On Wed, 9 Mar 2011, Roger Basford wrote:

On 09/03/2011 07:28, Tim Wescott wrote:
On Tue, 08 Mar 2011 19:49:15 -0800, 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.

Used to work, doesn't work now -- sounds like a bad switch.

Exciter works fine, amp blows toobs -- sounds like something on the 'big
sparks' end of the amplifier.

No degradation noticed (or mentioned, at least), then suddenly things go
"blam"! -- sounds like a bad switch.

Bad contact on one of the bandswitch segments on the high power side of
the amp? That'd make the tuning be way off, which would load the tube
way wrong, which would make 'that expensive smell'.

Dunno -- things to check.

I'd say, that as it was working fine on the lower bands, there is a good
chance that the neutralisation

SB-200 was grounded-grid. Did not have neutralization (I had a SB-200).

wasn't set right for Ten and the amplifier
went into self-oscillation, destroying one of the 572s.

Roger/G3VKM