This was covered at length on the forum in the past and is also on W8JI's
While a ferrite loaded choke will perform well in a low power transmitter
such as the single 6146 which the original magazine article was about it
wont hold up in a QRO amp for the reasons mentioned below by John.
----- Original Message -----
From: "John Lyles" <firstname.lastname@example.org>
Sent: Thursday, March 06, 2008 10:29 AM
Subject: [Amps] ferrite in RF chokes for PAs
> 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.
>> Date: Thu, 06 Mar 2008 07:56:34 +0100
>> From: Angel Vilaseca <email@example.com>
>> Subject: Re: [Amps] Ferrite (was: how to wind an HF broadband 10:1
>> To: Manfred Mornhinweg <firstname.lastname@example.org>
>> Cc: email@example.com
>> Message-ID: <47CF95A2.firstname.lastname@example.org>
>> 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
>> Angel Vilaseca HB9SLV
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