[Heath] SB-200 problems...solved...

[Kenneth G. Gordon]

On 19 Sep 2011 at 22:45, Wilson Lamb wrote:

> Well, you're sorta right.  I got the diagram and looked.  The shunt is
> not really in series with the equalizing resistors.  It's in series
> with the ground side of the PS, so all the plate current goes through
> it to get back to the negative side.

Yes. Of course you are correct. My mistake in not explaining 
properly. Thanks.

> Watch out.  If you open the shunt, you'll have B+ at the gap and
> you'll blow the meter and who knows what else.  If anything comes
> loose in your multimeter circuit, B+ will be present on the rest!

Oh, yes! I knew that, and that is why I was VERY careful to make sure 
all was connected properly before I even plugged the amp into AC 
power.

As I said in another post, the result was that the external 
milliammeter read 68.8 mA idle current, which is, at least, in the 
ball-park.

This proved, given that all components have been proven to be well 
within spec, that the inaccuracy lies within the plate meter itself.

> Your trimmer should be fine.

Yes. I think that will work to "calibrate" the meter, at least.

I am still not really certain where the inaccuracy comes from. 
Perhaps the meter itself is losing its magnetism. I don't know. But 
at least I have discovered the source of the problem that has been 
bugging me since I got this amp, and now have a solution.

I still have some other "issues" with this particular amp which I am 
almost certain are related to the tubes I have to use.

With considerable help from Carl KM1H I have a handle on those and 
will address those as soon as I have the time.

Thanks for your help, and for the suggestions from the others here.

I still have to deal with the Harbach soft-switch problem. I may 
build my own interface, or dump that method completely and go to the 
QSK circuit devised by Rich Measures. I have all the parts.

Ken W7EKB