Re: [Amps] Ferrite-loaded plate choke

[R.Measures <r@somis.org>]

On Jan 8, 2005, at 3:15 AM, Angel Vilaseca wrote:

> Hi,
>
> I am currently building my first HF amp using a couple of 4cx 250b.
>
> I have found in the literature that the plate choke is quite a critical
> component and I found a few kinds of plate chokes designs.
> Recently I found this text (see below) on the Internet.
>
> Now I saw that the use of ferrite cores is classical in filament 
> chokes,
> but I never saw a ferrite core recommended in a plate choke.
>
> What do the experts on the list think about using a ferrite core in a
> plate choke?

Angel -- There really aren't any experts on AMPS.
> Plate choke: High RF current rules this circuit, and makes the design 
> of
> the plate choke much more difficult. The trends have been to reduce the
> inductance to the bare minimum required to block RF out of the B+ line.

Quite true.

> However, this trend in design has serious drawbacks due to the tendency
> of the plate choke to resonate at certain frequencies within bands that
> the amp operates on. I have found that increasing the inductance to 
> very
> high levels by using a ferrite core works well. The tough part of using
> ferrite in a plate choke is insulation, enough to prevent an RF arc to
> the core from happening. If enough insulation is installed, a ferrite
> rod will give a large amount of inductance with a relatively small
> number of turns.

What about placing the ferrite rod core in ABS, TFE, or polyethylene 
tubing?

> The lower number of turns for a high inductance gives a
> much lower "distributed capacitance", which is a major factor in choke
> resonance at high frequencies. My experiments have proved that for an
> amp operating 160-10 meters, a plate choke with an inductance of
> 800-1200µH is entirely stable on all HF ham bands. The problem with a
> choke with this level of inductance and able to carry the high level of
> DC current would be very large, and have a high level of distributed
> capacitance. A hollow core of an insulating coil form can hold a
> 1/2"dia. ferrite rod about 3-4" long. This rod will bring the typical
> inductance of a 90µH choke, up to 1200µH, with the distributed
> capacitance of the original 90µH choke. However, the higher level of
> inductance would cause any resonances to be far out of any HF ham 
> bands.

My guess is that the 0.5" ferrite rod material you used has a Mu of 
125.  This material distorts RF waveforms above 13MHz -- i. e., 
generates harmonics -- and suffers from heat-loss as well, which is 
probably why Mu 125 material is rated at 10MHz max.   In the 1950s, 
Collins Radio mfg a silver-ribbon roller-L that used a powdered iron 
core to increase L -- probably because powdered-iron is much less prone 
to saturate than ferrite.  However, the tradeoff with powdered-iron is 
lower Mu.  .  .  .  Mo' is not always mo' betta.  cheerz.
> 73
>
> Angel Vilaseca HB9SLV

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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