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Re: [Amps] Plate coupling capacitor value

Subject: Re: [Amps] Plate coupling capacitor value
From: Manfred Mornhinweg <>
Date: Mon, 11 Sep 2017 17:50:19 +0000
List-post: <>
Bill, Carl,

There is a better way to tune 10 and 12 meters despite high tube
output and stray capacitance. A small coil in the anode lead will
create an "L" network which reduces the plate impedance on those bands
and permits use of a larger tune capacitor.

Yes, that's right. And the same technique can very well be extended, so that the matching network becomes a cascade of several L networks. The nicest thing about this configuration is that the total Q of such a matching network can be lower than what's required to implement the full impedance transformation in a single L network, let alone a single PI. This lower Q provides a larger bandwidth, without having to retune.

You surely know those hybrid ICs used as power amplifier chains in VHF and UHF radios. These typically have such cascaded L networks, with 3 or 4 stages. This allows a 12V 80W module, for example, to cover 136 to 174MHz with nearly flat performance.

The actual circuit was to tap the coil

And this technique too is commonly used in tuned solid state amps. The coil is implemented as a single trace on the printed board (a microstripline), and several chip capacitors are soldered between the microstripline and the ground foil, at precisely calculated points. I have witnessed non-RF-savvy electronic engineers looking at such amplifiers and scratching their heads about how that works...


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