[Johnson] 250-39 TR switch.

[Roy Morgan]

On Oct 23, 2008, at 10:34 PM, Rodger wrote:

> ... T/R switches are rated for pretty high power that is only true  
> when the antenna is well matched to the transmission line, that is  
> low SWR.  Otherwise the voltage the T/R tube sees may be much higher  
> than it can handle

Agreed, this could be a problem.  The tube in the Johnson unit is, I  
think, the 6BL7, a dissimilar dual triode octal tube which would seem  
to be quite strong at the grid.  Other T/R switches use 7 or 9 pin  
mini tubes and are likely much less robust against high RF voltages.   
The RF voltage, RMS  mind you - not peak, can be the SWR times the  
voltage expected for 50 ohms at the output power level.  Add  
modulation peaks to that and it will frighten any 6AU6 or 6AK5 you can  
use!

I recently got a B&W T/R switch that uses such a small receiving  
tube.  If I remember right, all B&W transmitters were in the 100 to  
150 watt class or below.


> ...  If you want to experience full break in CW like it existed in  
> days past and are willing to put up with the drawbacks then you  
> might want to play with one.  But otherwise a relay is going to be a  
> better choice.

Of course there is the sought after state of real QSK.  With  
boatanchor gear, this usually involves modifications at the receiver  
and two antennas.  With modern equipment using linears, it's PIN  
diodes or vacuum relays, or both.

Sooo many projects, sooo little time!

Roy

Roy Morgan
k1lky at earthlink.net
529 Cobb St.
Groton NY, 13073