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Flashover in a tube of an unprotected RF amplifier.

The powerful spark induced scorching and inside a hole in the glass. The PA had no additional protection and had not been operational for years. My inspection yielded a burned grid resistor and a partially conductive varistor on.

Irregularities in the schematic. The AL- H is sold worldwide and is quite a well-built “budget” amplifierbut by no means easy to repair. As radio amateur time is free and one can modify amegitron random time. For decadesI advise to mount a resistor and fuse in series with the anode choke as flash over protection for tubesrectifiers and transformers.

It is a cheapsimple and proven effective method. Arcing is a momentary short flashover in one of the transmitter tubes. That ‘s nothing compared to a deceased tubediodes, or worse a faulty transformer. The wire-wound resistor also acts as a replacement for inductor L7.

Faulty Ameritron AL-811H

If the PA is not used for a long time and goes back into operationremove the fuse and let the amplifier an hour long al811h idle. Given its damage in the depicted tubeI think it’s caused by parasitic oscillationswhich led to a high voltage.


The energy could not leave via the high impedance of the plate circuitand so chose the shortest path via the tube. The shield of the cable also serves as al-811h supply line to the central ground point in the tubes.

T he least IMD is produced with that power.

Because there size s little space was to the valve sockets. Not close to L4 but far away on the PSU board. I believe it’s asking for troubleand it is better to shorten the RF path by installing an additional 4n7 capacitor at the lower end of L4.

The long wire to be replaced by coaxial cable RGso that the length is converted into a small capacitance in parallel with the load capacitor.

The size of the 10 m coil has become too small due to mounting method. The inductance of the long supply wires can not be ignored.

ALH Modifications

The shielding of the cable is only grounded at the side of C With this methodthe distance between S1 and C26 changed to a small capacitance manul parallel with the load capacitor and the total inductance of the coil 10 m is thereby reduced. On the other bandthe SWR will be high and is an internal tuner required for full ouput. Connect ameeritron amplifier to a dummy load and adjust for maximum ouput.


Test and adjust the SWR also on the other bands. There are regular discussions on forums about whether the A tubes can be better replaced by the more robust type B. My preference would be given to the latter because I have more experience with it. Therefore, a sumiere test was done with the two types. After each modification the amplifier was firmly tested and plates never becames red.

Please note that the output in the table is partly achieved by exceeding the maximum grid and anode current. In reality, no way to predict how the circuits actually behave. I used as “earth” the screw at the bottom of the anode choke. Turn per band load and plate control clock and anti clockwise and watch if there is any anode or grid current. The suppression of the third- order harmonic is the difference between marker 3D and 4D.

A tentative conclusion is that this AL – H does not distort the signal of the drive stage.