|[AMRadio] 4-1000A Grounded Grid Class-C|
ranchorobbo at gmail.com
Fri Oct 4 07:12:54 EDT 2013
Thanks Bernie; okay on the cathode drive feed through--makes sense. I
didn't think about that. I'm working on a pair of 3-400Z modulated by
572Bs. I'll see what happens.
On Fri, Oct 4, 2013 at 1:28 AM, Bernie Doran
<qedconsultants at embarqmail.com> wrote:
> 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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