RF Power Transistor Replacement

[woddah]

I have a 20W linear RF Power Amplifier using two BLW83 transistors.
The amplifier is transformer coupled at input and out and operates from 30 to 88MHz on power supplies ranging from 18 to 30VDC.
The BLW83 is no longer available from the original manufacturer and BLW83 devices from other manufacturers have not performed at all.
I have tried MRF426 devices in the circuit and they function fine at all supply levels above 20V.
Below 20V a spurious causes excessive current draw and "on channel" broadband noise.
Has anyone any thoughts on taming these beasts?

 

[zappedagain]

A quick look at the data sheets shows the parts are very similar, except that the MRF426 runs a bit hotter. At 20W you might be running into thermal issues.
Also, the data sheet that I found for the MRF426 states that 30 MHz is the top end of the frequency range.
It sounds like you might have some oscillations starting that look wide-band. I can't offer more than that without seeing the circuit.

 

[woddah]

Thanks for your response zappedagain.
While testing I don't think I will run into thermal trouble as I am testing at room temp and the original box was rated for continuous operation at 70 C, operational use might be a whole new story though.
Appologies to, I got the operating frequency range wrong (must have been asleep) it should be 1 to 30 MHz.