On 04/06/2011 11:05 PM, K1HL wrote:
On Mar 19, 3:13 pm, Kenneth wrote:
On 03/09/2011 01:05 PM, Michael Black wrote:



On Wed, 9 Mar 2011, Tim Shoppa wrote:

On Mar 8, 10:49 pm, 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

The SB200 was made before the 11 meter regulation came into being. The
SB201 was the re-designed amplifier that covered this. However, being a
kit Heath complied with the law by leaving out the 10 meter switch label
on the front panel and not including in the instructions the steps to
connect the wires to the band switch. All the required parts were
actually in the kit. If you sent the company a photo copy of your ham
ticket (or even a QSL card) they would send you the missing page in the
manual. But it only required a half a brain to figure it out on your own.

Other companies complied with the law in different ways. One linear
only required removing a screw from the band switch that served as a
stop to prevent the amp from going into the 10 meter position! My
Kenwood linear required adding one or two capacitors in the front end
and adjusting a coil. I also had to add a decal to the front panel for
10 meters (I used a dymo label).


Thanks everybody for your ideas. Turns out that the most likely
answer had to do with one of the two 572B's being internally
defective. I ordered another matched pair (goodbye $100) and tried
again; but as suggested, first to confirm it worked on 15 and 20,
which it did. Then, with great trepidation went to ten meters, this
time keeping the exciter at a low output. Tuned quickly, never more
than 5-10 secs before giving it a rest. Waddya know, the darn thing
tuned just like it does on the other bands. Began to raise power,
watched the plates for cherry - everything good with output to about
400W. Since the incident I have carefully etched fine pencil marks on
loading and plate tuning so I can go back to resonance very quickly.
Even did same on the exciter.

Sometimes you never know exactly what happened, but Art Sowers hit it
right. Amp with previous set of tubes always ran on 10M. When it
failed filament went out, breakers tripped, flash of light, pop of
noise - probably bad tube that worked ok at lower frequencies.

Thanks again!!
Harry K1HL
Where did you get the new 572B's? I've heard that no matter WHO sells
them or what label is on the bottle, they are all made in the same
factory in China today. I suppose some of the importers may test them
better and have in their contract that they can return tubes that fail
their tests to the factory for credit, so where you buy them from might
make a difference, but you are still getting the same tubes.

If you are REAL lucky you might still be able to find some NOS US made
572B's, but they won't be cheap. The Svetlana 572B's are NOT the same
spec's as the ones used in most ham linears. They are AUDIO tubes!
They will work in a ham linear but you must keep the plate voltage at
2000 or less, maybe apply some bias, and be happy with less power.