On Mon, 13 Sep 2004 22:50:08 +0200, J M Noeding
wrote:

I suppose it is to be used for a linear amplifier,

Apparently this was needed for ATV, however, I have no idea if it was
for AM/VSB-ATV or FM-ATV.

and then it is a not a good idea to use stabilized voltage.

This is certainly true for tube amplifiers which can tolerate high
standby voltages.

However, especially with bipolar RF transistors, if you first run them
at a lower voltage, high current and high dissipation and thus finally
with a high junction temperature and then reduce the loading, the
power supply voltage is increasing to the standby value, you are going
to have secondary breakdown problems.

When operating with an unregulated power supply, you must make sure
that during idle condition and maximum mains voltage, the storage
capacitor voltage is always less than the maximum allowed Vce for the
RF transistors. However, when loading the amplifier, the power supply
voltage drops 10-30 %, thus the RF output would drop even to one half
of what would otherwise be available from these RF transistors,
assuming adequate heatsinks.

It makes perfectly sense to use cheap series pass regulator
transistors in order to get the maximum power out of the expensive RF
transistors by running them all the time close to the maximum allowed
voltage.

Even if you accept the hum problems associated with unregulated
supplies, at least use some simple series pass regulator to limit the
standby voltage to the maximum allowed for the RF transistors. When
the capacitor voltage is below this maximum, simply put this regulator
into saturation, with less than 1 V drop, thus the amplifier Vcc
follows quite closely the capacitor voltage during discharge. The
dissipation in these series pass transistors would be quite low (low
current at high voltage difference, only the saturation voltage at
high current).

It might have been needed
for 14V, although Atlas 210-X was operated without - with a varying
voltage between 11-16V, but is not the same problem for 24V.
With maximum voltage of +32V the output may drop down towards 24V, and
no problem at all - apart from setting a limit for the power input

Only problem that you must use 32 V RF transistors (64 V for collector
modulated AM). With a regulated 24 V supply, you could use 24 V (48 V
AM) RF-transistors and get the same RF output assuming identical power
dissipation and current handling figures. The price for higher voltage
transistors is usually higher.

With AM/VSB-ATV this will cause truncation of the sync pulses and
possibly hum bars in the received signal (since the receiver a.g.c is
derived only from the "constant" amplitude synch pulses :-). On FM-ATV
there could be two noise bands across the screen.

Paul OH3LWR