[Amps] Another SB200

[Tom Rauch]

> 1. Looking at the meter i can see that it has been opened 
> and worked on so
> without testing i am going to assume it is cased. Is there 
> any other meter i
> can replace it with or where can i find a replacement.

Check the connections to the srews through the rear of the 
meter case by tightening up on the nuts agains the case a 
tiny bit. Some Heath meters developed bad connections there, 
although you could have a bad coil.

> 2. It has 2 Svetlana tubes and from what i hear that might 
> not be the wisest
> choice as far as replacements go.

The Svetlana 572's were all very old stock and gassy. 
Ameritron made the mistake of buying Svetlana 572's when the 
AL572 was released, and out of every 100 tubes well over 
half were unserviceable with gas right out of the box. This 
made the tubes arc upon power up. Some tubes would "heal" 
after a few arcs, but we had to build a special test fixture 
to run new tubes and attempt to getter or age them. We still 
wound up with about half bad. Out of the ones we tested we 
had about 50% of the amps in the field fail with bad tubes.

When Ameritron pressured Svetlana they finally caughed up 
the fact that the tubes were old stock and there was nothing 
they could do about better aging or anything else.

Out of several hundred tubes nearly all failed despite 
testing and use of many protection devices in the amp.

>This might prompt me to retrofit using a
> GI7B. What is the input impedance of the 2 572B's?

About 80-100 ohms for a pair.

> 3. Without replacing the existing transformer is there a 
> way i can piggyback
> another transformer to up the voltage a tad to give me 
> about 2500 on key
> down?

You could, but it would not be a wise thing to do. The SB200 
transformer already has marginal secondary insulation. 
Anything you do to boost the voltage would take headroom out 
of that winding's insulation.

73 Tom