I have a Drake L-4B, which I purchased new in about 1975. Recently I have
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

John,
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

Why push it ?

The difference between 500 and a 1000 watts is only 3db

Its not worth stressing out the power supply and tubes.


"John, N9JG" wrote in message
t...
I have a Drake L-4B, which I purchased new in about 1975. Recently I have
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

Thanks for the reply. Your suggestion about the HV caps has already been
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" wrote in message
...
John,
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

FWIW, Although the Drake manual for the L4B says that tuning/operation
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 wrote:
I have a Drake L-4B, which I purchased new in about 1975. Recently I have
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

Probably a good idea since I don't think I'd like the smell of a burned out
plate transformer and rf choke.

"K3HVG" wrote in message
...
FWIW, Although the Drake manual for the L4B says that tuning/operation
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 wrote:
I have a Drake L-4B, which I purchased new in about 1975. Recently I have
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

Before I read Jeep's response, I was getting ready to respond that 400 watts
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

Does the L4B use the Eimac tube chimney and matching tube socket?

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.

K3HVG wrote:

FWIW, Although the Drake manual for the L4B says that tuning/operation
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 wrote:

I have a Drake L-4B, which I purchased new in about 1975. Recently I
have 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

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

"Dave" wrote in message
. ..
Does the L4B use the Eimac tube chimney and matching tube socket?

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.

K3HVG wrote:

FWIW, Although the Drake manual for the L4B says that tuning/operation
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 wrote:

I have a Drake L-4B, which I purchased new in about 1975. Recently I
have 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

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