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In Reply to: RE: Explain yourself... posted by Valva Actual on March 27, 2021 at 20:06:51
The 6cg7 can be used in place of a 12au7. The MU is the same. The trasnconductance is the same. The plate resistance is the same.
The pinout is different and the 6cg7 can only be run on 6.3 volts for the heater but both tubes have operating points that are very similar except the 6cg7 is a lot more linear. In a lot of circuits you could just drop the 6cg7 in as long as the heater supply can handle it.
Look at the 6cg7 running at 200vdc plate at 14ma. Now imagine a horizontal load line like you would get with a CCS plate load. That's about as linear as a tube gets. You could use a wide range of load line angles and this would stay linear as there is a large area where the grid line spacing stays symmetrical.
Now look at the 12au7. There isn't anywhere that you can run it and have it be even close to that linear. If the plate voltage doesn't move the same amount (following the load line) for a given increase in grid voltage vs. the same amount of decrease in grid voltage, that will cause an asymmetrical output wave form and that is the definition of harmonic distortion.
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Edits: 03/28/21Follow Ups:
" ...The pinout is different and the 6cg7 can only be run on 6.3 volts for the heater... "
that does not sound like a simple drop in.
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