but the point is with a horizontal, or nearly horizontal load line, one can get very close to a stage gain equal to the mu of the tube.

Just a note, for triodes a horizontal load line produces the most stage gain with the least amount harmonic distortion.

Tre'

Have Fun and Enjoy the Music
"Still Working the Problem"
- http://www.pimmlabs.com/web/ccs_performance.htm (Open in New Window)
- RE: Yup, but the 5751 can be used in place of a 12AX7 so long as - Jim McShane 09:34:41 12/21/15 (0)
  Of course using a CCS will "horizontalize" the load line with the tremendously high impedance. And while plate curves may indeed not reflect presence of a resistance in the cathode that is because the plate curves are not the same thing as tube performance in the circuit with the cathode resistance and next stage influencing it.
  
  Since the original post was regarding a phono stage I'm going on the assumption that it did not use Gary Pimm's circuit, that it did have some cathode resistance, and that there was a stage driven by the output of the 12AX7 based stage. While the other stuff is interesting it is likely not applicable to the specific situation the original poster was asking about.
  
  But thanks again for sharing your knowledge!

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