[BC] Substitute driver tube for FM-2.5K

[Xmitters@aol.com]

In a message dated 3/27/07 1:43:18 PM Central Daylight Time, 
broadcast-request at radiolists.net writes:

<< The "H" and at least some of the "K"'s had a coaxial connection from the 
IPA to the PA cavity.
 
 Get a good exciter with at least 30 watts of drive, pull the IPA out, and 
feed the exciter directly into the PA.
 
 Works fine.
 
 Remember that Collins (later Continental) had a similar design 2.5 kw 
transmitter which had no IPA.  Instead their exciter had 20 watts driving the final.
 
 I have a 2.5 "H" that developed a short in the IPA socket.  Was blowing the 
fuse on the 700 volt IPA plate supply (which also supplies screen voltage to 
the final).  This happened on a weekend, of course.  At the time I was using an 
MX-15 to get  1100 watts out (small Class A with a high AHAAT).  To get 
something up and running I snipped & taped the plate connection to the IPA & ran 
the MX directly into the PA.  Had to turn the exciter's output down because the 
IPA was acting more like a buffer than an amplifier.
 
 Now, of course, you need more drive than that to run the transmitter at full 
power. The MX, at full bore, will only get you half power.  We subsequently 
got a mod on this station to increase power to near equivalent 6 kw A, which 
required a TPO of 2450 watts. So the MX was replaced with an 802A, then a much 
newer BE FX-50.  About 25 watts gives me the required TPO.
 -- 
 
  >>

Be sure to have a 1/4 wave length of cable between the PA and your high power 
exciter. That will make your substitute driver see something closer to 50 
Ohms.  The Harris seminar I went to a lifetime ago, they told us that the IPA 
likes to see a 50 Ohm load and this cable length helps accomplish that. 

When you bypass the IPA, be sure to jumper out the underdrive relay contacts; 
that relay is activated by IPA grid current. 


Jeff Glass
Northern Illinois University
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