I run the IRF 510 essentially at the "class B threshold"(leakage
idling current but nowhere near linear range), simply because I could
not drive it hard enough to run it without bias like in the AM BC band
transmitter(Oct 03-Jan 05). Envelope distortion is not a factor when
frequency modulation is in use, of course. Stage gain with 1W drive and
10-13W out is essentially 10 db-not great, but I've seen a hell of a
lot of RF transistors speced with gain from 10-13db.

If you want to use the IRF 510 for SSB, I have no idea what the IMD
will be,but I've used them at audio as modulators for the old AM rig.
If anything, your stage gain will be the same or greater. I used 3V
bias, generating leakage current only in the FM rig. For good linearity
at audio(and presumably for SSB as well), you need to bias them further
forward than threshold(they are NOT like darlingtons at audio for
this!), and you will get more heat. I've had them on the scope at
audio with sine wave drive, and they needed to go well into AB1, so
much so that they made measurabgly more heat than darlington bipolars
did for an audio amp.

No way will they be MORE linear at RF than audio, so run them in AB1
if you want AM linear or SSB. CW or FM, just set the bias pot for
maximum OUTPUT, and you will get about 3V when running in the VHF
range, as turn-off time seems to lower output with more bias. As a
result, they like to be "centered" between "on" and "off" for maximum
efficiency, which still sucks at VHF.

I would not be entirely suprised if an IRF 510 amp proves a CW and
FM only deal at 6M, and I would bet on it at 2!

On the other hand, efficiency at 6M will be better than at 3M, so you
should be able to stay within acceptable dissipation, even if AB1 bias
is CUTTING output with the same drive as "threshold B" bias. ****-SSB's
a low duty-cycle mode, while at FM the duty cycle is 100%! This should
compensate for the extra heat of AB1 operation necessary to make the
IRF 510 linear at ANY frequency, even audio.

When I used the IRF 510 in the AM band as a drain-modulated Class C
pair, they could make 55W carrier but only sustain 35-40W carrier for
30 minutes operation. Years ago, there was a well-published design for
a broadband HF aming this device, accepting as much as 80W pep at 40M!


Spurious outputs, BTW, are as undesired for what we do as they are
for amateur work. If you get spurs and they bring the FCC, you can
probably resolve the issue. For a pirate, spurs can cause mean MUCH
more serious "difficulties!" I have a lot more trouble chasing them
out of exciters than amps, but always check out both very carefully.

Voltage used in the FM rig is 30V, the AM rig was 17 volts B+, going
to 34 at 100% mod. With tuned input and output circuits stability is
excellent, and unlike multi-2N222 drivers I've never had to chase hash
or oscillation with the IRF 510. Of course, a big bulky heatsink
serves as a groundplane, with all parts screwed to it.

Glad to hear I'm not the only one to push the IRF 510 above 30MHZ!




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

Cut to the chase. The only thing the IRF5xx series has going for it
is it's dirt cheap. The problem is that coaxing them to deliver is
more pain than better devices. Good devices in reasonable circuits
have one advantage, repeatability of performance.

As to MW am service, I'd use the heavier devices and run them class E
in the several hundred watts to KW range. Even the IRF510 in class E
is good for more than 50W for a pair and should run reasonably cool.
Even the really big E amps only need a few watts as excitation.
This works because they are being used as switches, what they were
designed to do. A class E amp can be "plate modulated" in the usual
way but a PWM modulator will be far more efficient. There are a
number of people running class E Am phone on 160, 75 and 40m and
they sound very good.

For decent RF designs that withstand impossible swrs a good RF rated
device is hard to beat. The real problem is finding a source that
doesn't mark them up 500%. However there are some devices out
there that are like the venerable 6146 they do the job and can be had
at less than 0.40$/watt. For 6m the MRF492 for example rated
power out at 12V of 70W and most deliver 80-90. For the price it's
cheap.

For 2m I'd just grab a MRF247, easy 75watts at 2m class C or B and
a pair with Wilkerson splitter/combiners deliver a heafty 145watts on
the Bird. They work well for 2m SSB service with decent IMD. the
last pair(matched) I bought as for a commercial amp repair only cost
70$. For that kind of power that's cheap.


Allison
KB1GMX