Johnson Thunderbolt Questions

Kenneth D. Grimm grimm at LYNCHBURG.NET
Thu Jul 22 21:06:20 EDT 1999

Bill Neubauer wrote:
> I am thinking about getting a Johnson Thunderbolt amplifier to use with
> a SX-117/HT-44 combo.  There are a few questions I would like to pose to
> the BA community:
>     -reliability of the Thunderbolt amplifier?
>     - expected output?
>     -strengths/weaknesses/design problems of that model?
>     -price range for a complete/working (not "collector quality") unit?
>     -any other comments-- pro and con?
> Tnx and 73s-- Bill//KC4ZZ

Reliability - It is built like a battleship with very heavy duty
Expected output - As I recall it was something like 650 watts DC input
and so maybe 1200 watts or so PEP output.  Price - Don't have a clue.

Other comments - Well it was not very efficient.  The plate voltage was
too low.  It was AB1/2 grid driven so if you had a 100 watt exciter you
needed an attenuator pad to absorb the excess drive.  It had the power
supply built in, so it weighed about 100 lbs or so.

It was my first commercial amp and I loved it, but I soon discovered the
beauty of grounded grid and higher voltage and although I kept mine for
many years, it received relatively little use.  The only parts ever
replaced were the 866A mercury vapor rectifiers which I changed out for
3B28s to reduce the hash in the receiver.  The 4-400s also gradually
weakened and I replaced them a time or two.  I had fun with my
Thunderbolt and once upon a time even modulated the screens to produce
an interesting AM signal!

Ken K4XL
grimm at

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