I think the 717A has the same drawback as the 6AC7 - it's not a variable mu tube. For certain conditions, it's fine, but twiddly. In the case of the BC-348, I think a useful outboard project would be a passive (or very low gain) preselector. The best homebrew treatment I've found is in the receiver chapter of the Fifth Edition of the Radio Communication Handbook, published by the RSGB (Radio Society of Great Britain). Those handbooks are head and shoulders above the ARRL handbooks for vintage work. In a search I noticed several offshoots of that data turned into home projects.

Or, if you want to get one the easy way, see if you can find one of a number of HF preselectors available online, like the MFJ-1045 or -1046 (either one would be a welcome addition to a BC-348). Ameco made some as well.
- Mike KC4TOS

On 6/20/2023 12:02 AM, Hubert Miller wrote:

One old kink was to plug in 717A tube in the RF. I don’t know if this tube will work in both RF slots

though. 717A are quite uncommon now but used to be all over the place. I found the 6AC7 to be

bad news; sharp cutoff, not so good for strong signals. My feeling now is if you want to modify the

active devices, go all out and make plug in ss devices. One thing that still is a BC-348 mystery to me

is how much the “shorted turn selectivity kink” actually improves selectivity. Also to change the last

IF winding to normal inductance and coupling and not overcoupled. I mean, quantify the result. It’s

on my project list but still way down there.

-Hue Miller