The quest of the perfect MOPA could happily end on this circuit. It has, really, everyhting's needed. Neutralizazion, wide band operation, provision for a VFO, high amplification tubes so that low drive is necessary, even when the signal frequency is multiplied to transmit on the higher HF, and adjustable PA drive level.
As compared to the 12BY7/6DQ6 MOPA, this one delivers a higher power, but uses a couple of tubes a bit more expensive than the former. This schematic allows quadruplication of the injected frequency, due to the high amplification of the 6AG7. Which tubes is, roughly, a stronger version of the 12BY7. Recently I've discovered the reason why the Novice transmitters, like the above one, were conceived to quadruple the frequency: radio crystals around 9.5 MHz, so to go n 28 MHz by triplication, were rare and expensive.
High order of frequency multiplication are demanding in that they require high amplification and high rated power. Things are tougher if all the freuency multiplication burden is assigned to one tube only. The 6AG7 is perphaps the most suitable to all this. The 12BY7 comes a second, but it does need a rather high amplification PA after, and in fact the 12BY7/6DQ6 MOPA fulfills the requirement.
Radioman, Jan 2016
Posted by Radioman | Permanent Link