Last year I picked up a L-4B from an estate sale. About a month ago I
tried to fire it up. Looks like I got some problems to repair first. I
did hear a thump come from the transformer of the L-4PS as it charged
the high voltage cap bank. No biggie, also I did see the 3-500's light
up in the RF deck.
What I did not see was any indication of High Voltage being present on
the L-4B's voltage meter. Fingers crossed I do not have a blown meter.
What should I see for pin to pin resistance of the High Voltage meter?
Also what resistance should I see for the plate current meter? This has
me puzzled, I would think that the meter should have shown that high
voltage was present. I presume that this is a proper assumption. Correct?
Since I suspected there was problem, I stopped there and did not attempt
to "key" the amplifier and see if it would tune up. I pulled the 240VAC
power plug and let it set for a few days. After which I opened the high
voltage power supply section and inspected it internally. The power
supply has the original paper caps that came with it from the factory. I
did not see any blown diodes or smoked resistors, the paper caps still
had their guts which is a good sign.
After some thought and having good service from Harbach Electronics with
a Clipperton-L power supply rebuild kit I decided to order one for my
L-4PS so I could get those old caps and diodes replaced. I also ordered
a new rebuild kit for the front panel power switches and a soft start
kit for the amplifier too.
As for the Should I or shouldn't I question...
I am going to operate this amplifier with an out board QSK switch and
bias control. I am thinking about hard grounding the grids and
installing a 50k ohm resistor in the B- lead to cut off the tube instead
of relying on the +150Vdc (or so) cathode voltage as designed from the
factory. I have heard "internet stories" claiming that the L-4B can
become unstable if the grids are hard grounded. Something about the gain
of the tubes becoming higher and unstable. Is this true or not? I do
want to use this amplifier on all bands including 10 meters.
My bottom line goal is to run this amplifier at about 750 to 800 W rms
out and enjoy it for many years to come. Looking for some technically
sound advice and objective criticism. Sorry for being long winded. Thanks!
73, Don KO7i
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