Post
by Albert H » Thu Aug 31, 2017 2:27 am
As I remember,those PAs have 2N6083 or 2N6084 power transistors in them. The amplifier probably won't do much with 1 Watt of drive - you might not even get your Watt out! If you put 5 Watts into it, it should do about 30 Watts out. These are very old RF power transistors (from the 1970s) back from the days when 6dB of gain was thought to be a lot. They have the advantage that they're reasonably robust - they're quite difficult to blow up!
Back when I first started building transistorised VHF FM rigs, we used to assume that low level stages in the exciter would have (roughly) 10dB gain per stage - that's 10 X power gain. 1mW would be amplified to 10mW, 10mW to 100mW and so on. A typical rig in those days would have many more stages than one today:
We used to start at ¼f - so the oscillator would be between 22 and 27 MHz. I preferred the Colpitts oscillator, though others would use Clapp or even Vackar circuits. This would then go through an un-tuned buffer stage to provide some isolation between the oscillator and subsequent stages, into the first frequency doubler stage. This would get us to ½f, and would be critically tuned. There would then be a second doubler to get to the output frequency. I used double-tuned filtering at this point to ensure that I had really clean drive. All the transistors up to this point would have been either BSX20 or BF494. The output of the second doubler would be around 10mW of the required output frequency. This would go to a BSX20 amplifier stage, giving about 100mW. This final BSX20 would need a little heatsink.
The 100mW driver would usually be separately boxed, and would usually have its own 12V regulator. The next stage would get us up to about a Watt, using either a 2N4427 or a 2N3866 according to the supply voltage of the rig - battery powered stuff was usually run at 13V8, and mains powered rigs could have the luxury of 28V supplies. Assuming we're at 13V8, the next stage would be something like a PT8810 or a 2N6081 to get to about 8 Watts, and the final would be a 2N6084 for around 40 Watts.
If we were at 28V, the stage gain would be higher, and we'd use a 2N3375 into a weird little stud-mounted device called a 587BLY. I built huge numbers of PAs with the 3866, 3375, 587BLY line-up. They were entirely consistent - three stages would take 100mW in and give 90 Watts out at 28V supply. If you could give it a bit more supply voltage, you could get the magic 100 Watts. I also built large numbers of PAs with pairs of 587BLYs or even four paralleled for nearly 200 Watts! Other favourite devices were the SD1019 and SD1127 which were (sort of) second generation devices, with more power gain, but were much more expensive.
These days, we're spoilt. I have a PA on the bench at the moment that needs just 4 Watts of drive for 1500 Watts output - no-tune, high efficiency and just a single stage! My current basic rig design uses an at-frequency oscillator that develops 800mW in a single stage. This drives a single small 12V FET for powers between 4 and 18 Watts (according to requirement). It's almost like the old valve days, when we could develop high powers with just two or three active devices!
"Why is my rig humming?"
"Because it doesn't know the words!" 
Bloody nora, typical. 
