Film-Tech Forum ARCHIVE: Ballentyne Power Amplifiers

This is topic Ballentyne Power Amplifiers in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Paul G. Thompson (Member # 655) on 05-06-2001, 12:26 AM:

20 years ago, I serviced a bunch of Ballentyne Power amplifiers used in a drive-in applications in Wisconsin.

I believe they had 866 rectifiers. Does anyone know the Model Number of that amplifier, and where I can get a schematic of those? They were extremely powerful amplifiers. What was the tube number of the finals?

There is nothing in the manual section of Film-Tech.

Thanks - Paul

Posted by Gordon McLeod (Member # 33) on 05-06-2001, 10:32 AM:

THe most common amp they made was the PD50

Posted by Ronnie Martin (Member # 663) on 05-06-2001, 11:19 AM:

Paul,
I believe those were 805s for the final stage.
I know they had somewhere in the neighborhood of 1300 volts on their plates.

Best I can remember that was a MX-20 used with a MX-24 pre-amp.

Later dude,
Ronnie

Posted by Paul G. Thompson (Member # 655) on 05-06-2001, 05:24 PM:

Thank you, gentlemen. Much appreciated.

Posted by Daryl Lund (Member # 356) on 05-06-2001, 05:36 PM:

Paul E-mail Kenny Layton, When I was a GM for ACTIII He rebuilt about twenty of those amps. He had set up a test bench for and might have rebuilt more than twenty.

Posted by Paul G. Thompson (Member # 655) on 05-06-2001, 10:19 PM:

Daryl, it would not surprise me. Those things liked to arc off in a nice blue and orange ZAP in humid weather. They were a very poor design....but setting the 866 sockets to stand above the chassis on HV insulators seemed to cure most of the problems. Then, it was just a matter of replacing the bottles....

Posted by Joe Schmidt (Member # 822) on 05-06-2001, 11:16 PM:

I'm quite sure that MX-20 is correct, and somewhere I think I have a schematic of this critter but it is buried in my archives of gazillions boxes file folders in my warehouse. Email me as last resort if search is unsuccessful w/ Ken and I'll try.

Yes, it is not such a good design. One fundamental fault in all of these that I've seen, is the time-delay for kicking in the plate power is way too short. 866 mercury-vapor rectifiers must have at least a 30-seconds MINIMUM warm-up per specs first and if you can let it have a minute, better. I'd disconnect that time-delay relay and have a manual switch on the plate xfmr, wire it so filaments must warm up first and put warning sign. If you change out the 866's to new ones you must give a 30 MINUTES minimum initial warm-up. No wonder things in that amp pop like popcorn!

All Ballantyne sound was lousy, but beautiful mechanically, which probably helped to sell it. I often felt this amp needed a bigger and more rugged output xfmr too instead of that dinky little thing. As with other theatre equipment mfrs, Ballantyne just didn't connect up with the right audio people to do their sound for them, and have better stuff inside that nice metalwork.

If you can find any and overhaul them, the Simplex XL 60-watt amp with 807 finals is a much better amp. Particularly, Change all the coupling capacitors to new and overrate to the final, @1000vdc. You can get very, very good-->excellent drive-in sound in a Simplex system by installing solar cells replacing photocell, provide DC exciter lamp power, and be sure the preamps are getting proper plate voltage and not too low, it sometimes is. Given this, exciter lamps can be run @8volts, won't burn out, speech intelligibilty will be so good you don't have to push the volume so high.

Good Luck!

Posted by John Eickhof (Member # 323) on 05-08-2001, 11:45 PM:

Paul, it is the MX-20 they made two different versions though, anyway, I ahve one in my shop! (for the collection) I am sure I have drawings too! Let me know what you need!
John
PS...i'm home for a few days too!

------------------
John Eickhof President, Chief Slave
Northwest Theatre Equipment Co., Inc.
P.O.Box 258
Wendell, ID. 83355-0258
208-536-5489
email: jeickhof@nteequip.com