Replace transistors in MiniPA amplifier kit

Question

I recently purchased a MiniPA 100-watt HF amplifier kit. It is a class AB push-pull amplifier and uses the MRF 9120 transistor. While doing some reading on this amplifier kit I noticed some discussion of using a pair of the more powerful MRF300 transistors, which could theoretically increase the power output to 600 watts.

My question is whether one could actually replace the MRF 9120 with a pair of MRF300s in this kit, and if so, how? Would the changes be doable by an inexperienced kit builder or would they be more complex?

In general, when thinking about building/modifying amplifiers, how can I reason about component-swapping - what are the most important things to check?

Answer 1

I didn't look up the differences between the two transistors; but the main question I have is, Will that heatsink dissipate 600 watts?

It's not likely that the stock heatsink in a 100 watt amplifier will, thus you risk destroying the bigger transistors.

After looking at the photos, what about the transformers and other components? Looks pretty risky, IMO.

I think your best bet is to spend the money on a higher and/or higher-gain antenna. You'll hear better too.

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In general, when thinking about building/modifying amplifiers, how can I reason about component-swapping - what are the most important things to check?

Well, there's really not a simple answer for that. If you would have asked that as a standalone question, it probably would have been marked as Too Broad, because there are countless different amplifier circuits, and often a lot of fine details. Sorry. :)

Answer 2

The link to the amplifier is dead, so I don't know what the schematic diagram looks like.

Out of the two transistors, if I were to design an HF/6m/2m power amp today, I like MRF300 much better. However, those transistors have a number of important differences, and I would consider they are not directly replaceable. There would be a number of adjustments to be made, and they are tricky enough that I would not recommend doing it if you had to ask what to do.

Those transistors have very different gain, input characteristics and the optimal loadline/supply voltage allocation. Basically, it would be more like re-designing the amp in key areas.