[AMRadio] A productive Morning

Geoff/W5OMR ars.w5omr at gmail.com
Tue Feb 12 11:59:40 EST 2008

Good morning, troops!

Today, I was able to wake up, walk, talk and complain about things I can 
-not- change!
Today, I am TOO Blessed, to be Stressed!

So, while we're on the positive side of things, I think I've managed to 
incorporate new/old technology out in the ham-shack.

Hear me out.

The final is home-brewed.  I'm sure you already knew that, and it runs a 
pair of 250TH's in push-pull, class C.

Well, what you may not know is that now I have -two- RF finals, that 
incorporate the 250TH tubes.  The original was built by a dear friend of 
W5BU/Dan(sk) back in the mid 50's.  This one I'm using now, was built by 
Mike/W5IMF in the early/mid 60's.  Thanks to my good friend, Duie/K5KZQ, 
I was able to obtain this good ol' boat-anchor and put it back on the 
air after 40 years of collecting dust.  Works great, too ;-)

NORMALLY, I would drive it with the Viking II, like I do the Titanic 
final, but the grid input is different on the 'new' final.  In order to 
maintain around 125mA of grid drive (250TH's require 45mA per, for Class 
C.  That's 90mA for a pair.  Running another 15mA of grid drive on each 
ain't gonna hurt them big jugs ;-)) Using the Viking II, this wasn't an 
issue for the plug-in coil and swinging link arrangement on the original 
final.  On the 'new' one, I've got to slip in the 160m coil, re-dip the 
final, unload it as much as possible and then back off of the grid 
drive.  You know what happens to tubes in Class C when they don't have 
sufficient grid drive, and full plate voltage... run-a-way current 
shoots sky-high and there's nothing to stop it.  There's protective bias 
on the 6146 finals of the Viking II, and I'm -sure- that's the only 
thing that's kept those tubes from shooting up outta their sockets like 
little glass rockets! 

What I have done in the past, with this 'new' final, was to use my 
little Kenwood TS-450S, all band/all mode xcvr as the RF driver.  But, 
it takes around 35~40w to get the grid drive to the 125mA level.  Kinda 
hard to tame a Viking II down to that level, and it's kinda hard on a 
solid-state rig, with the long ol' buzzard transmissions I tend to make 
(so I'm told) ;-)


A while back, Rick/K5IAR and I made a deal on a Globe Linear.  This 
thing uses 4 sweep tubes, something like 6DQ6's or some such.  So, I got 
it out, hooked it up, fed RF into the darn thing, and hung it on an 
antenna.  It'll produce around 200w output with 50w of drive from the 
solid-state exciter.  Not bad.  Then, I backed off on the carrier and 
power controls on the little solid-state exciter and got the exciter/amp 
lash-up down to a more 'reasonable' level for driving the final.  Then, 
I connected it to the final, made some minor tuning adjustments on the 
grid input and voila(!) the solid-state exciter is making around 4 or 5 
watts output, (no danger of hurting anything there) the Globe linear 
amplifier is making enough output at about 1/4th (or less) of it's 
maximum output, and that drives the 250TH grids right to 125mA, and it 
-stays- there.  Nothing is wavering.  Kewl!

All that's left to do now, is find a set of relay contacts that I can 
use to key the exciter, or use the exciter to key a relay that will, in 
turn, activate the 110vAC T/R relay.  I think the last thing I want to 
do, is try to make/break 110vAC out of the back of the solid state rig 
;-)   I think what I -really- need to try to find, is one of the dual 
SO-239 Dow-Key T/R Relays, and that way I could also use the Solid-state 
rig as the station receiver, as well.   The Globe Linear does -not- have 
T/R switching in it.

Currently, the original T/R relay is doing a good job of switching the 
antenna between the RCA AR-88 (Thanks to Jim/WD5JKO for the receiver) 
and the transmitting side of the relay.

With it's 'direct frequency entry' keypad, I'd like to be able to use 
the Kenwood as a receiver, also.  Maybe instead of, as opposed to 'in 
addition to'.
(no, the AR-88 is -not- for sale).

So, after the story, what I'm asking for is; does anyone have a dual 
SO-239 Dow-Key T/R relay they would be willing to part with?

As always, 'deal' mails should be sent off-list.  Discussion about the 
setup would be encouraged to be conducted -on-list-.

Thanks for your time, patience and consideration.

Driving your AM Rig without a scope, 
is like driving your car at night, without headlights. (K4KYV)

73 = Best Regards,

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