[BC] RE: substitute driver tube for FM-2.5K ?

[Tom Taggart]

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.
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