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