On 28 Sep 2005 19:04:34 -0700, "WSQT" wrote:

We pirates have made the common, ordinary IRF 510 MOSFET work much
higher than the 6M ham band-like at 88MHZ! For several months WSQT
Guerilla Radio(The Squat in DC) was on 87.9 at 10W with a single IRF
510 as the final. Details are in an old post to alt.radio.pirate. We
later went up to a dual 6146 tube final for more range, and are now

snipped

Several questions. Operating mode? Class A, B, C, D, E?
Stage gain?

Answer makes a difference. SSB required at worst class B
and class C would cause way to much distortion. While FM
is typically class C as there are no amplitude variations. This
makes a great difference is operating characteristics and
power out.

Since SSB and most amateur uses are narrow band by nature
any spurious outputs are undesired. Amps using the IRF510 at
HF have been built but their purity (IMD and harmonics) is only
adaquate to barely so and tends to degrade as frequency is
increased.

FYI: to feed an IRF510 as broad band you need to match the
50ohm input to less than 2 ohms reactive at VHF. This is
hard to do for greater than octave frequency ranges as it
is all reactive.

With that I've run an IRF510 at 6m (50mhz) linear (class ab1)
with a stage gain of around 9db and with 24V around 4W.
It was difficult to match, unstable tending to oscillate, and
prone to thermal runaway (case has high termal resistance).

A scrap final from a CB (2SC1307) gave me 7W, clean at 12V
and far less stability and matching issues plus cleaner IMD.
For the same effort a MRF492 at 50mhz easily delivers
90w at 12V and 11db gain as linear. That same 2sc1307
class C will deliver 12w and would drive a MRF492 to
100w class C.

To put it bluntly, for the effort to get 10W out of a IRF510
I can do far better with a 22$ transistor and still be ahead.

Allison
KB1GMX