an interesting question came last days from John, W8BCR regarding the PL-15...
"So I have an mx-21s radio. I have a line on a pl-15 model amp. Are these compatible? Or do I have to find a pl-21 model amp? Any info would be helpful! Thanks, John W8BCR"
The PL-15 is dedicated for MX-15 transceiver from Pico series. (together with VX-15 ext. VFO) This amplifier is designed for 300mW exciter as the MX-15 radio has this power output level.
So in order to avoid over-driving the PA by PL-21S radio or damage the transistor you should change the components inside or if you would not like to do anything inside of the PA (to keep it collectible)then I propose to try use the external RF attenuator in the input of the PL-15 to jump your exciting power from MX-21S handheld down to approx. 300mW.
The best way is to measure the real power output from your MX-21S into dummy load (I am expecting something around 1.5W out) and then make an appropriate attenuator to get 300mW output for your PL-15 amplifier.
The rest of the both amps is same including the cables so I am confident it should work for you.
Hope it help,
73 - Petr, OK1RP