please post following
gzook at YAHOO.COM
Wed Sep 19 21:25:45 EDT 2007
I know that others on this reflector are getting tired
of our exchanges. Therefore, I will try to answer you
since I now realize that you are asking legitimate
questions rather than being argumentative (which your
previous posts seemed to be).
First of all, a good number of the "CB" type "linears"
are operating the sweep tubes outside of their
ratings. This not only affects tube life but also
affects the linearity of the signal.
Next, a good number of those "linears" are not biased
correctly for linear operation. Often they are
operating in Class "C" which definitely is NOT a
"linear" mode. The power output (showing on a
wattmeter) is definitely higher with Class "C" but the
quality of the signal is definitely pretty lousey.
The input and output circuits are often a "bare
minimum" and offer little, if any, harmonic (or sub
harmonic) filtering. Of course harmonic energy
appears in the wattmeter reading so the output "power"
of the amplifier is greater and "looks better" on the
wattmeter even if this energy is not contributing to
the desired signal.
Spurious emissions (both near frequency and VHF) are
often present due to insufficient supression
techniques. Again, these "show up" on the wattmeter
and "add" to the "claimed" power output of the unit.
The majority of these "CB" amplifiers when operated at
"published" power levels produce VERY wide signals
(often several MHz wide) and thus cause interference
to many other stations not even working in the same
band. This is often very evident even if the basic
design is reasonable. If the drive level is reduced,
and the actual input (and output) power is adjusted so
that the amplifier remains within the linear range
then the quality of the signal is much better and the
technical specifications of the FCC can be met.
Modifications necessary to many of these "CB" type
amplifiers require a change in the bias applied to the
tube thus changing the Class of operation. Sometimes
things like regulating the screens of the tubes is
required. Additional circuit bypassing is sometimes
required to eliminate spurious emissions. Also,
adding things like plate supressors (i.e. a small coil
around a resistor) will help eliminate VHF spurs and
are missing in a lot of units.
The power supply is often operating on the "ragged
edge". Often higher quality components are just not
used. But, since the wattmeter reading is what the
desired result and not the quality of the signal,
using higher quality components is of little interest
to the manufacturer. The primary "aim" of a lot of
the manufacturers is to maximize profits by minimizing
Since these amplifiers are designed to be used with
like 3 or 4 watts maximum drive they have to be grid
driven. This requires considerably closer attention
to the circuit design. For amateur use, rebuilding
the amplifier to use grounded-grid will allow a better
use of the components. However, the drive level will
definitely be MUCH greater.
Now there are definitely a relatively few "CB"
amplifier designs that are "decent". Unfortunately,
most are trying for a high wattmeter reading with
little, or no, thought of signal quality.
I hope that this is closer to the information that you
were seeking. Unfortunately, I misunderstood your
original comments and I definitely thought that you
were trying to be argumentive.
--- RAY FRIESS <rayfrijr at MSN.COM> wrote:
Well excuse me for trying to get some information from
someone who is "generally" regarded as knowing what he
is talking about when it comes to technical issues. I
am NOT "fixated" on the Palomar 300 or on any other
type of amplifier. I used that as an example because I
happen to have two of them and they seem to be, from
my examination of the schematics, typical or
representative of the CB type amps that have been
discussed here. I didn't realize my inquiries would
raise such ire. I'll confine my discussions on this
topic with a few people here in this state who are
also generally regarded as knowing what they are
talking about and are willing to calmly answer my
Fussy? Opinionated? Impossible to please? Perfect. Join Yahoo!'s user panel and lay it on us. http://surveylink.yahoo.com/gmrs/yahoo_panel_invite.asp?a=7
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