basic info provided by Bill Cohn
Year Model Chassis Tube Lineup Notes 1942 7G605 7B04 1LN5, 1LA6, 1LN5, 1LE3 Available with either 1LD5, 3Q5G, 117Z6G Sailboat or Bomber Grill Cloth 1946 8G005 8C40 1LN5, 1LC6, 1LN5, 1LE3 Has Push Pull Audio 1LD5, 1LB4, 1LB4, 117Z6G/GT Output. 1947 8G005TZ 8C40TZ 1LN5, 1LC6, 1LN5, 1LE3 Change in Bandswitch ILD5, ILB4, 1LB4, 117Z6GT and coils 1948- 8G005TZ1 8C40TZ1 1LN5, 1LA6, 1LN5, 1LE3 Change in bandswitch 1949 8G005TZ2 (?) 1LD5, 1LB4, 1LB4, 117Z3 and coils and tubes 1950- G500 5G40 1U4, 1L6, 1U4, 1S5, 3V4 First model with five Apr 1951 min tubes and selenium rectifier 1951- H500 5H40 1U4, 1L6, 1U4, 1S5, 3V4 Changed tuning bands 1953 1U5 replaced 1S5 1st revision to 7. Band 2 is 4-8 Mc 1954 L600 6L40 1U4, 1L6, 1U4, 1U5, 3V4, 50A1 First slide rule model L600L 6L41 with filament balast 1955 R600 6R40 Earphone jack on front R600L 6R41 1956 T600 6T40, 6T40R Radio/Phono Switch T600L 6T41, 6T41R 1957 Y600 6T40Z Y600L 6T41Z 1958 A600 6A40 2nd band 4-9Mc A600L 6A41 1959- B600 6A40 1962 B600L 6A41 Military R520 & R520A same as -600s
Note: 1LA6 and 1LC6 are listed as substitutes for each other. When possible always use 1LA6 since 1LC6 is lower rated tube. 1U6 is sometimes listed as a substitute for the 1L6. If this is done, additional parallel filament resistor on the order of 56 ohms is necessary to compensate for lower filament current.
A 1R5 may be directly substituted for 1L6 however performance above 9 MHz will be sharply degraded and realignment is necessary.
1S5 and 1U5 may be substituted if socket is rewired to match. 1T4 has said to have been used as substitute for 1U4.
Of all the tubes used by the TransOceanic line, only the 1L6 pentagrid converter, essential for all minature tube models, is both scarce and expensive (the 50A1 ballast tube is merely expensive) today. Being the most complex tube used in the TO, combining both oscillator and mixer functions, the 1L6 has always been expensive.
The use of the "early" circuit in a postwar tube has been questioned many times particularly since the "late" 1R5 was released in 1940. Most likely the 1L6 was chaosen since it is electrically identical to the 1LA6 used in the 7G and 8G series. If the 1R5 had been specified, it would have been necessary to redesign the complex coil tower and it was probably simple less expensive at the time for Zenith to have Sylvania produce an "obsolete" tube.
Surprisingly, the available and inexpensive 1R5 can be plugged into a 1L6 socket and the TransOcenic will play - on Broadcast band and up to about 10 Mc just down about 20 db in sensitivity since internal capacitance is considerably different than the 1L6. A good alignment can recover much of this loss however.
Inducing the 1R5 to oscillate above 10 Mc is another matter though it has been seen in cases where the tube is overdriven with increased filament voltage - over 1.8v is necessary and the life of the tube will probably be drastically shortened.
When the tubed TransOceanics were new and battery life was a big selling point, Zenith used to pitch 200 hours of use from a battery. To achieve good operation from a low battery, Zenith used a 9v A+ battery. According to the TM 11-877 for the R-520/URR, 8.4v is expected with AC operation.
I have found that a minature tube TransOceanic will play well with 7.5v A+. With the six filaments (the 3V4 counts as two), this equates to 1.25vdc across the filaments as compared to a 1.2v minimum specified by RCA for this series of tubes and can extend tube life considerably at the expense of lowering the sensitivity by a few db - I have not noticed any audible loss.
Consequently, when a chassis is removed, I make a habit of adding a bypass 7.5v 1w Zener diode from the switched A+ to A- (the switched side is used to avoid battery loss if so connected).
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