This project is by no means over. Coming up, I hope:
Splitting the project in two.
(1) I will soon have enough components to build a "museum replica" of the original ARRL project, with no modifications whatsoever. This will use an original Bud chassis (not the LMC/Heeger modular chassis used in my first build), the original Millen dials (re-built to the correct specification by the actual Millen company, which is still in business) and, as we have discovered, the receiver won't work well on any band above 40 meters. It won't have a cabinet, because the original design didn't have one, either. I will label the front panel so that it is indistinguishable from the labeled receiver in the ARRL book, "How to Become a Radio Amateur," 1968 edition. This receiver will be "hot" on 40 meters and fun to use in "nostalgic" QSOs.
(2) There will be a duplicate receiver, using the existing LMB chassis, which I will now feel free to alter/improve to my heart's content, replacing and improving the temporary "back panel" experiment. I am very keen to see how far I can push this simple design, once I am freed from the (admittedly absurd!) constraints of staying "loyal" to the exact original ARRL design. This receiver will have the following additional features, at the very least:
(a) Six-pin Amphenol coil forms instead of the original smaller Millen five-pin forms. The extra pin will be used to allow for a different value of C9 on each band. Experimentation has determined that this is the only way to obtain decent multiband performance, beyond just "monoband" 40 meters. C9 is is connected between the source of Q1 (the FET regenerative detector) and ground.
(b) To reduce AC hum, the set will use the existing cabinet that I built, with the addition of a hinged door in the top left-hand end of the cabinet, to facilitate coil-changing.
(c) The set will have a preliminary RF stage in front of the regenerative detector, mainly to add isolation that will eliminate parasitic emissions from the antenna and frequency variations caused by the antenna being swung in the wind. An antenna trimmer and an RF gain control will be added to enable control of overload (regenerative sets are very easily overloaded; experimentation has show that the addition of the aforesaid controls is effective in dealing with this problem).
(d) There will be an extra audio stage to drive the speaker that's mounted in the top of the cabinet. Interconnections will be improved so that the transmitter's sidetone can be fed to the receiver and be audible in the headphones and/or the speaker. Transmit/receive switching will be added permitting easy muting of the receiver during transmit.
(e) In order to add this new circuitry, to make extra room on the chassis I am thinking of (i) eliminating the broadcast-band filter that is in the original design; in my experience so far, BC breakthrough at my location is only an issue on 160m, a band I have no intention of using, for the time being, and using the freed space for the new, one-transistor RF stage; (ii) removing the huge D-size battery holders underneath the chassis, which provide several years of power (I have never changed the batteries since last summer when the set was built!) and replacing them with C-size holders, which should still provide lots of battery life; this should free up enough extra room to build the audio amplifier, even with its big, 1950s-era transformers.
(f) I will eventually be building an audio filter for CW, but that will almost certainly be an external affair, so that it can be used later with other receivers.
I will also be building an entirely new, different receiver (my first superhet) at some undefined future time, and will start a new thread when that gets under way. This will be a tube receiver. It may be a variation of the "Novice Q5er" published in CQ magazine and authored by W6TNS, using a homebrewed crystal-controlled converter in front of a restored WWII-era military BC-453 receiver; or it may be something entirely homebrewed. Haven't decided yet.
Martin,
you might find the 2 transistor regen in RCA Ham Tips, Vol 20, number 1, Jan-Feb 1960 of interest.
http://n4trb.com/AmateurRadio/RCA_Ham_Tips/issues/rcahamtips2001.pdf
73
Peter G3RZP
Peter: belatedly reading the article that you linked to, that does indeed look interesting, especially for portable operation.
73 de Martin, KB1WSY