Tubes of the Trans-Oceanics 1941-1962


Tube Chart


Comments on the 1L6

Pampering Filaments

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Tube Chart - chassis 7B04 - 6A40

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

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

Substituting Tubes

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.

1L6 Tubes

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.

Pampering Filaments

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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