Tube Depot shows the 50 C-5 to be $30 the each. There are a number of versions of the 12AX7 tubes prices ranging from $10 to $250 a piece, I presume the $10 one would be fine, but I really wouldn't know.
https://tubedepot.com/products/50c5A couple speculations I've made concerning these transmitters.. Based on the copyrights in the manuals, the Knight broadcaster came out in 1953 but did not show up in the Allied radio catalogs until 1954.. Then in 1956 the alternate provision now known as 15.219 was created, but not finalized until in 1957. The next Knight model which added amplifier circuitry came out in 1958 which included in the manual a FCC certification to attach to the bottom of the transmitter.
It stands to reason that the original model was designed to conform with 15.209, and the later model been designed to conform with 15.219 and it's 10-ft limit. Now, with that in mind let's consider the revisions to 15.219 in 1957, which at that time was known as 15.204....
[Docket No. 9288; FCC 57-790]
...The Commission proposed in its
November 1956 Notice that low power
communication devices be operated on
frequencies below 1600 kc. In the band
10-1600 kc, the proposal contained field
strength limitations and, alternatively,
power input and antenna length limits
for the bands 160-190 kc and 510-1600
kc. No comments directed to these par-
ticular limitations have been received.
Recently, however, additional radiation
data were obtained for low power com-
munication devices operating in the
band 510-1600 kc. The data indicate
the advisability of reducing the power in
order to further limit the area within
which interference may occur. There-
fore, the maximum allowable input
power in the band 510-1600 kc has been
reduced from 200 to 100 milliwatts as
set forth in Section 15.204 of the ap-
pended rules.
I couldn't help but to suspect that the Knight Broadcaster was somehow behind that "additional radiation data"" which the FCC had recently acquired, resulting in the decision to lower the power from 200 to 100 mw, it was after all the most iconic part 15 transmitter of the 1950s and 60s thus the most likely culprit! So I set out to confirm this theory.....
So much for gut feelings, only months prior to the FCC reducing the power limit, they did conduct feild strength measurements on a part 15 device in their lab, but it wasn't a Knight Broadcaster, it was a "Radio-Talkie Radio-Vox ". They have manufactured about 20,000 of these walkie talkies and now had the federal trade commission investigating them four false claims, which is what prompted these lab tests to occur. Although I don't really see how it influenced the reduction to 100 mw, it's just too much of a coincidence to have not been the culprit. Not to get too far off track, but here's a brief excerpt from the report...
...................
Office of Chief Engineer
Laboratory Division
Tests of Radio-Talkie Radio-Vox
Manufactured by Western Radio, Inc.
April 16, 1957
.......The instructions with the instrument suggest an additional antenna
of 50 to 100 feet be used as well as connections to various
other objects, such as lamps, telephones, bedsprings, large
metal windows, doors, screens, etc., as well as to “outside
aerial wires of any type.” Provision is also made for a
ground connection. The transmitter itself consists of a transistor supplied
by a 6-volt battery. The unit is designed to operate in the lower part of the standard broadcast band and is tunable.......
Conclusions:
At 820 kilocycles, the Part 15 limit is 15 microvolts per
meter at 190 feet. At 950 kilocycles, the 15 microvolt limit
applies at 165 feet. The values obtained in (c) and (d)
above are well outside those limits. The unit tested was
within the limits when used only with the whip antenna
and extension provided, or with short extension thereof.
................................
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