----- Original Message -----
From: "KD7QAE" <KD7QAE@ARRL.NET>
To: <Amps@contesting.com>
Sent: Thursday, August 25, 2005 9:33 PM
Subject: Re: [Amps] plate choke
> So, many of the plate chokes I have seen described in home brew
> amplifiers and those in use in Ameritron, Henry, Alpha and QRO are
> single layer wound solenoids, many like the one I built. These do not
> exhibit resonant peaks (High Z) in the ham bands as far as I can tell
> based on my limited experience. I am very confused by the assertions
> that I need to build something materially different than these examples
> or face a very inefficient and perhaps flaming amplifier.
>
Well before today, Tomm, I would have thought the same, but
after looking at Orr's Handbook, I am beginning to think
otherwise. For instance Orr describes an "HF Radio Frequency
Choke for Power Amplifiers" with a "2000-Watt PEP Rating"
covering "3.5 to 30 MHz" as follows:
"110 turns #26 enamel, space wound wire diameter
on a 4" long on a 1" diameter x 6" long ceramic form.
Series resonant at 25 MHz. (78uH)."
78uH at 3.5 MHz would be equivalent to an inductive
reactance XL of :
XL = 6.28 x 78 x 3.5 = 1714 ohms
assuming no effect from parallel resonance. For a plate load
resistance of say 2000 ohms and Ql = 10, the Xc of the plate
tuning capacitor would be around 200 ohms (227pF). Placing
a 1714 ohms of XL in parallel with Xc = 200 ohms would
presumeably produce a small but noticeable shift in tuning
(~+26pf offset in the plate tuning cap to bring the plate-tank
back into resonance). If the effective inductance of this
plate choke looks higher at 3.5 MHz due to the effects of
a proximate parallel resonance, the effect on the plate tuning
would be reduced.
Single layer inductors tpyically go thru a parallel resonance
before they go through a series resonance, so this plate choke
which has a series resonance at 25 MHz would likely have
at least one parallel resonances below that frequency. Whether
or not this parallel resonance would be broad enough to
produce multi-100K ohm shunt impedance over a significant
portion of the HF band is what is not clear to me.
BTW, the data plot shown in Orr that sounds similar to what
K6XK described (e.g. >multi-100Kohm parallel resistance
with multiple parallel and series resonances) is for a multi-stage
"Pie-wound" RF choke, not the single layer plate chokes he
describes. I don't know whether a single-layer plate choke
would exhibit similar behavior to the multi-section pie-wound
inductors or not. I would love to know, however.
I have an Ameritron single layer plate choke in my old 811A
homebrew amp. Maybe I will pull it out and try making some
measurements on it.
73 de Mike, W4EF.......................
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