Hello, I have three EB104 "kits" from Communication Concepts coming.
A friend and I are are building them (one for his Dad). I have been
studying web pages related to the Granberg EB104 amp and the necessary
surrounding components. This is my first RF project although I am no
stranger to homebrew electronics projects. Here are a few questions.
Swamping Resistor:
I looked at the Ameritron ALS-600 schematic since it is based on the
same Granberg application notes. They use what I have seen described
as a swamping resistor (35 ohms, 50W) on the input of the amplifier
(in series with the input transformer) to knock down the amplitude of
the incoming signal. I would also like to make the amp less sensitive
since I believe it only takes about 10W to drive the amp to full
power. The resistor obviously drops voltage across it when
transmitting, but it seems like a pi or T network should be used to
keep the input/output Z at 50 ohms. Is this just a case where it was
cheaper to do it this way than use a pi, or is there a reason for it?
Output filter:
I like the idea of using a diplexer instead of a basic low pass filter
since you can burn up a lot of the harmonics by sending it through a
high pass filter and into a 50ohm power resistor. K6IF has a circuit
board on his 1500W EB104 based page
http://www.k6if.com/ssamp/page6.html The circuit board PCB file is
Express PCBs software (which I am familiar with) and it would be nice
to order several boards to make filter building easy. Has anybody
used his filter board or have any comments on it?
Power:
I am not sure how to determine how large of transformer is needed for
the job. If the amp was producing sine waves all day, I would
probably want to assume 600W @ 50% efficiency so a >1200kV
transformer. But the main use (voice) has a very low average power
with peaks reaching 600W so is a 800KV to 1000KV transformer about
right? Toroid of Maryland has a 820KV toroid that will give me right
about 50V (of course the nice thing about a toroid is you can always
drop or add a few volts by adding a few turns).
Thanks,
Matt
N6EAJ
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