>Hi all,
>
>A few weeks ago, I mentioned that I had an AL80-A that I had modified for a
>higher B+ voltage, mainly for the educational value. After putting it back
>into service after a 7 yr hiatus, I had some fireworks and asked for
>advice. I thank everyone who responded. That aspect has now been tamed. I
>did some other mods, such as added a DC "brushless" fan in place of the AC
>fan, which moves more air and is much quieter, but must be shielded from
>the RF deck, lest it stop when I transmit.
>
? Did you try bypassing + to at the DC input?
>I hadn't had much chance to use the amp immediately after my mod, because I
>relocated to Oklahoma from Colorado. So, I was fuzzy on its operating
>characteristics. About all I recall is that, after the mod, I wasn't very
>happy with it on the low ends of 160 m and 80 m, but that 75 m and all the
>other bands seemed fine. The move, new job, and fresh biological harmonics
>left little time for radio, and it, along with all my other
>equipmentcollected dust .
>
>When I started this, about 7 years ago, that I fully realized that 1)
>modifying perfectly fine, working commercial equipment is often a fool's
>gambit, 2) raising the B+ from 2700 V to 3700 V (key-down) is pushing the
>components to their limits, and 3) an extra 1000 V nets me only a nominal
>1.4 dB increase in power output, which is of no practical
>significance.
? amen to that, but don't mention this to a rabid DXer or he/she is
likely to start frothing.
> Yet, in the face of all that, I had the itch. I wanted to
>*do* this, and see, hands-on, how all of this stuff works. I don't have the
>tools, and only barely the metalworking skills, to fabricate the hardware
>needed for a complete homebrew amplifier,
? The 20db, tetrode-with-handles amplifier on my Web site was built with
a vise, a coping saw, a drill, a square and other basic hand tools.
>and I don't have the
>facilities. So, I was left with fixing stuff that ain't (yet) broke.
>
>After the mod, I knew that the output impedance of the 3-500Z would be
>higher, but decided to "just see" how things went. Sure enough, all the
>knob positions were different and I couldn't tune the amp well on the low
>end of 80 and 160 m. It was as if I needed more tune and especially load
>capacitance. Fiddling a bit with the equations and a simple program showed
>me that the change in output impedance could not be easily ignored on the
>low bands, especially.
>
>The output network (a Pi-L or L-L, take your choice) now had too high a
>loaded Q (probably around 16-18 or so) and needed far more overall
>capacitance because the inductance of the Pi part remained constant. I
>guesstimated how much to move the taps to increase the inductance and bring
>the loaded Q (and so capacitance requirements) back to a reasonable range
>for the low bands (40 - 160) and everything worked great, except
>160. There are no unused turns on the Pi inductor, so I had to add some.
>Along the way, I found that the resistor in my parasitic suppressor had
>been baked to a value of ~10 ohms, and so I replaced that.
>
? more DC volts = more amplification. The most destructive parasite I
ever encountered was at c.45MHz in a 12MHz 80kW PEP amp that ran enough
anode V to make S9+15db X-rays.
>...
cheers, Kim
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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