Matching input of PA that contained several tubes GU50
with a transceiver is not a simple matter as it seems at first
look. Sometimes for simplicity of the design PA the driver power
from a transceiver going straight away to several connected to
bridge cathodes of the GU50.
resistance of a PA containing 2 tubes GU50 (that
is close to 50- Ohm at major amateur HF- ranges) is differed from
input resistance of a PA with 3 tubes GU50 (the resistance lower
in 1.5 times compare to 2xGU50- sample), and of course differ
from input resistance of 4xGU50- PA (the resistance lower in 2
times compare to 2xGU50- sample). So, the input resistance for
those PAs with 3 and 4 tubes is differ the 50-Ohm that requires
proper matching of the PA with driving transceiver.
Radio club "Arktika"
Photo Credit Line:
Figure 1 shows
a simple autotransformer that helps match input impedance of the
PA to 50-Ohm in wide frequencies range.
The autotransformer was made on a Russian ferrite toroid
with permeability 60. It is possible to use Amidon
T-50-2 (OR t-50-6). At some cases it is better to use 2 toroids
putting together. Coil contains 12 turns of wire in diameter 2-mm
(12-AWG). Tap is chosen for best matching of the transceiver with
the PA in wide frequency range. Experience shows that best matching
for 3xGU50 PA gives tap from 8th turn (from "cold"
end). For 2xGU50 PA best matching gives tap from 5th
turn (from "cold" end). Anyway, try to find best tap
and best quantity of coils in the autotransformer for your design
of the PA.