FYM match: matching tetrode inputs on 144 MHz
Dr.
R. den Besten, PG0A/PA3FYM
Introduction
Building
medium and/or high power tube amplifiers is a creative process and is a lot of
fun. Besides knowledge of electronics, mechanical insight is almost obligatory
to bring these kind of projects to a satisfying end. And, don't forget the guts
when powering up your contrapsion for the first time !
When I made
my first Y681 (a water cooled 4CX400J with approx. 1200W plate dissipation,
i.e. the 'secret' PE1MFB-design) tetrode amplifier in 1988, I considered the
'standard' input/grid circuits of other designs too complicated. I decided to
make something simple and most certainly reinvented a wheel in a period where
internet was accessible to the scientific incrowd only.
FYM-match
When a
tetrode is being driven in a grounded cathode configuration, the input
impedance is very high. The majority of the grid circuits have a 1/4 wave
approach with two variable capacitors: tune and load. In 1988 I decided to use a 1/2 wave circuit with a coupling link.
Actually, a 1/2 wave circuit consists of two 1/4 wave parts of which one part
is 'inside the tube' almost completely. This means that the high current, i.e.
low impedance, point is somewhere in 'the middle'. Empirically I made a coil of
3 turns and loaded the part which was closest to control grid with a hairpin. By
transmitting into the cold tube I tuned the circuit and manouvering the hairpin
resulted in a reflection damping of almost 50 dB. I found out that heating the
tube with its filaments has a minor influence.
Example: AM6154 / GRT21 ITT VHF 144 MHz
amplifier
As an
example the modification of the grid circuit of the well known surplus
amplifier AM6154 is discussed.
Please
refer to the AM6155 VHF+
Conversion Repository for the original input circuit of the AM6154. With
the FYM match the 1/4 wave grid line and input loading capacitor are discarded
(literally). In figure 1 the control grid modification is presented
schematically. The two carbon (!) resistors of 2k2/2W are damping the control
grid to avoid oscillation of the amplifier and decrease the overall gain of the
amplifier somewhat.
Figure 1. AM6154
input circuit using the FYM tetrode match.
The 1 nF capacitor is for safety purposes. It serves
as a precaution for not destroying the tube when the input tune capacitor will
have a shortcut to ground for whatever reason. I considered the plate distance
of the input tuning capacitor of the AM6154 too small.
In figures 2 and 3 this story is visualized.
Figure 2. Picture
of AM6154 input circuit using the FYM tetrode match.
Figure 3. Picture
of 4CX600JA input circuit using the FYM tetrode match.