Any advice on converting a RCA BTA-250M to 80 meters?

[AE2EA]

I've acquired a RCA BTA-250M, 250 watt AM broadcast transmitter with the goal of using it on 160 and / or 80 meters. It's 750 pounds of pure AM joy, all 813s and 807s.The 866 rectifiers have been replaced. I have it running on 1550KHz into a dummy load right now and have added a WA1FFL buffer / amplifier so I can drive it with a solid state VFO. 160 will be an easy reach but I'm not so sure about 80 meters. Does anyone have any pointers to send my way, things I might not want to learn the hard way?

Here's a link to the BTA-250M manual, courtesy of the BroadcastHistory web site:
http://bh.hallikainen.org//wiki/uploads/RcaBta250m.pdf

Made a short video about the project so far, for the AWA YouTube channel:
https://youtu.be/f8kMEOcyifU

Thanks,
Mark ~ AE2EA

[Steve - K4HX]

I have a BTA-1M. Been wondering the same thing. Looks like it could be modified to the standard Pi-Net output pretty easily. The lower level stages could be a bit trickier.

[KK7UV]

Your manual indicates P.A. plate load of 1440v and 250mA, which should be around 2900 ohms in Class C.
You can plug that into a Pi or Pi-El network design with a 50-ohm antenna load to see what values you need for the output tank.

The output coils can be reconfigured to get what you need for either Pi or Pi-El lash-up.

Your C118 tuning cap is 250pF which will be plenty for 80m operation, but should leave the C123 padder in place to get on 160m.  C117 padder is not needed.

The cluster of C120,121,122 are not needed for Pi/Pi-El, but can be calculated with the network design equations/software.

Does your VFO and Hagerty buffer have enough power to drive the PA grids?  Looks like you only need about 6 watts of drive (?).

[AE2EA]

The Hagerty buffer output replaces the crystal and drives the 807 oscillator. Here's a link to the original article in CQ:
http://www.wa1ffl.com/VFO_driver_amp.pdf

Thanks for your comments on the required mods!

73, Mark

[KB5MD]

Replace the output network with a standard pi network connected as close as physically possible to the power output tubes.  Getting rid of the long wires in the original output will allow operation all the way up to 40 meters.  I have a BTA 1R1 that I run on 40.

[AE2EA]

I've finally wrenched my mid 1950s RCA BTA-250M , 250 watt AM broadcast transmitter into the 160 meter band. This transmitter is all 807s (oscillator, RF driver, push-pull modulator driver) and 813s (push-pull modulator PA, parallel RF PA). It wasn't a tough project, but other diversion kept getting my attention. I'm now basking in the warm glow of those directly heated 813s. Before I put it on the air I'll need a base loading coil for my Marconi "T" antenna, which is just the vertical portion of the feedline for my 80 meter dipole. The old girl works great into a dummy load, about 270 watts AM carrier out. Next...a move up to 80 meters.

I've posted a trilogy of videos on the project the AWA Youtube page, the third installment went up today:

https://www.youtube.com/playlist?list=PLLTogcYJemH4D6wynJj1z6PA01q38B1IQ

Thanks for the helpful advice, I'll need it for the 80 meter conversion.

73, Mark ~ AE2EA

[W3GMS]

Excellent job Mark. 

Now get it down on 75M and you will be good to go on the AWA PM/AM net! 

We should try to work on 160M some night as well.

Joe-W3GMS