开云体育

If you want to be bulletproof while using RD16HHF1's (Vds-max of 50v)
you definitely want to keep the power supply for the finals down around 12vdc.?
The IRF510's have a Vds-max of 100v, and so can work fine with a 25vdc power supply.
If they have an adequate heat sink.

Looking at the uBitx schematic, I believe we could remove L8 and L9, and instead
supply 12v to a center tap of the T11 primary (many push-pull amps are designed this way).?
The circuit functions the same, but this arrangement is easier to visualize.??

When the bottom FET is turned on, pin 1 of the T11 primary is grounded.
With the center tap at a constant 12vdc, pin 6 of T11 (the other end of the primary)
jumps up to 24vdc.? ?On the next half cycle the top FET turns on, pin 6 of T11?
gets grounded, and pin 1 of T11 shoots up to 24vdc.

So Vds of each FET regularly reaches a max of twice the power supply voltage.
It can be somewhat more because the transformer at T11 is not ideal.
If we push that power supply to 25vdc, then each FET sees 50 volts, the absolute
max for an RD16HHF1.? Not good.

Running without the antenna, the voltages at the primary should remain about the same.
It's like running a 6.3v filament transformer without a load.?
But nothing is ideal, voltage spikes on the FET drains will rise up some.

The old Bitx40 with its single IRF510 in the final is considerably different.
Where the uBitx's T11 works as a "forward transformer", the output transformer T7
of the Bitx40 works as a "flyback transformer".? With no load, the voltages around
a flyback transformer head for infinity.? This is where you really have to worry about
what happens when the antenna is left disconnected, and why you often see a zener
diode placed on the output of an RF amp.? In the case of the Bitx40? operating at 7mhz,
the Miller effect happens to keep those voltage spikes on the drain in check
because they get coupled to the gate as negative feedback to slow things down.
Here's a dimly remembered conversation about this;??/g/BITX20/message/41057

Jerry, KE7ER