Consider how many of this very popular amp are still used regularly without
making "Internet Changes" to join a few other lemmings.
Carl
KM1H
----- Original Message -----
From: "Don Jones" <ko7i@comcast.net>
To: <amps@contesting.com>
Sent: Saturday, March 29, 2014 9:12 PM
Subject: [Amps] Drake L-4B Should I or Shouldn't I?
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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