[WRL] LA-1 Selenium Rectifier Replacements

[Garey Barrell]

Walt -

When all else fails....  :-)

I just picked up the manual from BAMA. 

Class C operation is at -13V, unregulated.  This is the 6.3V filament 
voltage rectified with a selenium rectifier, and then loaded with two 
220 ohm resistors in series (R4 & R7), with additional bias derived from 
RF drive.  Class C bias isn't all that critical anyway, so just put a 
1N4007 in place of SR-1.

For Class B, the bias voltage is specified as -3.2V.  There is a 3.3V 
Zener, but low voltage zeners are sometimes a little hard to find.   I 
would replace SR-2 through SR-4 with a string of five (5) 1N4007 diodes 
in series.  The anode end of the string would go to ground, i.e., the 
same as the selenium string.

73, Garey - K4OAH
Atlanta

Drake B & C-Line Service CDs
<http://www.k4oah.com>



Walter S. Szachara wrote:
> Good morning to all...
>
> I have a Globe LA-1 linear amplifier that I am trying to refurbish.  I am
> looking for suitable replacements for the for the 65 ma. 25V. selenium
> rectifiers used in the bias circuitry.  The voltage drop across the selenium
> rectifiers is important as it apparently generates the two bias voltage
> levels necessary for Class B or Class C operation.  The manual describes the
> theory as:
>
> "A regulated bias voltage is applied to the final tube grids for obtaining
> maximum linearity for Class B operation.  Class C operation is obtained by
> increasing the bias to the final grids...The bias voltage for Class B linear
> operation is regulated by heavy bleeding and the use of reverse-connected
> selenium rectifiers.  The back resistance of the rectifiers varies with
> applied drive and bias in such a manner that grid bias on the amplifiers is
> held to a very close tolerance.  The fixed bias voltage for the R.F.
> amplifier is obtained by rectifying the 6.3VAC filament voltage with a
> selenium rectifier."
>
> Does anyone on the list have any recommendations on suitable replacements
> for the stated selenium rectifiers?
>
> Thanks...
>
> Regards,
>
> Walt, W2OKF
>
>
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