Topband: Antenna matching question

[Tom W8JI]

Hi Ray,

All of this is traceable to real causes, and can be repeated in tests and 
measurements. This is something I learned through experience in 1970 
building homebrew amps, and it repeats in everything from antenna tuners to 
amplifiers.


>I have burn up two output band switches on my Ameritron AL1200 while on
> 160m. The amp uses a single 3CX1200A7 tube. My swr may have been high, but
> less that 3:1.

The constant SWR, despite claims,  has very little to do with it. SWR issues 
can cause excessive voltage from the antenna port to ground, but that 
voltage makes little difference in voltage across the switch. The large 
voltage variation SWR or load impedance creates is voltage across the 
loading control. This is very easy to see with an analaysis of the matching 
network.

Near the HV end of the tank, or the radio port end of a tuner, voltage 
doesn't change at all with SWR. There are variations with different networks 
in tuners, but not in amplifiers.

The real problems in switchs are the contact-to-contact and contact-to-rotor 
voltage differences as the anode voltage swing is progressivly reduced 
through the tank. This is a function of anode voltage swing and what taps 
are selected. In an HF amplifier of proper tank inductance values, you have 
about 50% of the peak anode voltage swing between the 80 meter tap and any 
adjacent taps.

In a properly loaded AB or B class amp the peak anode voltage is about 90% 
or less of the anode voltage, so with PROPER tuning you would have about 3kV 
peak. This can go to 6kV or more if you improperly tune the amplifier.

See this link:
http://www.w8ji.com/demonstation.htm

This means contact to contact voltage might be 1.5kV when properly loaded.

The AL1200, like most amplifiers I do, is designed so the plate tuning cap 
fails at less voltage than the switch fails at. This way when the amp is 
mistuned, the tuning cap will arc instead of the switch.  There is also an 
intentional spark gap.

The cap has about 50 to 75 % margin to flashover in normal operation, and 
the switch worse case has significantly more headroom **IF** it is installed 
and wiored correctly.

> Could the discussion above be the same for the AL1200 amp? I'm tired of
> replacing the band switch. I looked at replacing it with a larger switch,
> but there is not enough room.

Send me a picture of where it failed, and I can tell you why it failed. It 
normally will NOT fail unless the wiring is dressed poorly. There also were 
a  few incorrectly manufactured wafers that required a wiring change. It is 
very abnormal to have a switch failure unless:

1) The load is being lost while transmitting at high power, and the switch 
(usually through a contact or wiring error) has less breakdown than the 
plate capacitor breakdown.

2) The wiring is dressed wrong, has sharp points, or the contacts are not 
properly aligned. There is a contact alignment procedure that MUST be 
followed.

If you just ordered a wafer and put it in, and do not understand how to 
align the contacts or did not use rounded points on connections and good 
lead dress, or if you got one of the revised wafers that has a shorting wire 
missing, then you can easily have repeated failures.

We have a bunch of AL1200's here. I have some pretty careless ops here for 
contests, and have had one operator who consistently melts down the plate 
tuning capacitor from grossly improper tuning, but it never hurts a switch.

Send me a picture of the arc area before you move any wires.

73 Tom