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Originally Posted by Neil Purling
Am I correct in believing that for a regenerative detector one should have a varaiable mu RF pentode?
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No on both accounts.
Every valve you will encounter is variable mu to some degree or other. The changes you get from varying the anode or screen grid voltage on a non vari-mu valve is quite sufficient to control regeneration.
Nor does the valve have to be a type intended for RF operation. Audio valves will do quite nicely.
As I mentioned previously, then with regenerative receivers less is more. It is an all too common mistake to use too 'hot' valves for the detector, and doing so is a major contributor to many of the ailments, which can plague the species. You will in particular
not get better sensitivity, smoother regen control nor less hum from using a 'strong' RF valve, quite the contrary.
Frequency pulling, another common problem associated with regens in the presence of strong signals, is at least in part directly associated with the vari-mu characteristics of a valve.
My preferred choice for a detector below 10MHz (Ie. for most any sane design
) is the EF86 audio valve. I am presently putting the final touches on my little 'reference' design for an experimental testbed, even though I have been forced by
external events to postpone the work for now.
Frank N.
Edit: The pentode in the ECL80 requires around 5Vrms for full power output, somewhat dependent on screen grid voltage and anode load impedance. But 5V or perhaps a bit less should be a reasonable target to aim for.