I wondering about your comment about filtering. If you look at the tank
circuit of these amps, you see a pi network that is usually no different than
any other amp, except the coil is small turns wise because it is for 11 meters
(and close to 10 meters as well). So what kind of "filtering" are you talking
about?
If the pi network is not different than, say, a 30L-1 or an SB 200 or any other
amp with a pi network, why would harmonics be any more of a problem than with
other amps?
Ray wa7itz
----- Original Message -----
From: Tom Rauch<mailto:w8ji@contesting.com>
To: chasm@texas.net<mailto:chasm@texas.net> ;
amps@contesting.com<mailto:amps@contesting.com>
Sent: Tuesday, October 09, 2007 9:42 AM
Subject: Re: [Amps] CB Amps or full spectrum???
There are at least four major problems with the amps, not
two.
1.) Filtering. They need filters to reduce harmonics.
2.) Power output. These amps are all really ratty at rated
power, and it has nothing to do with bias. ANY transistor,
especially one running at 12 volts, can't be pushed anywhere
near the saturated power or the IM goes to heck. A typical
rule of thumb is if the amp starts to limit power at 100
watts, the useful peak output for reasonable IMD will about
50-75 watts.
This is why the FT1000MP's and TS440's and such we use will
run 150 watts or more output if someone gets inside and
screws up the power limiting controls. Even if left alone at
100watts, they are marginal for IM products. Bump them up
another 20-50% and they go to heck fast. It's all part of
the CB mentality. While a tube amp, in particular a cathode
driven triode, can be driven to near the point of power
limiting without objectionable IM products a bipolar
transistor cannot.
You might get away with a somewhat trashy 400 watt amp on a
small inefficient mobile antenna, but not when it is 1500
watts out and especially not on a big antenna!!! So if the
amp runs 1000 watts carrier before it starts to get into
heavy gain compression it's probably good for 500 - 700
watts, depending on how clean you want it.
This has NOTHING to do with bias, although the bias voltage
stability under full drive can affect the IM a little bit.
3.) Idling or low drive bias. CB amps don't usually have
any. Replacing it or installing it for SSB service requires
a regulated bias source that has very low source impedance.
The idle bias sets the cross-over or low level distortion
products. They are still harmful, but not as much as the
full drive saturation above causes.
4.) Transformers. CB amps often have only transformers good
at upper HF.
5.) Feedback. Many CB amps don't have negative feedback that
reduces distortion and stabilizes the amp.
73 Tom
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