100 Watt PA

[Albert H]

Here's a 100W PA using a pair of BLW60s. It needed 8 Watts of drive for 100 Watts at 15V supply. It draws a shockingly high current at 15V and the positive lead from my bench supply got so hot that it discoloured! For best results, the transistors should be matched, but we found that transistors from the same batch number were good enough.

100 Watts the hard way.jpg

It needs a HUGE heatsink and a blower!

[sinus trouble]

That is an absolute BEAST!!
It looks to me like parallel device config? or is it push pull style?
Its great to see some of your work Mr Albert! Excellent!!

[Analyser]

It's parallel Sinus.

Why did you bias it Albert?

[sinus trouble]

Cheers Mr Analyser!

[Albert H]

Analyser - It was originally Class C, but I found that I could tweak out the differences between poorly matched devices if it was biased up a bit. Also, it gave more gain that way. At 98 MHz, in Class C (with carefully matched transistors) I needed about 11W of drive for the magic 100W output. Biased, I only needed 8 Watts - I could drive it with a nice, cheap 2SC1971 at 15V. That PA is about 30 years old. I was having a clear-out and found a couple of them. They were really prototypes, never used seriously. They drew much too much current!

[Analyser]

Yes, putting those low voltage devices in parallel must be a complete nightmare, the collector impedances are so low any imbalance would probably cause huge current draw and massive inefficiency.
I would probably chuck it in the bin but keep the trimmers- they look nice!

[Shedbuilt]

but keep the trimmers- they look nice!

Yes. Jackson air spaced trimmers. I seem to remember buying quite a few of that type from John Birkett - in the early / mid nineties.