I have an L in pretty good condition that I've been using as an
experimentation amplifier with an eye toward eventually building my own.
I've spent quite a few hours with it so here's my two cents....
> 1. Since I intend to run this amp on SSB very conservatively to prolong
> component life ( and my pocketbook), should I run it in the CW
> (low power)
> position (approx 1500V @ 670 Ma) or in the SSB (high power)
> position (approx
> 2500V @ 400Ma)?
The bias on the tube in low voltage operates the tubes closer to class B.
In high voltage, the tubes are operating in AB. My SSB signal is cleaner
when operating in the high voltage position. The low voltage position is
better suited to CW or RTTY.
>
> 2. Are there any drawbacks to conservative operation of this amp
> (other than
> the obvious foregoing of 1-2 dB)?
You can run the amp at the higher voltage and lower the exciter level if
you're concerned about smoking your antenna or disrupting TV service. The
plates will run cooler.
> 3. What modifications are cost effective for this amp?
I've done the following things to the L:
1. Installed a Peter Dahl replacement transformer. This was pretty costly
and was necessitated by the demise of the original transformer. The new
unit is more robust than the original.
2. Switched the unit to 220 volts. 110 was just not enough...
3. Converted the 12 volt relay/lamp supply to full bridge and then post
regulated with a LDO regulator. This mod will generate a stable 12 volts so
that the pilot lamps don't dim when you key the amp and keeps the supply
noise out of your keying rig. This mod was more aesthetic than anything
else.
4. Installed a 12 volt powered (very quiet) blower that uses the above
regulated supply. The AC blower in my unit rattled and shook probably due
to bad bearings. I also shed the big AC dropping resistor that was used to
lower the AC fan speed.
5. Installed new parasitic suppressors. My old ones were severely deformed
from years of abuse. I bought mine from Al Harbach since he's right down
the street.
6. Recalculated the output PI network. The original L schematic called for
an 800 pF output C. My unit had a much smaller 325Pf unit in it. From the
available space in the chassis, it appears to be a manufacturing change and
not a mod that was installed by a previous owner. The smaller output C was
always in the maximum position. Based on the calculations, I added 300 pF
across the output C and added an additional 1000 pF to the 160 M tap. This
radically improved the tuning of the unit and bumped the power on all bands.
I now get the following key down in SSB mode(for short periods, of course):
160M 1100 W
80M 1300 W
40M 1300 W
20M 1200 W
15M 1000 W
I removed the block in the switch to open up the unit for 10 meters but it
runs away when I key it. I'm in the process of adding neutralization to fix
this one.
I'm also building up a tuned input circuit as mine was an older unit that
did not have one. I drive the rig with a TS950. The built in antenna tuner
matches the rig to the amp pretty nicely so for this configuration, I really
don't need it. But, this was a good chance to build up an input PI network
and see how it works.
Hope this helps,
Steve Iezzi KT4FY
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