[Collins] FW: RE KWM2A

[Dr. Gerald N. Johnson]

On Thu, 2008-03-27 at 18:09 +1030, Frank Woolfe wrote:
>  
> Can anyone shed some light on the reason of changing V13 (1st RX Mixer/Xtal
> Osc) in the KWM2A from a 6U8A to a 6EA8?
> 
>  
> 
> Does the 6EA8 have better performance lower noise etc; and when was the
> change implemented. It appears that there is no physical difference between
> the two tubes or in circuit, so all I can surmise is that it is a
> performance issue.

The pentode section of the 6EA8 has a little bit higher transconductance
which makes for lower first mixer noise and higher first mixer gain. But
not much.

I hadn't noticed it it, but many have said the 6U8A was a looser with
short life and that was improved with the 6EA8.

Otherwise the tube data is virtually identical. RF and IF tubes in the
S-line receivers are run at high currents for adequate gain and so
slight weaknesses show up sooner. Previous (75A, 51J, R390()) receivers
had one more IF stage (which makes IF strip bypassing and layout a whole
lot more critical) so the tubes didn't have to be run so hard.

I know the 75S-3B/C has 6EA8, I know the S-1/2 has 6U8A. That sets some
loose bounds. I don't know about those in between.

I don't find admission of the change in the list of changes that are in
the last KWM-2 manual on-line. But the schematic shows 6U8A in some
stages, 6EA8 in others. V2, 11, and 12 are shows as 6U8A and V13 as 6EA8
in that schematic. Nothing about 6EA8 in the schematic notes.

Looks like the 6U8A was in all versions of the 32S through the Rockwell
32S-3A.

I doubt the performance differences of good tubes can be measured in the
field. It may well be only that the working lifetimes of 6U8A in some
applications are short, so I'd say use whatever you have interchangeably
(not neglecting to align the inputs and outputs of the changed tubes no
matter which type is used) but expect the 6U8A performance to fall off
sooner than the 6EA8 performance.

Remember that tubes are in sockets because they have finite lives.
> 
>  
> 
> Frank VK5MFW
> 

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer