[Amps] Parasitic suppressors on GI-7B.

[Peter Chadwick]

VK3ZL asked abput parasitic suppressors.........
At the risk of another AMPS reflector war....
If the parasitic circuitry of the anode is resonant below that of the grid, it won't oscillate. The GI7B has the capability of a very low inductance grid return, so there's a good chance that if you ground the grid through a collet type arrangement, you won't need parasitic suppression. If you do need suppressors, then there are various approaches. Remember that the resistors heat up mainly because of fundamental current, although the harmonic currents have an effect, because the shunt inductance reactance rises with frequency. In Class AB, the second harmonic anode current is about 6 dB down on the fundamental current. In my experience, an awful lot of parasitic suppressors have problems because the resistors are being pushed to their limit or beyond in terms of dissipation. Additionally, they're usually operating at a high temperature. The usual classic approach has been to use carbon composition resistors of the 2 watt size, with two or three in parallel, and this really isn't enough in my opinion in a 1000 watt or so amplifier. A 'globar' type is much better. Under rated carbon resistors tend to go high in value if they don't actually go open because of the heat - arguably, that can be the cause of things going bad with age.
As to whether you use wire or tape for the coils, it's pretty well up to you. You need enough inductance to move the anode parasitic resonance to below the grid resonance, and the resistors are there to damp out other resonances and reduce the anode load impedance at VHF. If the parasitics are suppressed, then there's only fundamental and harmonic currents in the parasitic suppressors. I have seen suppressors with just an inductance. Too much inductance leads to the resistors heating up on 10metres: too little, and the amp has VHF parasitics.
Some say that you can determine the parasitic frequencies with a dip meter: I've never had much luck with that approach. Some say that suppressor should be wound with nicrome wire, another approach that I personally disagree with. I would build the GI7B amplifier with a collet grounding the grid, leave room for a parasitic suppressor in each anode circuit, and try it without. The resistor size can be a problem, and I don't know where to get the globar or carborundum type resistors - they look like a piece of carbon rod about 3/8inch diameter and 1-1/2 inches long and are rated at about 15 or 20 watts. I got mine years ago when they were available! Usual value is about 47ohms. You need to run as much current as possible when testing, and reducing the HV to keep dissipation in limits can help.

Hope this is of help
73
Peter G3RZP