On Fri, 10 Feb 2006 13:40:49 GMT, George
wrote:

I have a HP201C oscillator, which mostly works. But, on the one freq range that
I need (x1), the output dies after a short period. This sometimes takes just
seconds, sometimes a few minutes. Other freq ranges are OK.

It turned out to be a bad tube. The grid on the 6AU6 developed a soft short (
1M) to the cathode when it got hot. This was enough to kill the loop gain on
the x1 range, where the bridge resistors were 8.3M, but didn't disrupt the other
ranges, where the bridge resistors were smaller.

George