I have an Ameritron AL-80A that I've modified for a higher B+ voltage
(about 3600 VDC at 400 mA) and it generally works quite well. I've also
added new, much quieter, brushless DC fan which now works works fine after
some shielding between it and the RF deck, etc. However, I'd noticed that
the amp never loaded up well on 80 and 160 m after the mod: it was as if
there weren't enough tune or load capacitance. My perception was right on,
because the original B+ was about 2700 VDC at 400 mA, so (according to
Eimac's data sheet) the output impedance was around 3500 ohms. At 3600
VDC, it's closer to 5000 ohms. This changes things quite a bit for the low
bands.
To maintain the same loaded Q under these new conditions, the pi part
of
the output network (it's a pi-L) needs more inductance and less capacitance
for the same output impedance (the ideal inductance needed for the L isn't
as critical, though it changes slightly). Without increasing the
inductance, the capacitance needed for resonance rises significantly, as
does the loaded Q.
I found it easy enough to guesstimate how much to move the taps on 40
and
80 m, but 160 m isn't as straightforward, because there are no taps to
move: it's all there is. So, I added a small inductance -- about 10 uH,
or 13 turns of an AirDux 1609A I had on hand. There isn't enough room to
add this coil to the end of the existing inductor, extending it's length,
so I placed it perpendicular to the 160 m coil. This also places it
perpendicular to the 10-15-20 m coil a bit more than an inch from the end.
Afterwards, 40, 80 and 160 m all tune much better and efficiency seems to
be up. 20 and 15 m are as good as ever, but 10 m is now wacky.
Initially, everything on 10 m tunes up fine but, after about 15 s, the
output begins to drop. The output continues to drop and the plate current
rises. I know it sounds nuts, but overall, the match goes to pot, and I am
completely unable to tune the output! No combination results in a dip in
plate current or significant output. Obviously, plate dissipation rapidly
goes through the roof.
I'd guess that something is getting hot. It's not the parasitic
suppressor, since the resistor looks fine, and is without any
discoloration. I haven't screwed up the nerve to quickly rip the cover off,
throw a short across the B+ supply, and go touching everything to find the
suspected hot item, and I haven't operated the amp with the cover
removed. Upon inspection, I see no visual evidence of anything hot: no
discoloration and no melted solder. I haven't monitored it closely, but the
input impedance appears to remain normal.
Is the problem in the proximity of the new coil to the 10 m coil?
Since I
know how much inductance is needed, I can make this onto a powdered
iron-core toroid, which can be placed well out of the way if it would help.
All was fine on 10 m before I added this last coil.
What's up? I'm open to any ideas as to what this is and how I might
remedy the problem.
All the best,
Kim Elmore, N5OP
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