Hi Angel,
Here is a link to an old posting which describes how to use a "standard" RF
choke with 4CX250's.
http://lists.contesting.com/archives/html/Amps/1998-10/msg00480.html
If you fabricate the entire Pi-L coil from RG-402 then the effective
impedance at the plate choke will be circa 50 ohms. I am using this latter
method with a pair of 4CX600's in an amp currently under constuction.
73 & Good morning,
Marv WC6W
-- Angel Vilaseca <avilaseca@bluewin.ch> 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?
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.
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. 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.
73
Angel Vilaseca HB9SLV
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