Toroid tank coil question - link vs. pi-net?

[WB3JOK]

My 4X4-125A amp (Command Sets just can't put out the STRAP) is gradually taking shape on a 17x17x3 chassis with a 12.25" rackmount panel for a front panel. I am now building the plate tank circuit. The tank L is three stacked T200 toroid cores, appropriately insulated and wound with spaced bare #12 wire. If this isn't high-Q enough, I may redo it closewound with Teflon-insulated silver wire.

It occurred to me that instead of a "standard" pi-network, I could tune the tank with a typical air variable, but wind a couple of turns of Teflon-insulated HV wire through the toroids for a link coupling. (I assume a variable "load" cap would then go in series with the link to tune out its reactance).

As previously posted on the "lost" board, I want to use this amp on 75m and 40m without a bandswitch... output loads will be 50-75 ohms. Will this be a problem with the link having too many, or too few turns, on one or the other?

Could I also use link coupling for neutralization?

thanks
Charles

[WA1GFZ]

When you drive an iron core hard the permability goes down. Stick to Ui of 6 to 10 or you will have problems. Don't snort burning teflon as it will kill you! 4X 4-125s will be a low Z output. You will not need much inductance anyway so I would stick to 1/4 inch copper tubing.

[WB3JOK]

When you drive an iron core hard the permability goes down. Stick to Ui of 6 to 10 or you will have problems. Don't snort burning teflon as it will kill you! 4X 4-125s will be a low Z output. You will not need much inductance anyway so I would stick to 1/4 inch copper tubing.

Thanks for your reply. The cores have a μ=8 so three of them should be ok at the relatively modest power output. (I expect to run 3000v @ 250ma, maybe 250W or so carrier output, which isn't a particularly low plate Z). I had thought about running the HV through the tank coil to eliminate the plate choke, but the dc bias might indeed lead to saturation of the cores, and also that opens up a new can of worms with regard to resonances.

I calculated the required tank inductance to be around 13 μH which takes 17 turns or so on the toroid stack - 1/4" tubing won't fit, and even #10 would be getting close on the inside of the turns. I suppose I could go back to a single-layer "air" coil but I was hoping to minimize the strong RF fields atop the chassis, as well as the size of the coil.

Since I don't plan to use the amp above 7.3 MHz, and will only be generating 250W, or 1KW PEP for AM (maybe a little more for SSB), is a conductor that large really necessary at 40m? Most of the designs I've seen have really heavy wire/strap for use on 10m or above.

What do you think about the link coupling, whether to a toroid or an air-wound tank coil?

thanks
Charles