Vaughns Magnetrons
TYPE 2J21 & 725A
After completing the
2J55 project a more versatile experimental set up was built several months later in order to get a better understanding
of the magnetron performance curves and to get a better feel on how it operates. An electromagnet was constructed in order to vary
the B field and a variable high voltage power supply was used for varying the pulsed cathode voltage. I decided to stay with the X
Band type magnetrons without the permanent magnets. After checking the Web out I decided first to get the 2J21 from
Surplus Sales
of Nebraska to get started then later after replenishing my beer budget I got the more expensive 725 about 6 months later. I am happy
to say that both seem to work considering the fact that even if unused in their orginal package they can "outgas" over time making
them useless.
2J21
"The operating specifications has been somewhat difficuit to find on the Web and took some digging. I checked out http://www.tubecollector.org/list.php
and discovered that the various views of the 2J36 appeared identical to the 2J21 so I assumed its operational parameters given, "9085.5MHz
+/- 82.5MHz. Heater 6.3V/1.5A, anode 14kV at 12A, 14kW pulse output", might be close to the 2J21. The 1951 Radio Amateur Hand Book
also provides 2J36 data including the operating magnetic flux (2500 gauss) Then I discovered the Bell Journel (Full text of The Bell
System technical journal.htm bellsystemtechni25amerrich.djvu) available at the Archives Web does provide most of the 2J21 specifications
states a peak output power of 15 kw "at an efficiency of from 12 to 15 per cent".Regarding the 725 "..the requirement of interchangeability
with the 2J21. Thus good operation at 10 kv. and 10 amps, input was necessary.....the magnetomotive force required to supply the field
for the 725A was almost identical to that required by the 2J21 and no extensive magnet redesign was necessary" When I was designing
an electromagnet to operate both , Electromagnet Part 3, also discovered the magnetic pole separation for the 2J21 is about twice
the 725. I realized that for a MMF of 8 KA turns that for pole diameter of 1" dia. gave approximately the operating flux for the required
pole separation ; 2.5KG for the 2J21 and 5.5KG for the 725. The photo at the right shows the 2J21that I purchased and associated magnetic
pole pieces. My guess is that its operating impedance is somewhere between 1000 to 1200 ohms. See sketched performance chart below.
725A
The photo at the right is one that I purchased from Surplus Sales of Nebraska also shown is the magnetic pole adaptors for the above
mentioned electromagnet. As far as available operating characteristics there is quite an abundance.
Based on the above and below references I decided to operate the magnetron at its nominal 12KV at 12 amps peak; pulse duration of about 500nsec (@ ~ 5500 gauss; see above Part 3). It appears that for the electrica pulse parameters the operating impedance is about 1 K ohms.
Some References:
> Philips 725A data sheet that I got on the Web 725a.pdf
The next several photos (below ) shows the orgional packaging that the 725A came in:
The sketch of the 725A magnetron performance chart was traced from "Microwave Theory and Techniques", R. Krauss & O. Skalnik, Van Nos. , 1953 pg. 722 Fig. 14-28. As can be seen when operated a 12KV at 12 amps (1K ohm operating Z) the magnetic flux is close to 5500 gauss.