Plate RF choke has an appreciable DC current component in it. This lowers the
effective mu of the ferrite, and makes it less effective as a choke. As a
matter of fact, the value of inductance will fluctuate with peak current
fluctuations. One could design for this, but besides being a 'moving target' in
the design, the flux density in the ferrite may drive it into saturation.
Remember that the choke will have RF voltage across it, and DC current going
through it. With normal chokes, the 'cold' value of inductance (and inductive
reactance or RF impedance near self resonance) is measured easily and is
predictably the same as when the amplifier is energized with HV and RF.
Ferrite-loaded inductors still do have parasitic resonances due to the stray
capacitance from wire turns. They tend to shift down in frequency so they still
have to be taken care of that they are not excited at the normal operating
frequencies. There is no free lunch here. Less turns of wire, for sure, but
also the resonances are lower due to the higher permeability in the coil's
medium.
Second reason is that spaced turns on a coil form are easy to insulate.
The good aspect of a filament choke is that two wires are bifilar wound, so
that the magnetic field of one wire is cancelled by the opposite magnetic field
in the other wire (since the current is always flowing in opposite direction
through the filament circuit). The ferrite sees only the RF voltage across it,
and not the DC magnetic field that would otherwise degrade the inductance
value. A torioid could be used also, just be careful that the hot end and the
cold end of the choke are not adjacent on the toroid. Also, the stray capacity
would be different, causing resonances that need to be figured, as before. Most
people just use a nice linear ferrite rod for this, as it makes the layout
simple, hot end near the tube socket, cold end near the AC line side. Many
years ago, before ferrite (BF), the cathode RF choke would have been just an
air wound inductance for each filament lead. Sometimes this is done in large
industrial and commercial systems, where ferrite is impractical
or too
expensive, and the frequency range of operation is small.
73
John
K5PRO
> Date: Thu, 06 Mar 2008 07:56:34 +0100
> From: Angel Vilaseca <avilaseca@bluewin.ch>
> Subject: Re: [Amps] Ferrite (was: how to wind an HF broadband 10:1
> transformer)
> To: Manfred Mornhinweg <mmornhin@gmx.net>
> Cc: amps@contesting.com
> Message-ID: <47CF95A2.2090800@bluewin.ch>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> Another situation where excessive wire length can lead to unwanted
> resonance effects (overheating) is the plate choke.
> If the plate choke was wound on a ferrite rod, or toroid, much less wire
> length would be needed.
>
> Why is this never seen in classical designs?
>
> On the other hand, a cathode choke wound on a ferrite rod IS a
> classical, but never a plate choke. Why?
>
> And why is a ferrite rod always used for the cathode choke, but never a
> toroid?
>
> 73
>
> Angel Vilaseca HB9SLV
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