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Hi Peter,
Indeed the 50V supply voltage must have been too high as a start for the amplifier if there was no any previous matching / tuning attempt at a lower supply voltage.
Regarding a setup process, here is a possibility. This text is going to be long, sorry, I think it is needed.
See a modified input circuit for the amplifier attached, I added a resistive divider ("a resistor Pi") ahead of the input Pi filter. The 470 Ohm potmeter can be any normal one, not a wire wound type, to be able to reduce the > 1 W RF power from the output of your preamp to 0.2-0.3 W level, not to overdrive the linear amplifier.
Next, remove the MOSFET, put a 360 pF across the 22 Ohm resistor and drive the input of the resistive diivider from the preamp. Adjust C1 of the Pi filter for max sine wave amplitude around 45 MHz, measure across the 22 Ohm resistor with the scope probe. Check by turning the 470 Ohm potmeter between what minimum and maximum range it can vary the RF voltage amplitude across the 22 Ohm. Leave the wiper at say 2-3 V peak to peak across the 22 Ohm, if this low amplitude is not possible (i.e. at the full 470 Ohm wiper position the voltage is still higher than that), then place say a 100 or 150 Ohm resistor in series with the potmeter and repeat this adjustment.
Now remove the preamplifier, i.e. no RF drive, and also remove the 360 pF cap which so far substituted the gate-source capacitance and put the MOSFET back to the circuit. Also, remove the choke from the drain and put a 50 Ohm dummy load instead, capable of the expected 15-20 W dissipation. It is ok you wish to see and check the signal at the drain but you have to have a load there too for feeding the MOSFET with the supply voltage.
You may wish to start with much lower than 50 V like say 10-12 V DC input only. First the DC operating point is to be adjusted with the Set Bias potmeter: you need to measure the DC drain current and set it first say to 200 mA only at the 10-12V supply voltage and no any RF drive yet. The wiper of the bias potmeter should be set in advance to start from about 2V because the minimum threshold voltage for this MOSFET is 2V. But your MOSFET may have say 3.2V threshold voltage from where drain current can start flowing at all, so slowly turn the bias pot up from the 2 V and watch the drain current rising to about 200 mA.
Now if you increase the supply voltage towards 40 or 50 V, still monitor the DC drain current, it will increase above the previous 200 mA of course (without touching the already set bias potmeter) and the heat dissipation at 50 V will increase beyond 10 - 12 W, a big heat sink is needed for the MOSFET of course. Naturally the same drain current will dissipate heat in the dummy load too, it should be able to dissipate it.
I suggest to run the MOSFET at this operating point for at least several minutes to check temperature and drain current stability (with still no RF drive input yet).
If all seem ok, then increase drain current up to 400-450 mA, supply voltage is at 50 V and see how heat sink temperature increases, hopefully a small ventilator would not be needed to cool it. Run the setup for several minutes at least.
Then, if you think, apply the smallest RF drive the 470 Ohm pot let through (as previously set above) and check the RF amplitude across the dummy load or across the drain and the negative rail, ideally they have the same voltage across them. Then you could increase RF drive by turning the 470 Ohm pot and monitor the drain current and the RF amplitude at the drain or the dummy load.
It is possible you need to retune a little the C1 cap in the input Pi filter when the supply voltage for the MOSFET is in the 40-50 V range because the gate-source capacitance changes to a lower value and this may affect input Pi filter matching.
It is possible the RF input drive to the MOSFET (depending on where the 470 Ohm pot is set) will increase drain current beyond the DC current previously set by the bias pot. Well, a small overdrive is not yet a problem but you can see this as a starting clipping (limiting) at the peaks of the drain-source voltage wave form. It is also possible you adjust DC drain current to say 600 mA to increase output power, this would allow an increased input drive too to achieve higher output, only the MOSFET dissipation is the limit.
If all seems ok, then so far the 0.5 A drain current at 50V supply voltage will provide roughly 12.5 W RF power in the 50 Ohm dummy load (the other 12.5 W is dissipated in the MOSFET). Such is the case for an ideal Class A power amplifier.
Next step is to agree on how you try to match the coil with the iron powder core to the drain of the MOSFET. The Pi filter at the output first sounds good but your coil will represent either a much higher than 50 Ohm impedance load when tuned to be a paralell resonant LC tank at 45 MHz or will represent a much lower than 50 Ohm load when tuned to be a series resonant LC circuit, so the output Pi filter should be thought over.
Gyula
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Topic: Towards a 45.525MHz 16 Watt Amp (Read 19662 times)
