I where trying to mod Chinese amp DIY kit
https://www.qsl.net/p/pa0oos//2017/RF_AMP_2078_SCH.pdf

Changing the pre driver 2SC3357 with BFR106 amp because of input gain, amplifier acting strange, specially with fast switching feature that where with 2SC3357, although I'm not sure that is the reason. I change also R15/R10 with this values 4,7k/4,7k. I need help because I'm new with modding things, what would be better alternatives to 2SC3357, or what I need to look in data sheets to get better fast switching, except the input gain, which was my initial thing

 

What dou you mean by "fast switching feature" and "input gain" and what is wrong with it?

I'm afraid this thing needs a complete redesign, modding will not get you far. There is no (power) matching between the individual amplification stages which makes the behaviour and the gain of each stage unpredictable. The final stage doesn't really work in class-AB due to very poor drain-to-drain magnetic coupling.

 

What dou you mean by "fast switching feature" and "input gain" and what is wrong with it?

I'm afraid this thing needs a complete redesign, modding will not get you far. There is no (power) matching between the individual amplification stages which makes the behaviour and the gain of each stage unpredictable. The final stage doesn't really work in class-AB due to very poor drain-to-drain magnetic coupling.

Gain on frequency that interest me

frequency RMS (mV)
20MHz 592
14MHz 574
3MHz 479

So I tried to maximize input, not touching attenuation in schematic, I change pre-amp I 2SC3357 with BFR106, and also base divider R15/R10 510/3.3KΩ with 4.7/4.7KΩ, also change base divider on second pre-amp 2SC19171 R5/R11/R13 56/56/56 with 560/560/220 source would be this video number 4 video in series 1-6 . I want to achieve some better performance bellow 8MHz, where my device has poor Tx, fast switching referring to changing frequency almost without idle, and think that originally I pre-amp 2SC3357 with rest of elements in schematics show better performance than with BFR106, IRF 530 after crossing 4A in working condition DC power 14.5V (58W) goes into avalanche effects in couple of seconds, no matter how good heat radiator or cooper you put, or put thermal pad bellow IRF fet. Specially affects my interests of 3MHz, where I'm having poor input RMS, but in the same time having SWR pretty high, which helping fet to go on haven even faster, do you have any recommendation, my configuration except changing elements mention before are the same as in schematics, and quiescent current per fet is 30mA.