[CQ-Contest] re: AL1500 efficiency and others....

Dick Frey k4xu at coinet.com
Wed Oct 1 23:44:45 EDT 1997

My design experience with amplifiers indicates one cause of poor efficiency
not noted in the literature. The impedance of the cathode network (which is
in series with the output in a GG amp) MUST exhibit a low impedance at all
harmonics of the operating frequency. If not, excessive harmonic current
will flow in the plate circuit which does not contribute to the fundamental
output power but kills the efficiency. I learned this lesson the hard way
while designing a commercial amateur amp.

Poorly designed amps might exhibit this because:

1.  There is no tuned input network at all. The current circulates through
the output network of the driver causing band-critical coax lengths to
2.  It uses a T-configured lowpass network. It's cheaper, only requires one
capacitor, but has a lousy stopband impedance.
3.  The mechanical layout or poor capacitor quality causes an otherwise
useful input filter to exhibit high impedance at frequencies beyond the
designed cutoff frequency.  (Most likely in your case)

Since you are in UK, do check the filament voltage!!!! The transformer was
NOT designed for operation from your 250V 50 Hz mains. Excessive filament
voltage, a problem in this amp under the best of conditions, is the most
common cause of tube failure in this amp. You will have difficulty
prosecuting a warranty replacement for your dead tube from Eimac because
they already know about this. "Improper application" is not covered.

Due to the nature of the tune capacitor and the mechanical configuration of
the PA compartment, the tank Q is very high on the upper bands. Under
CONTESTING conditions, this will lead to eventual failure of the band
switch contacts through detempering of the contact springs and/or
desoldering of the connections. I have personal and painful experience with
this one after an extended session on 15M RTTY during a CQWW.

This amplifier was apparently designed by a dilettante technician. It is
poorly made, poorly designed, and has a sorry service reputation -- but
otherwise it's a fine amp -- cheap, and well worth it.

After eviscerating whatever positive image you might have had of this
amplifier, I'd be interested in hearing any other technical comments you
receive on your query. 

Dick Frey	K4XU/7		k4xu at contesting.com

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