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Vintage threads from the first ten years

4CX250B's

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11/20/2003 5:03 PM

I just acquired a quad of 4CX250B's from one of my ham radio friends, and I want to build an amp out of them. only problem is the output transformer. in class AB1, the output is going to be over 1200 watts, and I can't find an output transformer to handle that much power. does anyone know where I can find laminations and other parts to build a custom transformer to handle up to 1500 watts? I wouldn't be opposed to using a toroidal output transformer, if it would be easier to get large toroids. I have had no trouble finding a power transformer, since hammond makes a good variety of high voltage transformers (I'll probably be using the 737 with a cap-only filter, and a separate supply for the screens).

11/21/2003 12:31 AM
Don Symes	A microwave oven?

11/21/2003 1:56 AM
dpcoyle	How about building a serious stereo amp? It would lessen the xmfr challenge. Dan

11/21/2003 3:35 PM

a pair of 4CX250B tubes has an output impedance of 11000 ohms. it would be much easier to wind a transformer with about 30% less turns of heavier wire and use all four tubes at 5500 ohms. I wouldn't be opposed to a 650W/channel stereo amp, but I'm a guitar player, not an audiophile. the only channel switching I need is in the preamp. if I wind my own transformer, I could even add an extra winding for ultralinear connection. maybe even have it switchable between ultralinear and tetrode mode. the screens only run at 350v, so finding a switch for the screens would be no big deal.

11/21/2003 5:34 PM
Bruce /Mission Amps	Sounds like a nutty and nearly unuseable project amp to me, but most of us old time ham guys who were building big butt tube amps know this guy: Peter Dahl. http://www.pwdahl.com/ Bruce

11/21/2003 2:19 AM
jeremy	just use 2 of em, or one single ended. that should lower the requirements a tad. or go conservative and run em only at 1000V. jeremy

11/21/2003 8:30 PM
elnonymous	4CX250's require forced air cooling that will be quite loud. It's a lot louder than using a muffin fan. Definitely not good for low volume playing. I agree with the advice from Mission Amps about contacting peter dahl xfmrs in El paso, TX. They primarily do custom HV transformers for PS's but seems like audio might be up their alley as well. Expect it to be expensive.

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