Roller inductor plate tank coil, short it or not?

[CaptDon]

I built a 3 to 30 Mhz amplifier using an Eimac 8930 (big radiator version of 4CX250B). The grid is untuned and swamped with 200 ohms at 10 watts. I have a hand wound un/un to match 50 ohms to 200 ohms to keep my Kenwood TS-690S happy. The plate circuit has a plate tuning capacitor, a roller inductor and a loading capacitor with some switch selectable additional loading capacitance. So now the big question, Should I set up the tank coil to short the unused section or not? If I shorted it wouldn't there be some frequency where it would have a huge negative effect on the 'tuned' part? Leaving it open (as I have currently done) would run the risk of a flashover although it hasn't happened yet. I used one of the B&W solenoid wound plate chokes (not the multi-pie style) and it seems to act like a dead short and want to burn up around the 15 meter (21Mhz) band. I do also own one of the multi-pie chokes but haven't tried it yet. Thanks for any thoughts on this. Cheers.

[szoftveres]

In theory, wouldn't flashover only happen when you didn't connect the antenna and the tank was purely reactive? I would imagine as long as there's proper (resistive) load presented to the tank, the voltage wouldn't go even close to breakdown (assuming properly sized parts).

Disclaimer: no experience with big rigs like this, just pure theory and speculation.

[CaptDon]

The open end of the tank could possibly become a high impedance open end of a step up winding?

[M0HZH]

Shorting inductor turns adds losses, don't do it unless you really have to. Most people leave the end unconnected and hope it wouldn't arc.

The best way to do it is to split the inductor in multiple sections so there isn't a single step-up inductor.

[T3sl4co1l]

Hm, what's SRF of the full winding (roller inductor at max setting)?

I think if it's higher than Fmax, you won't be in danger of at least exciting higher modes.  (So, no crazy voltages due to that, but more importantly, no resonant trap sucking down all your power and screwing up the tuning..!)  I suppose it better be, either way, else you'd have problems in the first place.

You will get some transformer action either way, but not as bad as you might think.  The coupling factor between end-to-end solenoids of modest length/diameter ratio is modest (say 0.3-0.7), which means the voltage won't be quite as high as the turns ratio might imply.

Leaving it open stands a better chance of having lower losses, assuming of course the voltage on the far end doesn't reach breakdown.  Shorting it depends on the rolling contact having low resistance to make a solid shorted turn, which is rather unsatisfying to me.  The short does increase available inductance ratio.

Tim

[Wallace Gasiewicz]

The Ham Amps I have seen have the tuning coil terminated Into the antenna. This is at the Ant tuning cap.
In your case this is where the "wiper" goes.
So the wiper bypasses part of the tuning coil.

[Bud]

You should scan your plate choke with a VNA to check for self-resonanses, or the situation may end up badly. If a resonance falls in a ham band, remove or add turns to shift it away. As to the tank coil I would leave the end unconnected if it does not arc. Shorting the unused section would likely reduce the working coil Q.