[Collins] KWS-1 V403 Mod

[Dr. Gerald N. Johnson]

My 4th edition KWS-1 manual owned previously by Forest Cummings who was 
one of the engineers on the KWS-1 shows a 6X4 was the tube for V403, the 
output ALC detector. It should be a bit more robust than the 6AL% but 
will have greater interelectrode capacitance reducing its efficiency as 
a rectifier at 30 MHz. In this edition of the manual its rectifying a 
small sample (coupled through a 1 pf and 22 pf capacitive divider) of 
the Rf plate voltage to make the ALC control voltage.

In the 7th edition the 6AL5 is looking at a voltage developed from grid 
current from the grid bias circuit to control ALC to keep the PA in 
class AB1. Probably makes for less distortion and is the fundamental ALC 
circuit used in the S line, KWM-2, and 30S-1.

Being that the two editions of the manual are so different, likely there 
is a serial number where all transmitters changed. KWS-1 serial numbers 
were issued sequentially unlike S-line serial numbers. Figure 4-17 shows 
the drastic differences in the ALC circuits in both manuals.

The 6X4 I say was NOT a mod, it was the way the factory built the 
earlier KWS-1. Its not as good an ALC because a lightly loaded PA will 
make more ALC voltage than a heavily loaded PA.

Digging you will probably find you need more feed through capacitors 
than are present to convert the circuit to a 6AL5 sampling the grid current.

KWS-1 SB3 introduces a mod for transmitters before SN 1083 to add a dual 
time constant ALC filter, doesn't admit the ALC voltage source is 
different. Nor does the 7th edition manual discuss any prior versions of 
the circuit.

73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.

On 3/22/2014 1:50 PM, Bill Riches wrote:
> Found two more mods in the RF deck of the butchered KWS-1 that I am working
> on.  A relay was installed to be able to use the RF amp as a linear for
> another rig.  Looks like they used a 500 watt iron to do the mod.  Removed
> that one!  They changed V403 from a 6AL5 to a 6X4.  Any ideas why?  Did not
> remove  that one yet. They did get the pins right though.  Have not fired it
> up yet.  Still waiting for the screen plate transformer to arrive by drone.
>
>
>
> 73,
>
>
>
> Bill, WA2DVU
>