[WRL] LA-1 Selenium Rectifier Replacements

[Garey Barrell]

Walt -

I'm not familiar with the LA-1, but it sounds like they are using the 
reverse breakdown of the selenium rectifiers as Zener diodes. 

Does the information you have specify what the bias voltage "should" be 
for each class of operation?  Should be a simple matter to replace the 
seleniums with appropriate Zener diodes.  Alternatively a simple LM723 
type regulator or three terminal regulator could provide for adjustable 
bias.  There is little or no current required from the bias voltage, so 
that's not a concern.

On the other hand, the bias supply in the Drake and other transmitters 
is not regulated, just loaded sufficiently to keep it at a stable 
voltage.   Again, no "load" current is required, so only line voltage 
variations would affect the bias voltage.

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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