> Firstly, if you do the sums for a Class AB tube amplifier
> with the usual Q values, you'll see that you don't meet
> either the FCC requirements for harmonics (46dB) nor the
> Radio Regulations requirements, which are 4 dB tighter at
> 50dB.>>
You keep saying that Peter, but it isn't true. A simple pi
network can easily make a class AB2 amp -55dB or more for
harmonics.
For example the AL-12 series of amps makes about -60dBc on
bands where it is a pi-net. It really isn't significantly
better on bands where it is a pi-L.
>> Which is why the manufacturers these days use a Pi-L
>> network.>>
Not that I know. I use a pi-L to extend matching range.
In the world of REAL antennas and real feedlines, the L
section often doesn't do much at all. It can even make
things worse.
> Biasing the 12 volt transistors is fun anyway. From a DC
> viewpoint, you'd like a constant current source of bias
> that reduced as the transistors got warm and the beta went
> up. But a constant current source is ideally an infinite
> impedance, and as soon as you apply RF, the rectification
> then alters the bias point>>
That's right. Many people miss this and think a simple diode
makes a dandy bias source.
In fact a simple diode almost never works well above a few
mA of base current. I've measured amps with a shunt diode
for bias and at full drive the base bias voltage is often
NEGATIVE by several volts. Unfortunately many people don't
measure dynamic regulation of screen or bias supplies, they
just look at zero drive bias current.
73 Tom
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|