MRF300 help!

[XXL]

Iv been using the mrf300 in the MRF101 amplifier. It does around 180-200w. but iv had 2 pop on me now. This happens after about 3 weeks on air. my setup is 1.5w input, The bias is set to 2.5v (actually turned it down to 2.3v this time) That equals to about 100ma, the same as the mrf101. The only thing iv changed is iv played around with the coils at the output to get it to match a bit better. It gets hot but not so hot that it burns my finger.

This device is meant to be 65:1 swr protected, so not sure whats going on. Can anyone shed some light ?
The mrf101 just isn't enough power now days and i refuse to use old devices now.

Thanks

[Albert H]

The problem with the MRF300 is getting rid of the heat. It's a very poor design.

[OldskoolPirate]

Is it the heat that’s killing it ? If so yes it’s a poor device.

The mrf101 runs perfect though. Obviously I know the amplifier design is meant for the 101 but the 101 gets just as hot.

[Shedbuilt]

Which one of the MRF101 amplifier designs / PCBs are you using with the MRF300 ?

[reverend]

I've seen one MRF300 amplifier that uses a thermistor in the bias network to turn down the bias when the thing heats up (R6 in the circuit below).

ex8mle_pa_mrf300an.jpg

[Shedbuilt]

I've seen one MRF300 amplifier that uses a thermistor in the bias network to turn down the bias when the thing heats up (R6 in the circuit below).

A sensible measure. Should help to keep the quiescent current relatively constant (if set up and thermally coupled correctly), which should also help to stabilise the heat and operating conditions.
I tend to think though, that going from the MRF101, to the MRF300 (and expecting the respective increase in power), needs more of a redesign than just changing the inductors. For a lumped component output network (at a high level, just looking at the differences in matching ~100W to ~300W, with the same VDS, and without any consideration of device specific characteristics), the inductors would become significantly smaller, and the capacitors (at least those involved in the impedance transformation), significantly larger. Various factors at play; network topology broadband / narrowband etc, but the "simple" impedance matching, differs by around a factor of 3; so it's not trivial.

[OldskoolPirate]

I've seen one MRF300 amplifier that uses a thermistor in the bias network to turn down the bias when the thing heats up (R6 in the circuit below).

ex8mle_pa_mrf300an.jpg

You got any pcb images of this ?

[yellowbeard]

I've seen one MRF300 amplifier that uses a thermistor in the bias network to turn down the bias when the thing heats up (R6 in the circuit below).

ex8mle_pa_mrf300an.jpg

You got any pcb images of this ?

https://70mhz.org/equipment/amplifiers/ ... ransistor/