[Collins] Collins military S line

[Dr. Gerald N. Johnson]

On 7/7/2014 7:39 PM, Carl wrote:
> If I had a full run of the tube manual into the 70's Id use a 6EH7 RF,
> 6ES8 as a Pullen mixer with AGC, 6DC6's in the IF and 6ES8's in the 2nd
> mixer and PD. Separate AGC loops would handle the different tube curves.
> This model would have the highest

Many a radio has "delayed" AGC to the RF stage, essentially a voltage 
divider so it sees less AGC voltage. Probably from use of remote cutoff 
tubes in the IF stages and a semi remote or sharp cutoff for the RF 
stage. Adding some threshold effect would enhance the AGC qualities but 
wreck the linearity of the S-meter reading, just like putting a sharp 
cutoff RF stage in most receivers.
>
> If I was stuck with tubes and circuits available during the different S
> Line production Id probably choose the 6BZ6 for RF, 12AT7's as straight
> mixers for early models then upgrade as better choices became available.
> Drake was smart to continually update the R4 series.

I don't know about tube availability, but I suspect early post war (WW 
II) designs were encouraged to use surplus tubes because they were 
available or on hand in large quantities and very cheap. Heathkit got 
into electronics that way with surplus 5BP1 CRTs and 6SN7s. I expect 
Collins had considerable quantities of tubes used in the ARC2 and other 
radios built during the war, which was a period of great manufacturing 
expansion for Collins. I don't know that to be a fact, though my first 
boss, Lloyd Winter (badge number 5) told me that in the 50s they had to 
redesign all the early post war broadcast transmitters because they had 
used surplus vernier reduction dials but had used up all the supply and 
couldn't get them any more. I would wonder if there wasn't a machinist 
in the place that could have made them in the quantities needed or 
specials couldn't be bought. Probably the cost was high. By then the 
style probably needed to be updated and Art did like to see style 
changes, even when they negated his demands for random S-line serial 
numbers. He didn't always keep his thoughts connected from one week to 
the next it would appear.
>
> In all cases the B+ would be no more than 150V except the audio amp
> which would be a 6V6 or 6BQ5/EL84.

I have thought it would have been nice thermally to have had a couple 
small (like 6AQ5 or 6AK6) tubes in pushpull so they could be run class 
AB or class B and have a significantly lower idling current than the 
single ended amps running class A. Costs a tube and a socket, and an 
extra wire on the output transformer plus s phase splitter and a couple 
extra coupling components. The audio quality and the heat dissipation 
would have been detectably better by ear and hot finger.
>
> Carl
> KM1H
>
73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association