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You reminded me I left out an important piece. I added 10 ohm
resistors in series with each of the gates. The circuit was
oscillating at 120 MHz, which is probably why the IRF530s blew. I
revised schematic is attached. For the 20 meter band, I made 2 air
core coils I wound on a 1" D piece of schedule 40 pipe using 9
turns 14 AWG wire. The pipe is just a winding form, not used in
That is about .9 nH each. The caps were 240 pF at the input at
output and 360 in the middle.
I posted another schematic yesterday with the LPF designs for all
I adjusted VR1 in the Ubitx to get the correct drive level. Of
course the uBitx's output is not uniform across the bands, so you
need to set it properly for each band. I had to drive the amp at
28 VRMS to get the full output. I'm not sure what the input
impedance is, so I can't measure the drive level in watts.
However, at the same setting the uBitx was putting out 24.4 volts
RMS into 50 ohms, or 12 watts.
On 6/10/2018 10:08 AM, Nick wrote:
This is good
information. I also picked up the same amplifier and the IRF530s
were quick to self distruct.
When testing into a 50ohm dummy load I was initially only getting
about 10 watts. The resistors that came with the unit were 100 ohm
instead of what I found in the online schematic of 150ohm .
I made a low pass filter for it but I think the toroids are a bit
small as they got really warm. I also made an attenuator pad at
the input as I read that the pa does not seem to like more then
about 2.5watts in. The pad was a 220, 22, 220 with about 4db
I found that with my o'scope the output waveform from the bitx was
very distorted into the amp which could explain the low power and
heating in my filter. In an attempt to figure out what was going
on, I modeled the amplifier in ltspice .It did not work well as
per the schematic I found. I had to increase the bias voltage and
put a 10 ohm resistor across the two mosfet gates to get a nice
input swr and a somewhat nice 70ish watt waveform out of it. This
was simulated at 7.2Mhz with the ltspice model of the irf530 and
all other components were ideal.
Since it looked good in simulation I attached the resistor and
started increasing the bias voltage. I was not using a very small
current limited power supply and while doing this there was a
small puff of smoke and my supply voltage dropped to nothing.
I suspect not paying attention to the drain current may have been
a problem. I think maybe they turned on and "shorted" at dc. Not
sure if I trust the components that came with the kit though as
they were in standard bubble wrap with no esd protection for the
MOSFETs. They could also be fake MOSFETs.
I don't currently have replacements available but I might try your
suggestion of using the 520s in parallel.
I appreciate your post,