3
With hard faults an ionized path occurs through and around the grid. This transfers a great deal of energy to the
tube filament. These arcs can be devastating to amplifier components and to the radio. Hard anode-to-filament
arcs produce a very sharp edge high voltage spike on the filament. The filament, if unprotected, reaches well over
1000 volts in microseconds. This arc is very much like a miniature lightning surge. The filament choke has
significant common mode series impedance. Along with bypass capacitors this forms a low-pass, only passing arc
frequencies below ~100 kHz through the filament winding to the bias and relay circuit. It takes a very hard
sustained arc to damage this path.
The second more troublesome path is through parallel 0.01 µF filament coupling capacitors to the tuned input. The
tuned input circuit, being a low-pass, rings. This stretches the pulse out into a several mS long series of lower-level
oscillations below the band selected. The pulse can then make it through the tuned input much like a brief high-
power transmitter of a few hundred watts into the radio. Worst case peak voltage level is an arc while on ten
meters, while worst case pulse length is on 160 meters.
The red line is the primary damaging high frequency arc path. The path goes through tubes, out through the
filament coupling capacitors. It then travels through the tuned input circuits to the RF input, and that is where your
radio is connected. The arc path is NOT out through the relay control line; a relay line isolation device will not
prevent or reduce arc damage. The arc path is NOT out through ALC; using or not using ALC will have no effect on
radio damage.
This kit does the following:
1.)
Adds two “150 volt” Gas Discharge Tubes from filament directly to chassis at the tubes. The
Littelfuse gas
discharge tubes (GDT) surge arrester devices protect personnel and
equipment from damaging high
Figure 1 primary arc path
