As per the posts, parallel it with a 10k.
I made some additional measurements of the PA50. One result was a bad IM3, which was only
at -15…-20dbc. Thats too low, if you use the amplifier in SSB you will - maybe - get a bad, distorted signal.
After tests with some OM the problem was confirmed, especially at 80m. This is not a problem with too much input, lower input doesn’t solve the problem !
Here my solution:
Adjust the BIAS up to 3.72V for both IRF530N transistors (PTT must be active for adjustment). After this adjustment I got now an IM3 of -35…-40dbc. That is much better, not perfect, but now the signal in SSB is ok and clear. Don’t try to get more output as 50W. The IRF530N are not RF transisitors, but work for this little power amplifier.Tests after the modification now confirmed a largely clean signal without distortion.
My solution has already been examined in detail here IRF530 30w Mosfet Amplifier – F5NPV – V85NPV and here PD7MAA HOMEPAGE: DIY kits 70W SSB linear HF Power Amplifier For YAESU FT-817 KX3 , the PA50 has a very similar concept.
I agree with PD7MAA, as he wrote:
“At 3 Watt input the output was about 50 Watt at 20m. But the modulation was as expected distorded in ssb mode at the recommended 2.7V bias voltage. At 3.7V ( gate ) bias the modulation is acceptable but far from perfect.”.
Before my adjustment I measure 3.57V BIAS from factory, where I need to agree, this is too low for a good signal in SSB. Now I’m happy and satisfied with this little amplifier.
73 Heiko, DL1BZ