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Formula for output trans match?

2/23/1999 8:59 AM
Formula for output trans match?
Can someone provide a usable formula for establishing the Primary Imp. for a output Xformer. How about example: 2 el34s push-pull at 424 vdc.  
I hope this is not a dumb question, I'm just not sure that I'm doing it right. Thanks RJH
2/23/1999 10:11 AM

There really isn't a simple formula to determine what load impedance is suitable for which tubes. These things are usually determined by the tube designer/manufacturer by actual testing & measuring. It's rather complex and results vary with tube characteristics, operating voltages, power output and distortion characteristics. There isn't one perfect load for each tube, either. Everything's a compromise.  
There is a way to determine a load graphically (along with some simple equations) using the manufacturer's published plate curves, and for instance the method is shown in the front pages of an RCA tube manual. It will take a while to go through this method, but it does help and I think it's worthwhile to get a feel for what's going on.  
Since most of the proper matches have long since been determined, usable loads are tabulated in tube manuals and even transformer manufacturers' catalogs.  
It's not a dumb question, and it's damn frustrating to grasp the operating characteristics of vacuum tubes. They look simple, but they are actually complex multi-dimensional devices. I've only scratched the surface myself and obtained a somewhat usable working knowledge that gets me by. There's still an awful lot I'd like to understand.  
Keep up your interest and don't get frustrated.
2/23/1999 10:14 AM

The "just-right" transformer impedance for a given pair of output tubes is not clearly determinable by a formula. For power pentodes like the EL34, it is usually around 1/9 of the internal plate resistance for best performance. Slightly smaller values give more power, slightly higher values tend to be best distortion. Randall Aiken's note earlier in the forum is a pretty clear statement.  
Most of the time the manufacturer built some, tested and tinkered with them and published a recommended value. For the EL34, this is probably 4000 to 4400 for best power, up to 6600 for best distortion at somewhat lower power. The tubes are tolerant of modest mismatches, so the exact value is a matter of taste and design.
2/25/1999 9:01 AM
Fred G.

See the thread below entitles "Plate voltage vs. power". I found it extremely helpful on this very same topic.
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