Paul raises a very interesting question, What prompted the evolution of device selection for RF amp and 1st mixer in Drake's receivers?
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12BZ6/6AU6 -> 12BZ6/6HS6 -> 6HS6/6HS6 -> 6BA6/6EJ7? ? if I have the sequence correct.
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ChatGPT tells me they went with 40673 MOSFETs at some point later on, but I doubt any of those ended up in the R-4x line. I'll be corrected if so.
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I'll throw out a hypothesis:
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The initial R-4 design was likely based on what they were used to using in earlier product lines. Getting the new receiver out the door quickly and into the market to support the company was probably very important.
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I remember in the 70s hams experimenting with various tubes to "soup up" the front ends of their radios. These new types had higher transconductance and lower noise than what the factory supplied -- 6BZ6/6CB6/6GM6/6HS6, etc., and were all sharp-cutoff pentodes. No doubt the engineers at Drake took notice of this and did their own research and experimentation, developing lower-noise, more sensitive receiver front-ends for weak signals. All well and good, until...
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Customers started complaining about IMD and front-end overload in crowded contest conditions. Probably around this time test equipment of high enough quality to measure these effects also became available. Engineers were sent back to the drawing board to see what could be done to fix that issue. Reaching back into the dusty pages of some old data books, someone remembered the old, reliable 6BA6, which had been used in TV receivers for years. It's a remote-cutoff pentode with lower transconductance, but better dynamic range capability than the high sensitivity tubes. The resulting design was a good compromise between sensitivity and IMD. I haven't heard of Drake front-ends becoming unstable, but almost certainly the 6BA6 was a more stable tube than the ones with higher transconductance.
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I wasn't present at Drake, I have no first-hand or historical knowledge, nor a deep technical understanding of receiver design, and all of this could be complete rubbish. But I toss it out to generate a discussion.?
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Dave
