Hmmm, I never figured that I would see the words "
Watkins-Johnson" on this site... I use some of their vintage gear in my line of work (RS-125, 8617B, 340-A7, and lots more. Even have some WJ goodies at home, and I have used an RS-111 as an FM mono tuner, talk about overkill Yellow_Light_Colorz_PDT_02
Now for sonething completely different, and way OT:
Speaking of tune substitutions, years and decades ago, I had a military surplus Collins R-392 HF receiver which I snagged at a now long-gone purveyor of such goods (John J Meshna) for chump change. Actually I bought two at the time, and re-sold the second one, recouping mu expenses...
This receiver ran off 24-28VDC, and the B+ voltage for all tubes inside was the same low DC voltatge. The audio output tube was a weird dual pentode in an octal bottle, a 26A7. Back then, these were hard to find, so I designed a simple plug-in transistor amp replacement, which actually worked and sounded quite good. Years later, I sold the set to a fellow ham, who still has it, along with a T-195 transmitter of the same vintage. Nowadays, I wish I'd kept that radio, the going price is over %650 -750 for one in reasonably good shape. 26A7s are sometimes available through AES and others.
http://www.mif.pg.gda.pl/homepages/fran ... 2/26A7.gif
http://www.mif.pg.gda.pl/homepages/fran ... 26A7GT.pdf
(edit: a better, more informative data sheet for these little cuties) Yellow_Light_Colorz_PDT_02 Check out the minimalist circuit on page 6 :o
Another receiver which used this tube, was the ARN-6 radio direction finder, which made an extremely sensitive and selective AM broadcast band receiver, with a few simple mods. This used two of the 26A7s in a PPP output stage, and had amazingly good, low-powered (1W), audio (and fairly deep bass) with a simple OPT change, to one which had a 4 or 8 ohm secondary. This RX was another all 24-28VDC design.
Perhaps a new design, low-powered, low B+ voltage audio amp could be created using these and other LV tubes. They can run on 48VDC B+ just fine, get a whopping 500 mW out of a PP pair :o
/ed B in NH