[Amps] ferrite in RF chokes for PAs

[jeremy-ca]

This was covered at length on the forum in the past and is also on W8JI's 
web site.

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.

Carl
KM1H


----- Original Message ----- 
From: "John Lyles" <jtml at losalamos.com>
To: <amps at contesting.com>
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.
> 73
> John
> K5PRO
>
>
>> Date: Thu, 06 Mar 2008 07:56:34 +0100
>> From: Angel Vilaseca <avilaseca at bluewin.ch>
>> Subject: Re: [Amps] Ferrite (was: how to wind an HF broadband 10:1
>> transformer)
>> To: Manfred Mornhinweg <mmornhin at gmx.net>
>> Cc: amps at contesting.com
>> Message-ID: <47CF95A2.2090800 at 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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