On Wed, 9 Mar 2011, Tim Shoppa wrote:

On Mar 8, 10:49*pm, K1HL wrote:
My SB-200 works perfectly 80-15 meters. *10 M opened the other day and
I found the SB-200 would not tune. *Within 30 seconds one of the
572B's blew out and the breakers both tripped. *My exciter, when used
without the SB-200, tunes perfectly on 10 - very low SWR, etc. - so
it's not an antenna issue.
It's got me stumped. *Built the linear in 1970 and never had a problem
til this.
Tnx - Harry

Had you ever used the SB-200 on 10 meters before?

For a good length of time, the FCC required that amps fail to operate
on 10M (really 11M was the target of the law but 10M got swept into
it) until a specific (available to "hams only" as if anyone else
should have a linear) mod had been applied. I forget when that law
went into effect but could it have been as far back as 1970?

I would have said something like that, but he said "built in 1970" so
unless he's off by about 7 years, it can't be the issue.

I thought the law was still in effect.

The problem was, anything that covered 10meters would cover 11meters,
they being adjacent. The issue was that while CB amplifiers were illegal,
lots of small companies would sell them as ham amplifiers, despite the
relatively low drive requirement (the same level that a CB set would put
out) and often not high power amplifiers. Even forty years ago, one
could look in the Lafayette catalog and see "illegal for Class-D CB"
amplifiers on the CB page, keeping Lafayette out of trouble at the time,
yet practically telling people "this is just what you need to boost your
power output on CB". And of course, since something illegal is about
making money, the amplifiers were often junk, so not only was there the
issue of high power where low power was the norm, but bad signals in
and out of the band.

Michael VE2BVW