Zetagi amp mod

[9H5JO]

I am going to get bushwhacked on this one but here goes:
I came to the hobby via cb. I have an old Zetagi 550p (4x mrf455) gives around 300w ssb with a 20w drive. Ok, it doesn't have filters. But if you replace the 6 level attenuator board with a decent 6 band pass filter-board (I can construct one) it should pass muster. The snag is it is supposed to be only usable on 20mhz and up. I want to use it on 20m n 40m if possible. Any ideas to make it a decent 300w amp on these freqs? I can provide the circuit which is available on the 'net.

[N8YX]

IIRC, the '455 is usable from 13-30MHz @ rated IMD and gain.

I'd have a really close look at the Application Notes for that device before doing anything. Gain compensation will undoubtedly factor into such a modification, thus necessitating rework of the biasing and output circuits.

Speaking of...compare the output transformer design and core types to those spec'd in the Notes. Is the mix and permeability suitable for lower HF operation w/o causing core saturation? Another possible gotcha.