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Re: [Amps] filament choke questions

To: Amps <>
Subject: Re: [Amps] filament choke questions
From: Roger Parsons <>
Date: Thu, 8 Oct 2009 04:43:29 -0700 (PDT)
List-post: <">>
Whilst the explanation below makes perfect sense, I have 1kW and 500W ex 
commercial FM (100 MHz) broadcast transmitters made by CCI - the former uses an 
8877 and the latter a 3CX800A7. Neither of them had filament chokes and I 
presume they worked OK! (The 1kW has been modified to form the basis of my 160m 
remote station and the 500W awaits a project.)

73 Roger

"Hi Paul
Just have survived strong wind of typhoon 18 here.

The purpose of the filament choke for 3CX800A7 is,
to keep filament's RF potential neutral, to avoid a
breakdown of cathode-heater insulation, and to reduce
loss of driving power.

2007 ARRL handbook pp18.20:
"The filament or “heater” in indirectly heated tubes such
as the 8877 must be very close to the cathode to heat the
cathode efficiently. A capacitance of several picofarads
exists between the two. Particularly at very high frequencies,
where these few picofarads represent a relatively low
reactance, RF drive intended for the cathode can be
capacitively coupled to the lossy filament and dissipated
as heat. To avoid this, above about 50 MHz, the filament
must be kept at a high RF impedance above ground.
The high impedance (represented by choke RFC1 in Fig
18.15) minimizes RF current flow in the filament circuit
so that RF dissipated in the filament becomes negligible.
The choke’s low-frequency resistance should be kept to a
minimum to lessen voltage drops in the high-current
filament circuit.
The choke most commonly used in this application is a
pair of heavy-gauge insulated wires, bifilar-wound
over a ferrite rod. The ferrite core raises the inductive
reactance throughout the HF region so that a minimum
of wire is needed, keeping filament-circuit voltage drops
low. The bifilar winding technique assures that both
filament terminals are at the same RF potential.
Below 30 MHz, the use of such a choke. The filament or
heater in indirectly heated tubes such as the 8877 must be
very close to the cathode to heat the cathode efficiently."

1/2 o.d. ferrite bar will work.
Material is not so matter. I am using a ferrite bar for
MF radio.

Self-supporting (no core) bifilar wound coil also
work, if it have no hole and sufficient inductance
around 50 MHz - I think about 2 uH is enough.

Also, non-bifilar, discrete coils also work.
Some circuits using indirect or direct heated tube
employ two discrete coils between the heater
terminal of its socket and and power supply feedthru
capacitors. 12 turns 1 mm enameled wire wound
1/2 inch OD self-supporting will work.

de Han JE1BMJ"


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