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Re: [Amps] Plate choke magic?

To: Tomm Aldridge <KD7QAE@arrl.net>
Subject: Re: [Amps] Plate choke magic?
From: Tomm Aldridge <KD7QAE@ARRL.NET>
Date: Sat, 29 Jan 2005 08:49:52 +0000
List-post: <mailto:amps@contesting.com>
More on the plate choke question. I just took a t200-3 core with pretty high RF losses (good) and an Al of 42.5 nH/T^2 and wound 154 turns on it to get about 1mH. Because it is a multi layer construction, am I to assume that distributed capacitance in the winding will resonate with the inductance in a manner that would come back to bite me? Maybe a T200-26 core with a much higher Al and HF losses would be better???

Tomm Aldridge wrote:
Thanks Will, that makes perfect and practical sense. Plate choke valuse for 1.8 through 30MHz amplifiers need to be high enough to present a large reactance at 1.8MHz with large defined as >> greater than the plate impedance, correct? And they must not produce any resonances withing the 1.8 to 30MHz band, correct? But, what about the fact that the tubes have gain well above 30MHz and well below it as well?

If I say that the plate Z is 2k ohms and therefore I want 20k ohms at 1.8MHz to satisfy the >> larger condition above, I get an inductance of 1.77mH. Looking at some plate chokes for QRO amps out there, I see values in the range of 200uH (a bit greater than the plate Z) to 500uH, much lower than I would consider to be an effective choke. A 300uH choke is approx 1" x 6" with 278 turns of 26AWG. Seems to be a reasonable DCR to be putting in a plate circuit. I calculate about 2.9 DC ohms with a large surface to distribute the losses. But why such a small inductance???

Tomm

Will Matney wrote:

Tom,

Actually, a ferrite core can be used if it's of the correct type of material. The material is determined by the frequency that the coil will operate at. There a couple of ferrite and iron powder types that would work. The reason most are air coils I would think is they are cheaper to make. An insulated form is all that's really needed. The air coil formula is then used to determine the number of turns for the amount of inductance wanted. The higher the frequency, the lesser amount of inductance is needed to block the RF, so the choke needs to be designed around the lesser frequency that will be encountered. Then you need to make sure the choke is not self-resonant at any frequency you wish to operate it on. This is done by using a grid dip meter and shorting the coils leads together. Any dip at any desired frequency means that the inductance will have to be changed slightly to move the resonance point to where it wont be encountered. Most of the time this is done by simply adding or shortening a few turns of wire. Those staggered windings on some chokes are done to stop self-resonance at a particular operating frequency, and are really several inductors being connected in series where Ltotal = L1 + L2 + L3, etc.. Hope this helps as an explanation.

Will



On Fri, 28 Jan 2005 23:42:10 +0000, Tomm Aldridge <KD7QAE@ARRL.NET> wrote:

Why are plate chokes seemingly black magic? Don't you just want a good decoupling of the PS from the Plate; i.e. lots of impedance from DC to Light and no resonances? How I get that should not be an issue but all teh plate chokes I see are long skinny and sometimes segmented single layer solenoids of questionable wire size. Why wouldn't a really lossy powdered metal toroid with a few fat turns on it work, assuming the inductance was high enough?

KD7QAE
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