> Interesting - I thought Ep dropped because of traditional voltage drop and
> because typical plate voltage supplies are not regulated to the degree
> that filament voltages are regulated - so plate voltage drop is a
> necessity? Or should I read the drop as an indicator of proper electron
> flow?
Here, Rich is talking about something different than the plate voltage
reading on the meter. During operation, the tubes acts like a variable
resistor between the high voltage supply and ground. The resistance changes
(primarily) as a function of the voltage on the grid. This is what develops
the voltage swing that is ultimately delivered as output.
When you think of the tube as a variable resistor connected between HV and
ground, you start to get an intuitive idea of what is causing the plate to
turn red, etc.
There is a very nice explanation of this at
http://www.w8ji.com/Vacuum_tube_amps.htm , and it also shows what is going
on in the tank circuit etc.
Ed
----- Original Message -----
From: <jsb@digistar.com>
To: "R. Measures" <r@somis.org>
Cc: "Joe Isabella" <n3ji@yahoo.com>; "Amps" <amps@contesting.com>
Sent: Friday, December 12, 2003 10:58 AM
Subject: Re: [Amps] 3-500Z plate durability
> On Fri, 12 Dec 2003, R. Measures wrote:
>
> > >I have to learn to be more careful though - i've been sloppy keeping
> > >the grid current under 260mA when tuning.
> > >
> > ** 3-500Zs have no grid-I rating. They have a grid-dissipation rating
> > of 20w -- which can be exceeded (as in glowing grid wires) for brief
> > periods without damage. Grounded-grid/cathode-driven amplifiers should
> > not be adjusted with the grid-I meter. Instead, they should be tuned
> > with max rated drive, preferably pulsed, and adjusted for max P-out with
> > the Tune and Load controls. With P-out peaked, grid-current will always
> > be ok because excessive grid-I will always rob electrons from the
> > output.
>
> I wonder why grid current is measured instead of dissipation - I would
> think dissipation and current measurements could be easily interchanged
> for display on a meter.
>
> > Also, because grid-I only flows when the anode is not very positive,
> > too little grid-I tells us that the anode-V is not swinging low enough
> > during the peak in cathode-emission.
>
> Interesting - I thought Ep dropped because of traditional voltage drop and
> because typical plate voltage supplies are not regulated to the degree
> that filament voltages are regulated - so plate voltage drop is a
> necessity? Or should I read the drop as an indicator of proper electron
> flow?
>
> > Naturally, when this is happening, P-out drops off. Thus, for a
> > cathode-driven amplifier, tune for P-max and relax.
>
> ... and be sure to stop applying additional drive when P-max is reached?
>
> My usual method for tuning is to apply drive, look for maximum output,
> continue increasing drive and tuning for max output until one of the smoke
> alarms goes off (meter shows max acceptable plate current and/or grid
> current).
>
> I have also been experimenting with touching up the load to lower grid
> current with the result of increasing power output. This seems to yield
> an additional 200-400 watts at best over 1000 tuned to peak output. I've
> mostly abandoned this practice for the most part because of the scant
> increase in signal strength by the receiving station (most of my operating
> is on CW), however it does seem to help on SSB - however I do not know if
> the additional 200-400 watts are comprised solely of of splatter with no
> beneficial yield to the receiving station on the intended frequency.
>
>
> thanks,
> jason
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