I see Hans has eliminated the option (PC board holes) to add zeners across the BS170 transistors.? I added 47V zeners to my QDXs and QMXs to protect against potential breakdown from bad SWR.? It seems to work.
This technique was common with npn power amplifiers. It's generally not employed with mosfet finals. I don't recall myself ever damaging a qrp mosfet final. Whether a zener even aids survival of mosfet finals maybe someone else can answer.?
As for economics bs170 devices are pennies instead of multiple dollars for few watt npn. Also zener diodes may be more scarce now.?
Yes I have transmitted into the wrong antenna before. Another approach is using a efhw and first insuring it's connected.?
Funny you should say that Bruce.? My physics professor at university was a huge Richard Feynman fan and had his famous quote "How do you know that?" pinned above his desk. We all make unqualified assumptions but engaging that quote has saved me significant embarrassment over the years.
I have had to replace BS170's in QDX and QMX four times before adding Zeners..
I was using an auto tuner before I realized they have big SWR spikes - possible cause of failure
I also found an intermittent in a coax cable? - possible cause of failure
No failures since I put the Zeners in - not proof they work, could be coincidence
Even a fast voltage breakdown of BS170 will probably cause permanent damage.
I am concerned that the Zener's capacitance may be detrimental during 6m operation.? I have been unable to find the Zener's capacitance, I may just see how the QMX+ works with and without Zeners
The datasheet from Diodes, Inc.? for Zeners in the family that includes the 47V 1W Zener 1N4756A shows capacitance information. The capacitance may be big enough to detune the 10m and 6m LPFs.
On Thu, Jun 6, 2024, 11:10?AM Chris KB1NLW via <chrisrey1=[email protected]> wrote:
What do I know?
I have had to replace BS170's in QDX and QMX four times before adding Zeners..
I was using an auto tuner before I realized they have big SWR spikes - possible cause of failure
I also found an intermittent in a coax cable? - possible cause of failure
No failures since I put the Zeners in - not proof they work, could be coincidence
Even a fast voltage breakdown of BS170 will probably cause permanent damage.
I am concerned that the Zener's capacitance may be detrimental during 6m operation.? I have been unable to find the Zener's capacitance, I may just see how the QMX+ works with and without Zeners
Steve Weber uses a 1N4756A diode in the QRP Guys FT8 transceiver.? ?We have sold over 1000, an to our knowledge,? never had a BS170 fail.? I know it's empirical,? but it does support what Chris is saying.?
Steve Weber uses a 1N4756A diode in the QRP Guys FT8 transceiver.? ?We have sold over 1000, an to our knowledge,? never had a BS170 fail.? I know it's empirical,? but it does support what Chris is saying.?
You must have been worried about it, your manual says: "Operating or testing into a bad SWR or no antenna will permanently damage the
three power amplifier BS170's. Be careful..."
That was cya for us.? I am interested in hearing why you don't? include one.? Is there a reason?
Originally I believed the BS170s in QDX/QMX would see less drain voltage anyway than in a QCX (with it's three parallel BS170) because QDX/QMX has a different PA design, it is push-pull so the voltages are half. And a QCX doesn't have zener diodes. Again QCX is a different design to yours because QCX is Class E.?
The capacitance of the diode could affect the LPF. Furthermore since the capacitance may vary with the RF cycle it may increase the harmonic content. It all would require some non-trivial testing to determine conclusively that there are no I'll effects. As yet I have tested QDX/QMX into shorts and open loads and it doesn't generate higher than spec voltages at the BS170 drains. As far as I'm aware nobody has yet demonstrated a load that can show more than +60V on the drains. Soz frankly it didn't become the highest priority for me among many competing priorities.?
QMX in any case has SWR protection and envelope shaping, and the SWR protection will protect the BS170s in any event. So it's really only QDX we're talking about.?
On QDX (not QMX) there's also the question of the very short dI/dT spike caused by the FT37-43 choke but IF that is an issue (also debatable) the better solution for that would be a small conventional diode across the choke, which has also been discussed here.?
Thank you Hans.? I knew that you would increase my level of understanding.? ?The student defers to the master,? again, lol.? Can't wait to get mine on the air.? 72, Doug?
Bruce Akhurst
Ted 2E0THH
Barb WB2CBA