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[Amps] A simple route to a 28V mobile HF amplifier?

Subject: [Amps] A simple route to a 28V mobile HF amplifier?
Date: Sun, 08 Jun 2014 11:41:40 -0500
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Lately I have been looking at transitioning my mobile HF amplifer from 12V to ...more. Currently it seems that 50V is all the rage; but that's not so simple to implement. 28V would be a lot simpler.

I have a number of DU2880U HF MOSFETs. They are 28V devices, 2-150 MHz. I'm guessing they must have been among the first 28V HF MOSFETs, because they are rated for only 80W output. What's intriguing about them is that they are physically identical to the common MRF454/458 and 2SC2290/2SC2879 BJTs.

It occurred to me that maybe the fastest and easiest way for me to get to 28V is to take an existing HF solid-state amplifier and substitute in the DU2880U devices. There are a few things that would have to be changed in the donor amplifier that I've thought of:

1. The existing control circuitry is all 12V based, but that could easily be managed in a battery-powered situation by wiring the DC feed as 0/+12/+24V. 2. Impedance levels are different, so the input and output transformers for each amplifier module would need to be changed. One plus is that the impedance ratios are more favorable at 28V. 3. RF circuit changes (for example, the input gain equalization), but those should be pretty simple. 4. Bias circuit would need to be modified for a higher voltage. But the current capability of the existing circuit would be way more than adequate.

Other than that, everything else should be reusable as-is...Low-pass filters, T/R relays, monitoring, input splitters and output combiners, cooling...and most importantly, all the chassis work and packaging is already done for me. As an added bonus, I'd have a set of BJTs to sell to offset incurred costs.

I do have a "parts" Ten-Tec Hercules II chassis that is 95% complete. The nice thing about the Herc II is that the construction is very well modularized, and I think it would lend itself well to such a modification.

Am I missing anything? Or just crazy? I would very much welcome any comments or pointers.
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