[Johnson] A question from A New BA operator.
john_walker_beasley
[email protected]
Thu, 21 Mar 2002 06:00:11 -0600
Thank You,
I printed it and put in my Valiant manual.
John
WA�STX
----- Original Message -----
From: "Guy Olinger, K2AV" <[email protected]>
To: <[email protected]>
Sent: Wednesday, March 20, 2002 9:01 PM
Subject: Re: [Johnson] A question from A New BA operator.
> 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" <[email protected]>
> To: <[email protected]>
> 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
> > <[email protected]> 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"
> > > <[email protected]>
> > > To: <[email protected]>
> > > 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."
> > > <[email protected]>
> > > > To: <[email protected]>
> > > > 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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