On Tue, 8 Mar 2011, 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

We need more diagnostics to come up with better suggestions (I read the
other responses, too).

1. How do you "tune" your amplifier? Are you looking at RF output as you
move the plate tuning knob?

2. Like someone else asked: had you ever had it on 10 meters in the past?

3. How did you perceive one of the 572s blow? (flash of light? filament go
out? noise? smoke/smell?).


Look inside, see if you have a cold solder joint on 10 meter position of
bandswitch?

Get one new 572B and restore to test on other bands, first.

Don't wait 30 seconds before you remove drive.

Use less drive (carrier) next time.

Another possibility is that there was some kind of "short" that developed
inside the tube.

Next time check for output on at least one other band before you try 10
meters again.

On 03/10/2011 12:20 PM, Art Sowers wrote:


On Tue, 8 Mar 2011, 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

We need more diagnostics to come up with better suggestions (I read the
other responses, too).

1. How do you "tune" your amplifier? Are you looking at RF output as you
move the plate tuning knob?

2. Like someone else asked: had you ever had it on 10 meters in the past?

3. How did you perceive one of the 572s blow? (flash of light? filament
go out? noise? smoke/smell?).


Look inside, see if you have a cold solder joint on 10 meter position of
bandswitch?

Get one new 572B and restore to test on other bands, first.

Don't wait 30 seconds before you remove drive.

Use less drive (carrier) next time.

Another possibility is that there was some kind of "short" that
developed inside the tube.

Next time check for output on at least one other band before you try 10
meters again.


Sounds like a parasitic oscillation problem.

On Mar 19, 3:15*pm, Kenneth Scharf wrote:
On 03/10/2011 12:20 PM, Art Sowers wrote:





On Tue, 8 Mar 2011, 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

We need more diagnostics to come up with better suggestions (I read the
other responses, too).

1. How do you "tune" your amplifier? Are you looking at RF output as you
move the plate tuning knob?

2. Like someone else asked: had you ever had it on 10 meters in the past?

3. How did you perceive one of the 572s blow? (flash of light? filament
go out? noise? smoke/smell?).

Look inside, see if you have a cold solder joint on 10 meter position of
bandswitch?

Get one new 572B and restore to test on other bands, first.

Don't wait 30 seconds before you remove drive.

Use less drive (carrier) next time.

Another possibility is that there was some kind of "short" that
developed inside the tube.

Next time check for output on at least one other band before you try 10
meters again.

Sounds like a parasitic oscillation problem.