Hi the Bias is a stabilised switch supply, the two IFR FETs will have a different turn on voltage hence two pots. They are on a bit of a knife edge.
These are cheap transistors pressed into service to form a good RF amplifier, however, only a few manufacturers will work correctly as not all IRF 510 have the same spec
They are switching transistors normally switched hard on or off in a switched mode power supply, however, as they have a low capatance they are ideal for a general purpose RF transistor. They are cheap less than ?1.
Most Transmitters have power control which throttlies back the drive when you have a mismatch, to save them from high voltages. The QRP LAB design does not, so they are vulnerable.
You must have a matched load to use them.
You could use a multi turn pot to set the Bias, or a very steady hand, they also will run away with heat as well.
It is a PA on a Budget, having said that mine has been working for 10 years with carful handling.
I build a second unit with a SWR bridge and the reflection port throttlies back the drive, this makes it very robust.
RF power transistors are very expensive ?25 each.
Make sure you are using the original make supplied with the kit. Vishay not sure how it it is spelt.
73
Andrew
Andrew Lenton