Evan,
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You are correct, I missed the 100pF cap.
Thanks.
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I had noticed that computed values for L508 and L512 were different than the ones Hans had said he simulated with.
I figured since I was simulating may as well go with those, and that final windings got adjusted when he was looking at hardware.
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I have made both adjustments to the simulation in the attached file QMX_sim03.asc
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Will try your 3:2 12v transformer, but haven't yet.
In simulation it may not make too much difference, I'd expect that all the voltages after the transformer just go down by that ratio.
From forum posts, it seems people have fewer failures of the BS170's with a 9vdc build, so I figured I'd start with that.
However, perhaps that's only because most people have built for 12vdc.
It does seem that with the higher currents of a 9vdc build we would have more trouble with overheating in digital modes.
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Here's the voltage readings from LT-Spice for my original QMX_sim01.asc simulation before and after your changes.
Curious that adding changing the L508 and L512 values brings the output down, but adding the 100pF cap brings it most of the way back up
Output: ?
? ? ? ? ?18.6v as in sim02
? ? ? ? ?15.6v when adjusting L508 and L512 to AL*(t**2) values of 1.875uH and 1.452uH
? ? ? ? ?18.4v when 100pF added
? ? ? ? ?18.4v when both changes made
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I then looked at that final version with both of your changes, and compared it with the same but without L502,
and also my VMCD and CMCD versions as in the attached file QMX_sim03.asc.
Since Vds-peak values can blow the BS170's, I also looked at peak voltage on one of the drains.
? stock (with both changes) ?vs L1-removed vs VMCD vs CMCD:
? ? Output? ? ?18.4 ?vs ?13.8 ? vs 14.4 ?vs ?18.5
? ? Vds-max? 33.8 ?vs ?18.5 ? vs 18.5 ?vs ?27.5
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? Output with L1 removed is 13.8v as shown, but goes higher to 14.1v if L508=1.83uH,L512=1.55uH)
? For VMCD, FET voltages are square and current is sinewave, for CMCD current is square and voltage is sinewave
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Note how much lower Vds-max is for L1-removed and VMCD.?
L1-removed is effectively VMCD, but using the stock LPF which I am still unsure of.
L1-removed and VMCD both also have low output power, which is a function of the square of the output voltages recorded above.
If we were to increase the transformer ratio of the L1-removed version such that Vds-max was equal to that of the stock QMX,
it seems the output voltage should increase to a value of 13.8*(33.8/18.4) = 25.35, which is way beyond the stock output voltage of 18.4
That's the reason I was sucked into this, removing L502 may help with the over-voltage issue at the BS170's
though this may bring on issues due to higher peak currents.
Like all things around RF amps, it's considerably more complicated than one might think at first glance.
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Anyways, I was up half the night here with a hacking cough, which is why I got that simulation done.
Probably time to get some sleep.
Given my current state, could well be plenty more errors in my attempts at simulation and in the above discussion.
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Jerry, KE7ER
