Hey guys...
The 8640B is a nice sig gen but NOT for testing high power FET amps! It
has a number of quirks / features that'll kill FETs. Ask me how I know....
Anyway...Ron... I'd be happy to take a look at your amp for 222. FETs
don't just go "pffffttt" for no reason. The protection circuitry attemps
to protect from outside issues like high rev power, overcurrent (which
I've never seen as an issue) etc. If there is some fundamental stability
or component issue with the amplifier the protection provides no protection
at all! The amplifier circuitry and design must be sound. I have been
running FETamplifiers both VDMOS and LDMOS since 2010 with ZERO additional
protection and have lost 1 FET on the 4 bands of amplifiers I have and that
was because the output chip caps for tuning were not rated for the current
they were seeing especially under JT65B 1 minute sequences! They turned to
a blob of melted ceramic and then the FET died as it was no longer tuned
properly (high vswr at the FET) that no protection would have protected
for.
Something is going on with your amp that is making it be right on the edge
of instability...most of the time it's OK....then there's that one instance
when something sets it off and the FET becomes a blown fuse.
Let's figure out how to get your amp in my hands and I'll give it a once
over...
73
Fred
On Wed, May 28, 2025, 7:43 AM David Olean <k1whs@metrocast.net> wrote:
> Hello GB Ron,
>
> Sorry to hear about the amplifier woes. The good news is that the
> typical gate failure does not usually result in smoke or burned
> component smell. Maybe something else let go. I was playing around with
> some 432 FET pallets. The things would put out 325 to 350 watts. I was
> runnng mine at 150 watts doing some tests and decided to turn it off.
> Rather than just turn off the drive power from the HP-8640B, I thought I
> would be smart and just turn down the drive with the output pot on the
> 8640B. That idea took out one of the FETs as there was a noise spike
> apparently made by the pot wiper, and one gate shorted out and pulled
> down the gate bias on both FETs. My 432 pallet turned into a brick. I
> think the best idea is to pick what is left of the brain of Mr. Fred
> K1FMS. He has so much experience with FETs and has plenty of ideas. He
> has mentioned that the gate bypassing on the Larcans is suspect. In
> broadcast service, they run 24 hours a day at full power and are very
> happy. When they run in amateur service, we are turning them on and off
> every few seconds and running them into all sorts of reactive loads that
> change as we turn and switch antennas. That is prime territory for
> parasitic belches and unforeseen behavior. I would think that Fred
> would help solve the problem as a public service just to keep you happy!
>
> He thinks that you need several chip caps in the gate bypass. The Larcan
> in question has a single chip cap there. A .001, an .01, and maybe a .1
> mfd chip there cannot hurt. I am going to do that mod to my 500 watt
> amp. Solving the WZ1V amplifier problem would be an excellent way to
> further the knowledge pool on FETs for amateur service. I am willing to
> lend a hand too. I just picked up a MRF151G FET from Fred, and it is
> sitting in its static free bag.
>
> I had a very good night on 222 MHz. The antenna repairs made a slight
> difference in performance. The grid current variations went away. I
> think I was seeing better signal strength too, but that is hard to tell.
> I worked K2RMX as my first contact. He was as loud as I have ever heard
> him. Next was KC2KAE, who was also an easy contact. PJ4MM got on and I
> ran with him around 23:00 UT and we worked with fair signals. Bacj]k to
> terrestrial signals and I worked some of wht you did, but missed KA1OJ
> and KA3FQS. I am not surprised as I was trying all sorts of modes that
> were not productive as a simple QSO on SSB or CW. I chased K3SK arund
> but could not detect anything on either FT8 or Q65B. We finally tried
> on meteors and we both copied each other, but did not get final RRRs.
> W5EME showed up in Louisiana and the Moon was setting on me so we
> stopped the K3SK sked and I looked for W5EME via EME. I must have been
> quite distracted, as I could not hear W5EME. After a few minutes, I
> noted that I was on the wrong frequency about 60 KHz up the band. Then
> a few more minutes passed and I realized that I was on 30 second
> sequences rather than 60 second sequences. Clearly my little brain cells
> were overloaded with all the activity and excitement. Once I corrected
> all of my errors, I did manage to work W5EME. I went back just above
> 222.100 and was looking for more stations. I tuned the rig to 222.105
> and asked "Is this frequency in use?" To my surprise I got an answer. I
> heard a big "NO". I started laughing. It was WB2ONA. Other stations
> worked include WC2K who was ungodly strong and a true S9. WA3NUF, W1AUV,
> WB2VVV, WA1MBA, N1JEZ, W9KXI, WA1NLG, K1TEO, N1SV, and W1XIV in FN34.
> I saw that W1AIM was on but I never did find Chip between all my skeds.
> I did work K9MRI via meteors for a nice contact. WA3EOQ also showed up
> at about 01:15 or so. He had a lousy condition on his end with lots of
> rain etc. We worked right away. It was an easy contact with weak signals
> on my end, but they were there and not fading below the noise. I never
> did get back to completing with K3SK. By 01:35 or so I was done and
> ready to have a fork stuck in me. It was a long day. I was up at our
> camp watching repairs to the plumbing done by a real plumbing crew. Last
> Fall, I had drained the water in the pipes, but I must have screwed up
> and did not open a drain valve near the shower. Water in the pipe froze
> and broke apart the ball valve and ripped the shower head off the
> shower. Fortunately that was all the damage. A new ball valve, so copper
> pipe and a new shower water valve and the rest of the system was ok. The
> camp now has water.
>
> A summary of 222 Night highlights from Mumbo Jumbo land. Tropo to
> WA3EOQ 502 miles, Meteor scatter to K9MRI 750 miles, EME to PJ4MM and
> W5EME both at 1500 miles or more. I almost completed with K3SK on
> meteors, but struck out with tropo scatter at 575 miles.
>
> 73
>
> Dave K1WHS
>
> On 5/27/2025 8:24 PM, Ron Klimas WZ1V wrote:
> > 222 conditions pretty good tonight, worked:
> > K2RMX FN20, KC2KAE FN20, K1WHS FN43, WC2KFM29,
> > WA3NUF FN20, K1TEO FN31, W1AUV FN32, WA1MBA FN51,
> > WA1NLG FN41, WB2VVV FN41, N1SV FN42, W9KXI FN12,
> > N1JEZ FN44, K3SK FM07, KA1OJ FN42, and KA3FQS FN20.
> > Unfortunately disaster struck after only an hour of fun,
> > I was calling on SSB when the amp made a PFFT noise,
> > output went to zero and then came the smell of smoke.
> > Obviously neither the W6PQL protection board
> > nor the fuses did anything. This if the 3rd time this
> > amp has fried itself for no good reason. I give up.
> >
> > 73 Ron WZ1V
> >
> >
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