Get rid of those big bleeders. Disconnect the 120 volt bias line from
the relay and put a 50k to 100k (1 or 2 watt is fine) resistor between
the center tap and B-. Use the relay contacts to short out the
resistor on transmit. The tubes will self bias themselves to cut off
the resistor will keep the cathodes from floating up too high when the
relay opens.

You will also loose the alc holdoff voltage but no need for alc
anyway.

73
Gary K4FMX


On Sun, 04 Mar 2007 20:55:25 GMT, (Rick) wrote:



Yes it does. Drake did an excellent job when they designed and manufactured
the L-4B.

Well, I'll say this- it sure is a pretty amp, with that lovely blue glow from
those meters and that warm glow from those beautiful filaments.
Mine was cranking my signal out in fine style the first 24 hours of the dx
contest, getting me easily across the pond to all over Europe from NJ on 80
meters and making me quite proud.
I wasn't even transmitting when all of a sudden..... ka-pow ! a flash of light
from the power supply, and I dove for the power switch, and unplugged the
line. Then the smell of burning something or other.

So far here's what I've determined, then we can debate how excellent it was
designed. There are 2 bleeder resistors up on top of the PS on either side of
the transformer. They are 50k at I guess about 50 watts, and if so, are
within spec even at 3000 volts. One of them opened up for some reason, and I
can't think of a eason for a bleeder to fail other than it just got to be too
old. Now, 120 volts is derived from the bottom of the bleeders through
another power resistor of about 5 k, I forget exactly. This positive 120
volts is used for the antenna relay and in standby is applied to the CT of the
filament transformer to cut off the tubes in standby mode. So when the
bleeder opened, the HV probably went up a little, and the tubes drew some
extra current, I don't know how much, or how long but eventually it blew a
little 0.82 ohm fuse resistor in the HV lead.

Still TBD is how much damage was done to the tubes due to the loss of bias.
I really don't like the way they get bias and will consider putting in a
little 100 volt transformer in the RF section to derive my own bias. In fact
it could even be redundant, fed in parallel with the original through a diode.

Now I am searching for a replacement bleeder, has to be the exact size as the
original because there isn't any extra room in the PS. Gee I'd like to put in
a bigger bleeder, because bleeders should not fail. Wonder if Drake
considered this senario when the L4 was designed. Maybe so, maybe they
figured no serious damage could come to the tubes and eventually the PS failed
safe and stopped before the house caught on fire, because this could have
happened when I wasn't in the shack.

Rick K2XT

Several comments about the bleeder resistor failure. Other than the finals,
those two bleeder resistors "bleed off" more heat than any other component.
My L-4B is about thirty years old, and I am guessing that the design
engineers probably used 20 years as a useful design life. You and I should
be happy that the resistors have lasted for 30 years. I am sure that the
life of these resistors are affected in large part by how often they are
cycled from room temperature to the operating temperature of perhaps 130 ±
20°C. Was the power to your L-4B cycled on a hourly basis, or was your
amplifier powered on for the entire day?

If you cannot find suitable resistors, perhaps you could replace them with
resistors of a larger ohmic value, and then bias the finals so that their
resting current is about the same as the value drawn by the original bleeder
resistors.

My last comment is in the form of a request. When and if you find
replacement resistors that are physically and electrically suitable, please
let me know the source so that I can buy two as spares.

John, N9JG

"Rick" wrote in message
...


Yes it does. Drake did an excellent job when they designed and
manufactured
the L-4B.

Well, I'll say this- it sure is a pretty amp, with that lovely blue glow
from
those meters and that warm glow from those beautiful filaments.
Mine was cranking my signal out in fine style the first 24 hours of the dx
contest, getting me easily across the pond to all over Europe from NJ on
80
meters and making me quite proud.
I wasn't even transmitting when all of a sudden..... ka-pow ! a flash of
light
from the power supply, and I dove for the power switch, and unplugged the
line. Then the smell of burning something or other.

So far here's what I've determined, then we can debate how excellent it
was
designed. There are 2 bleeder resistors up on top of the PS on either
side of
the transformer. They are 50k at I guess about 50 watts, and if so, are
within spec even at 3000 volts. One of them opened up for some reason,
and I
can't think of a eason for a bleeder to fail other than it just got to be
too
old. Now, 120 volts is derived from the bottom of the bleeders through
another power resistor of about 5 k, I forget exactly. This positive 120
volts is used for the antenna relay and in standby is applied to the CT of
the
filament transformer to cut off the tubes in standby mode. So when the
bleeder opened, the HV probably went up a little, and the tubes drew some
extra current, I don't know how much, or how long but eventually it blew a
little 0.82 ohm fuse resistor in the HV lead.

Still TBD is how much damage was done to the tubes due to the loss of
bias.
I really don't like the way they get bias and will consider putting in a
little 100 volt transformer in the RF section to derive my own bias. In
fact
it could even be redundant, fed in parallel with the original through a
diode.

Now I am searching for a replacement bleeder, has to be the exact size as
the
original because there isn't any extra room in the PS. Gee I'd like to
put in
a bigger bleeder, because bleeders should not fail. Wonder if Drake
considered this senario when the L4 was designed. Maybe so, maybe they
figured no serious damage could come to the tubes and eventually the PS
failed
safe and stopped before the house caught on fire, because this could have
happened when I wasn't in the shack.

Rick K2XT