On Sat, 2009-01-17 at 17:52 -0600, Bob McGraw - K4TAX wrote:
> Some companies are reportedly running 3500 to 3800 volts on their 3-500Z
> amps. I'll stand by the tube manufacturers ratings of 4000 volts as being
> absolute max with 3000 volts being typical.
You have to remember that in linear service the RF plate voltage will
reach about twice the DC. On a plate modulated PA the peak RF voltage
reaches 4 times the DC supply voltage. Many a tube (not necessarily
grounded grid linear tubes though) have a DC plate voltage rating for
plate modulation, 2/3 or 3/4 the CW and linear rating, yet at the
modulation peak the effective plate supply is twice the DC supply
voltage and many a linear PA maker has run the DC into that range.
> Plate current per tube should
> not exceed 400 ma. Based on these values, I believe that Tentec has
> designed the Centurion amplifier correctly and with a margin of safety and
> longevity while others may have pushed things to the edge. The result is
> likely shortened tube life, stressed components and the requirement for more
> expensive components to withstand the higher voltage.
Taking any PA and dropping the PO and drive back a mere ten percent
(less than 1 dB of output) can drastically lengthen the tube life. They
carried that to extreme at WWV where they run half ratings and get
100,000 hours tube life in continuous service.
>
> The real secret to keeping a 3-500Z amp alive is to carefully monitor and
> control grid current. Proper tuning and loading is the key to success in
> this area.
In m experience, one should monitor the grid current and adjust the
drive to keep it reasonable, but not use it as a tuning indicator. Its
easy to tune up a GG PA to show proper plate and grid currents and yet
to produce very little power output meaning the plate is dissipating way
more power than it should be. Tuning for maximum output gets the loading
for efficiency and linearity and adjusting the drive sets the grid and
plate currents. In a MARS station I saw a Henry 2K-3 go from 200 watts
output to about 2 KW with the same plate and grid currents when I tuned
it for maximum output.
> There is no doubt or question, a pair of 3-500Z tubes can be run
> to legal limit safely. Power supply components, high volume cooling and
> high efficiency tank components are key areas to making the tubes work at
> this level.
>
> 73
> Bob, K4TAX
>
73, Jerry, K0CQ
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