[Johnson] A question from A New BA operator.

[Guy Olinger, K2AV]

BA Drive control is to allow you to set the power from the driver
stage to give the design level of grid and plate current in the finals
when everything is tuned up. This is a practical necessity given the
varying output/sensitivity of tubes over their lifetime. It's not
really meant to make it easy to run at too high or too low levels.

If you took on the self-flagellating task of going through tube
characteristic graphs (not really recommended for reasons of sanity),
you would run into the difference between running tubes for efficiency
and running them for linearity. Early big tubes could barely muster
50% efficiency in linear service. LOTS of heat.

On the other hand, to quote an extreme that illustrates the principle,
there are things like 1500 watt CW output settings for a tube that
only has 300 watts plate dissipation, over 80% efficient. (See Eimac
data for 304TH, for example). The operation is anything but linear,
definitely, extremely class C.

With a tube operating in a mode like that, the thing that counts is
keeping all the tuning, bias, etc, in that narrow design range that
delivers efficiency at the fairly high plate and bias voltages and
matching circuit impedances. Change that at the full rated power and
the plates in the tube are smoke. That includes, at these extreme
settings, reducing the drive. That is because the tube is no longer
matched to the load, and a greater percentage of the power has to be
dissipated in the tube. It is possible to effectively mistune the
amplifier and raise the dissipation percent faster than the input
power is lowered. All depends on the tube characteristics and the
circuit specifics.

The easy, carefree way, without brain-curdling over tube curves, is to
tune at spec.

For modern tubes specifically designed for zero bias linear service
and with plate dissipation figures at the same magnitude as typical
output (as in 3CX1200A7 just for one) these considerations simply
don't apply until you reach tube max for grid dissipation. To run less
power, just reduce drive, no sweat. It's never going to do better than
65% or so anyway, and direct conduction forced air cooling can carry
away incredible amounts of heat without stressing the tube.


Also, as a practical BA tuning up procedure, keep the power quite well
down until you get all the knobs on frequency. Notice I DIDN'T say
down moderately.

Variable (as opposed to mode-switched CW or AM) screen voltage to the
finals in these rigs is usually a user mod. Careful how you use that
for all the reasons above.

Regards, Guy.

----- Original Message -----
From: "Ed Purvis" <wa4njy@yahoo.com>
To: <johnson@mailman.qth.net>
Sent: Wednesday, March 20, 2002 8:42 PM
Subject: Re: [Johnson] A question from A New BA operator.


>       It has been about 30 years since using the tube
>          rigs, so this is renewing some old practices.
>       Seems it is correct to run the final at full
>          efficency. But, what is the drive control
>       for? Limiting grid drive to safe level?
>
>        If I want to safely lower power can I drop the
>            screen voltage?
>                      tnx es keep up the chatter,
>                                Ed
>
>
>
> --- john_walker_beasley
> <john_walker_beasley@email.msn.com> wrote:
> > Cliff,
> >
> >     That is interesting.  I suppose this infers that
> > the unit should
> > be run at full steam all the time for it's own
> > protection ?
> >
> >     I am busy working on mine and I have reduced
> > coupling
> > to drop the output in the past.  Now I am concerned
> > that this
> > is not a good practice !
> >
> >     Thanks
> >     John
> >
> >
> >
> > ----- Original Message -----
> > From: "Dr. Clifford L. Coleman"
> > <psurg@globaleyes.net>
> > To: <johnson@mailman.qth.net>
> > Sent: Wednesday, March 20, 2002 3:23 PM
> > Subject: Re: [Johnson] A question from A New BA
> > operator.
> >
> >
> > > While restoring my valiant, I found(as the manual
> > suggests ) that less
> > > loaded causes high modulator current-so I tune it
> > up to full power.
> > cliff
> > > k9jts
> > > ----- Original Message -----
> > > From: "Mullin, Edward J."
> > <mulline@tycoelectronics.com>
> > > To: <johnson@mailman.qth.net>
> > > Sent: Wednesday, March 20, 2002 1:40 PM
> > > Subject: [Johnson] A question from A New BA
> > operator.
> > >
> > >
> > > > Hello All,
> > > >
> > > > I've been lurking on this mail list for a while,
> > as I've been
> > > > working on my Valiant.  So far I have gotten it
> > working again,
> > (recapping
> > > > and replacing a couple of tubes was all it
> > needed:)   Now I want to use
> > it
> > > > on the air.  (I've been testing into a dummy
> > load during restoration).
> > I'm
> > > > new to BA's (and Ham operations) in general, and
> >  I've been reading the
> > > > manuals alot.  What I want to know is: What is
> > the correct method for
> > > > transmitting at less than full power. The manual
> > is good at describing
> > the
> > > > tuning process, but it doesn't really cover this
> > information.(at least I
> > > > didn't figure out where it was.)
> > > >
> > > >
> > > > 73 de KB1HYS
> > > >
> > > >
> > > > > Edward Mullin
> > > > >
> > > > >
> > > > _______________________________________________
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> > >
> > >
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> > >
> >
> >
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>
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