It is much cheaper to buy a used SB220 for little money then trying to
collect parts for a homebrewed amp. If you are an old timer with a big junk
box or visit a lot of flea markets, you may be able to collect enough parts
to be able to build a much better amp with better parts than the commercials
could afford. And cheaper too. You may find an overrated transformer or a
vacuumcap that would be too expensive for a relatively lowpriced commercial
amp.
The charm with a homebrewed amp is that you can build an amp to meet some
special preferences. Lets look at my problem:
I travel a lot in summertime and like to operate portable from DX spots like
OH0. I have a bad back and no drivers license. So, the amp must be low
weight and easy to transport. Then there are two choices:
Build a low profile rf unit and separate power supply to be transported in
two suitcases. As it seems there are no high voltage switched power
supplies avaiable, will then use a low profile hi-voltage swedish toroid
transformer that still are on our surplus market.
Or, build a number of normal power supplies that could be staying all year
around in the qth:s I normally use and only carry the rf section with me.
Could use 3 himu triode connected 8072 tubes in paralell with homemade
external coolers similar to the EIMAC 8875s. Cooled by two flatpack blowers
for low weight and size. To further lower the cooling requirements could use
instead a supercathode, a DAF ( here we go again,hi!) or a carrier
controlled bias system to lower anode dissipation. The box could be made of
glass fibre laminate still giving rf shielding and low weight.
I need only need a few bands. But, for all band coverage I can use small
sized variable capacitors plus the the high voltage high current LCCs and
toroids I already have. That could be switched in with the hi power relays I
already have also.
The 8072s are quite cheap in Scandinavia - I have a number of them. The
control grid can take 100mA and one can assume the screen grid can take even
more. I have anumber of them but only one socket, so far. Anyone?
This system cannot be found on the commercial market and I must build it
myself. Transistorized amps still are no low weight and not considered.
What say - any comments or suggestions?
Happy New Year to all from Hans SM5KI
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