Re: QRP-Labs 50 W Amplifier - low power output

Hans Summers

Hi TIm

Your point 5 isn't just from left field it's off the field all together. There is nothing wrong with the supplied ferrites. I spent a vast amount of time on suppliers and material testing in order to choose the best ferrite for the job. I would not be supplying ferrites which are not up to the job. Many constructors have built this amplifier successfully and to spec. As with all QRP Labs kits, if it does not work there is a probability of at least 99% that it is due to an assembly error or fault, not a defective component. 

Regarding transistors, yes, replace them together. I wouldn't agree that the biasing scheme is crude, it is designed for a particular task and it accomplishes this perfectly. Separate bias for the two MOSFETs This is a Class C amp. The answer may be different if it was a Class AB amp where we were aiming for highest possible linearity. Nevertheless I would always recommend replacing both together, and would recommend that for any PA including the Class E finals in the QCX/QCX+. For the pennies these transistors cost it isn't worth taking chances. 

Sourcing the transistors does need care. A lot of cheap IRF510s are just some generic MOSFET onto which is printed various labels purely for selling them - such as IRF510, IRF520, IRF530... they don't necessarily have the correct characteristics of our beloved IRF510. The transistors used in the kit are Vishay brand and are reliable and correct. If you want to replace them then buy IRF510s from a reputable supplier. For example if you buy in US, Digikey IRF510s are also the same Vishay type and are known to be good. It doesn't have to be Vishay, just buy from a reputable place so you know it is is what it says it is. 

Regarding point 4... somewhat just off the Left Field boundary... do NOT add a capacitor and 100-ohm resistor at the wiper of the pot! You really will reduce the power output of the amp then, I guarantee it! I don't see what is confusing. The pot sets a DC bias that is applied to both gates. It works...

Good luck!

73 Hans G0UPL

On Sat, Aug 22, 2020 at 12:29 AM Timothy Fidler <engstr@...> wrote:
Point 5 . of the last  Troll post is from Left Field  (at least in terms of the OPTrafo only ) because all the indications are that the problem may be ahead of the gates of the Mosfets.

regards, TEF

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