Duty factor for Drake L-4B
John, N9JG
started to use RTTY mode with the L-4B, and I have some concerns about what
is the safe RTTY power level for this amplifier. The Drake easily puts out 1
kW into my antenna, but during an RTTY contest the transmission duty cycle
can approach 50%. During a long-winded rag chew, a transmission might be as
long as five minutes (just like the old AM days!). I am reasonably certain
that the L-4B (both the amplifier deck and the PS) can withstand long
transmissions at (say) 500 watts out, but my concern is with the 1000 watt
level.
John, N9JG
L Ron Hubbub
The difference between 500 and a 1000 watts is only 3db
Its not worth stressing out the power supply and tubes.
"John, N9JG" <jo...@Xremove.MeYgotwals.com> wrote in message
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Rick
The tubes are rated at a total of 1000 watts dissipation, so I'd say if your
input is 2kw and output is 1 kw you are ok.
The power supply might get a bit warm but I seriously doubt that it would fail
under these conditions. I hear that the electrolytics fail often in L4Bs at
about that age, so the heat in the PS might not be good for aging caps, so now
might be a good time to change them.
If you are ragchewing you could back the power down or maybe use the low
voltage setting, but for contesting you probably want all the power and dbs
you can get.
By the way, I have one here also and I need to get in there and change my caps
too. I recently added a transistor swiching circuit for the antenna relay to
make my transceiver happier.
Rick K2XT
John, N9JG
followed. The L-4B was in storage from 1990-2005, so before I put it back in
service, I replaced the original Eimac finals with Chinese graphite anode
3-500ZGs and replaced the HV diode and cap strings. Like you, I didn't want
to have an aging cap blow.
I followed the recommendation in my Orion manual and use an Amitron ARB-704
switching box to interface between the amp key output of my Orion
transceiver and the VOX relay input of the L-4B.
John, N9JG
"Rick" <rick...@hotmail.com> wrote in message
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K3HVG
for CW and RTTY are the same, in one of my L4B books, there's what looks
to be a 2nd or 3rd generation copy of a Drake letter that speaks to RTTY
activity. The Drake letter recommends that the power level for RTTY be
limited to 400 watts, citing the near 100% duty cycle, when key-down. I
guess you take your pick? I run mine at about 400 watts, just to be on
the conservative side.......
John, N9JG
plate transformer and rf choke.
"K3HVG" <je...@adelphia.net> wrote in message
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COLIN LAMB
of output is right. Solder melts and things fail. I think that is a
realistic number.
It is not the tube plate dissipation that is the limiting factor. It is
heat - both from the components and from the currents flowing through the
switch contacts and the coils.
Colin K7FM
Dave
The tube pins can get so hot the soldered connections on tubes without the
chimney/socket [i.e. Heathkit Cooling] can and do melt at 100% duty cycle at
full power.
John, N9JG
the L-4B.
"Dave" <W1...@comcast.net> wrote in message
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Rick
>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
Gary Schafer
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
John, N9JG
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 ą
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" <rick...@hotmail.com> wrote in message
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