On Sat, 2009-01-17 at 04:10 -0500, Phil Sussman wrote:
> Greetings,
>
> I am considering the purchase of a Centurion HF amp. After reviewing the
> specs,
> I'm wondering about the 3100vdc supply dropping to 2600vdc under 'full' load.
> Is the power supply heavy enough? My past experience with dual 3x500Z's has
> plate voltage holding at 2900vdc to 3000vdc typically under full load
> conditions.
Its the nature of a DC supply to soar with no load and to droop with
load. Has been since rectifiers and filter capacitors were invented.
With a light load the DC voltage soars to the peak of the AC to the
rectifier. With a moderate load, the DC voltage runs closer to the RMS
voltage as the filter capacitors supply the load between the voltage
peaks. With a heavy load the DC runs a little lower. Its hard to specify
precisely what these "light," "moderate," and "heavy" loads are without
knowing the load currents, the bleeder current, the capacitor sizes, the
rectifier impedance, and the transformer impedance. I presume there is
no filter choke, but if there is, its characteristics also affect the
computations.
In older days, the soaring was often prevented by keeping a load on the
supply with a bleeder resistor, might be dissipating as much as 20% of
the full power of the supply. But it gave good voltage regulation. Today
that's a lot of energy to dump all the time, and it makes little
difference to the amplifier operation if the voltage soars during
receive, so the bleeder resistors are chosen to bleed down the filter
capacitors after power is turned off, but not to prevent soaring. That
works, because in a linear tube, there is a need for an idling plate
current to get it linear and that zero signal idling plate current is
enough to pull the supply voltage off the soaring condition. So the
bleed current is put to use, and when the PA is driven the voltage
changes from no drive to full drive isn't so much.
Then while the wattmeter difference is easily seen between 1400 and 1500
watts, that's only a fraction (0.3) of a dB, not detectable at the
receiver in the face of 3 to 10 dB per second changes in propagation
loss if the received signal is a couple dB above the receiver and
antenna noise. If the received signal is equal to the noise, a dB or two
more transmitter power or reduced propagation path loss (antenna gain)
may improve the perceived signal to noise ratio far more than the
increase in received power.
>
> Comments certainly welcome.
>
> Thanks,
>
> de Phil - N8PS
>
73, Jerry, K0CQ
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