[AMRadio] 4-1000A Grounded Grid Class-C

Bernie Doran qedconsultants at embarqmail.com
Fri Oct 4 02:28:27 EDT 2013

Hi Rob: The problem with modulation is that a large percentage of the 
cathode driving power passes through and appears as output and thus 
unmodulated.  It has nothing to do with the voltages.

Certainly using the tube way under its ratings reduces the driving power, 
but what is the point? much better choices than a 4-1000 if the only desired 
power is the so called 375 watt or whatever it is for A.M.

For the same output, class C is always going to take significantly more 
power than class B. Makes no difference if the is grid driven or cathode 
driven.   Class B has a conduction angle of 180 degrees or more, class C 
usually is around 120 degrees and the tube must be driven at all times to 
near saturation so that the the plate current will be double during the 
peaks of modulation.  That brings up the problem common to all multi element 
tubes and high MU triodes, they do not modulate well with out playing some 
games, I E screen modulation and or driver modulation.   As an example look 
at the tube curves for a tetrode, take any point and double the voltage, the 
plate current hardly changes, that is why the screen voltage must also be 
modulated. In the case of a 4-1000 the screen current is rather high and the 
"best" way to modulate the screen is by a separate winding on the mod 
transformer or a dropping resistor from the plate supply. That becomes a big 
fat resistor  and some means must be provided to make sure the screen 
voltage never rises uncontrolled.  Under some conditions the screen current 
can be come negative and with a simple dropping resistor the screen rises to 
the plate voltage and goodbye tube.    The screen can not ever " float" for 
the same reason, one safe way is for another resistor placed directly from 
screen to ground.  The amount of current flowing through that resistor is 
subject to some variation but probably should be at least 1 1/2 times the 
max screen current. Running at the Eimac rating with 3kv on the plate, they 
do not like voltage below 3kv,  screen current is around 140 MA thus the 
resistors should carry at least about 210 MA plus. Near 300 Ma would be 
safer.  600 to 900 watts of heat!!   I think 4-1000a are really neat tubes, 
I have seven, but a bit difficult to work with.

Some triodes, Hi Mu, have issues with modulation.  "Worst" are the 3-XXX 
series, their curves resemble multi element tubes and even Eimac states that 
the 3-1000 should have driver modulation. ( that in grid driven 
configuration)  Look at the curves for low Mu triodes and one can easily see 
that the plate current doubles as the plate voltage doubles. The 304TL being 
my favorite.

Low Mu triodes simply make the best amplifier tube choice for AM service, 
regretfully the perceived need for rapid band switching has pushed them 
aside like open wire has been replaced by the inferior coax for feed lines. 
Bernie W8RPW

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