I suspect that I can us a 201A tube in place of a 200A in the detector circuit with no real problems?
Confirmation appreciated.
Thank you,
Mitch
It shouldn't damage the set, but it may not perform exactly the same way. The 200A is a cesium-gas-filled triode while the 201A is a hard-vacuum triode. So, their transfer functions could be different. I would expect the 200A's transfer function to be much less linear than that of the 201A due to its ionizing gas filling.
The A-K Model 20 service info on this set here at this site calls out three "choices" for the Detector tube: 301A, 300A, and 112A. While 301A is known to be electrically identical to the 201A, I can't see any evidence that the 300A is identical to the 200A. The 300A does not appear to be gas-filled. The final evaluation should rest on a detailed comparison of the two tube types' transfer functions, which I have not taken the time to do.
However, based on the apparent acceptability of the 301A in the Detector socket, it follows that a 201A would work there, as well, even though it was already considered an obsolete tube when the set was new. If anything, it looks like the 200A tube is out of place in this set.