In order to have a VHF parasitic oscillation in a HF amplifier three things are necessary: An amplifying device that has VHF gain, a feedback path from the outout to the input, and a VHF parasitic resonaant circuit, in the output. All electron tubes and all transistors have a feedback path. Example: 4, 572Bs have a feedback-C of 2.4 pF and they have a fair amount of gsain at 100MHz.

- note - @ 100MHz 2.4pF has c. &endash;j700ás of feedback from the output to the input..

The third requirement, the VHF resonant circuit, is a given in all HF amplifiers. This resonance is formed by the anode to grid C and the L of the lead between the anode and C-Tune.. For a 50kW amplifier a parasitic resonance c. 40MHaz is typical. For a 10kW amplifier 60MHz is average. For a 1kW amplifier 100 to 150 MHz is typical. . To find the parasitic resonant frequency yourself, unplug the amplofier from the maains, and couple a dip-meter to either side of the DC blocking C.

Launch

A VHF parasitic oscillation begins in the anode's VHF resonant parasitic circuit. Whenever the anode current abruptly changes this circuit rings at its resonant frequency, whereupon some of the VHF signal is fed back through the feedback-C to the input. If the tube can amplify at the parasitic resonance's frequency, an even larger signal appears at the anode where some of the now larger signal is fed back through C-feedback to the input where it is re-amplified by the tube. In short order the amplitude of the VHF signal becomes large. This would not be a problem if the VHF energy had a resistive load -- however, the typical output circuit is a HF low-pass Pi-network which blocks VHF - so the VHF energy runs amuck and causes arcing.

Avoiding Launch

There are two ways to avoid launching VHF parasitic-oscillations: 1. Reduce C-feedack and 2. Reduce VHF amplification. . Since there is no way to decrease C-feedback in a transistor or or in an electron tube, the only game in town is to reduce VHF gain. This can be done by decreasing the VHF RL seen by the anode - which is what a parallel L/R parasitic suppressor does. Tbus, a VHF suppressor does not suppress VHF parasitics, it suppresses VHF amplification at the parasitic resonance in the anode circuit so there is not enough VHF amplication to sustain oscillation, .

 

Parasitic suppressor Q:

Q is important for two reasons:

1. the ringing V across the VHF resonant anode circuit is proportional to Q, A lower R/L suppressor Q produces less ringing V to be fed back to the input and be amplified.

2. The lower the supppressor's Q the lower the VHF RL on the anode and the lower the VHF amplification.

The tradeoff with a lower Q suppressor is that it costs c. 0.1db of output at 28MHz.

--- congrats to F.E. Handy, W1DBM for realizing this in 1926.

- end

Rich, AG6K