[Radio Wrangler]

If it was designed to have current sampling places, they would be decoupled so that test cables wouldn't create RF problems. You'd still need short thick cables because of the current involved and the need to not disturb things with voltage drop, so special cables would need to be made up. DMMs would be poor choices, most go loopy in the presence of RF power, so you'd be looking at something expensive from Fluke or HP which get carefully filtered - or just use a classic passive meter like an AVO!

As for the currents, it would be nice to know the design values, but from knowing the supply voltage and the output power, a pretty good guess can be made, and anyone using it will get a working transmitter. It's a matter of re-doing the design calculations, and not particularly difficult.

Trying to get other people to just set a gate voltage introduces the variation of the transistors in terms of Vgs versus Id. That transistor has a data sheet stating there is 2 volts of variation in the threshold voltage (2 to 4 volts) and that's only at 250uA in the drain. It gets worse at higher currents!

So, setting the bias by just the gate voltage is very easy to do and very easy to describe, but it will only work for a fraction of the transistors of that type out there. Some people will be left with too low bias and very little output power, others will take enough current to blow fuses. You've been lucky that a figure suggested to you worked for yours. Not everyone will be so lucky.

1) Designing something is one thing.
2) Designing it so that it can use any part that is within spec for that type of transistor is harder.
3) Designing it so that you can adjust what needs to be adjusted is harder still.
4) And finally, writing a procedure that other people can follow to set one up properly is a right sod of a job!

That radio is OK on 1) above.
It's a fail on 2) because it needs matched pairs. Can't be relied to work with randomly chosen transistors
as for 3) it's not too bad, there is a sequence which will get it there, given matched transistors. Not matched on a hand-held tester or a multimeter, but matched at high current as they will be used.
number 4) is a complete wipeout. There is no info available on how to select transistors and set it up.

So, some of us can work out what current to set it up to, and a process for setting it up, but there's as much work involved as there was in designing these aspects in the first place. There is a class of circuits which can only be fixed with any certainty by someone that could have designed it right in the first place!

Someone I used to know had a radio and TV repair business. He kept getting asked to repair guitar amplifiers in the early sixties. He lost his rag with some of the junk that came his way. "I could do better than that!" was his thought. So he did. His name was Matt Mathias, a jewish escapee from Nazi Germany. His amplifiers were branded "Matamp" but a guy with a boutique music shop in London wanted to buy lots with his own branding on them and bright orange tolex covering. So when you see famous groups fronting "ORANGE' speakers and amps, it all came out of the thought 'I could do better than that!'. Better build quality than Marshall, too. Matt was a nice guy, and the people who called in at his place were a who's who of rock and pop.

So, you learn more from coming across a piece of iffy design than you do from an honest part failure in a good design. Ironic, really.

I've been enjoying a few days of sporadic-E propagation on 50MHz. 20metres is coming alive, I think the next sunspot cycle is starting.... yippee!

David