[AMRadio] AM Usage with Linear AMPS


John Coleman, ARS WA5BXO wa5bxo at pctechref.com
Mon Nov 22 22:29:43 EST 2004


I've got to get in on this HIHI.

	Some readers may not fully understand what you guys or talking
about.
So I will try to confuse it all some more. HIHI

	Plate dissipation is the heat that the plate of a tube is
emitting.
The manufacture of the tube has what is known as max plate dissipation
limits.  The 811A triode has a limit of about 65 watts.  This means if
the tube is in a circuit where it is putting out a steady carrier and
this circuit is 75% efficient at is max level this would be 4 X 65 input
power (Ep X Ip) and 3 X 65 output power (as measured by a RF Watt meter
with no reflected power).  This would be 260 watts input and 195 watts
output with 65 watts wasted in heat of the plate.  This would not be bad
for CW.

I don't think 75% efficiency is an achievable figure in a good linear
operation even on peaks.  As the drive level is reduced so is the output
level (linearly we hope) but the plate current will not drop as fast.
As a result the efficiency drops and it is possible for the plate
dissipation to be greater even though the plate current is less.  It is
not a good idea to operate at the point where the plate dissipation is
at its highest.  

In any linear amplifier audio or otherwise, proper design would be, to
not exceed the plate dissipation at any time.  How ever we all know that
plate dissipation limits can be exceeded for a short duration provided
that there is an equal or more amount of time that has a much less plate
dissipation than the rating and that the time intervals or close enough
to keep the overall temperature down on the tube.  Exceeding Plate
dissipation causes excessive secondary emissions and internal grid
leakage.  It is also the biggest cause of vacuum loss because of leaking
seals at the anode connection.  Poor vacuum causes more grid leakage and
emission loss due to corroded heaters.  Over temp on a tube is not good
even for a short period of time.  

	In a class B circuit with no input or output the plate
dissipation must be below the limit.    

	If the input is varying as it is with SSB or audio then the tube
will pass through an area of much higher dissipation.  This area is
generally (but not always) somewhere near the 50% mark of peak drive and
output.  This means, the tube could actually be hot with just a carrier
and, then cool down when the input is modulated with a tone.  

	As I recall 811s do not (or just barely) show color at 65 watts
dissipation.  This would be static 2000V and 32 ma or 1500 volts and
43ma.  

	Good practice would be to set bias some what below current for
65 watts dissipation.  As input carrier is introduced a output carrier
will also be created.  It should be a linear progression until top end
is met.  During this progression the tube will go through various levels
of efficiency and plate dissipation.  Some of the places will be very
close to or exceeding the limit of 65 watts.  

	A point of best operation would be where the output is 25 % or
less of the peak output POWER level (OR 50% of the RF output VOLTAGE)
and the plate dissipation (input power - output power) is less that 65
watts per 811A.

Good Luck to all
John WA5BXO            






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