In article , Robert Casey
writes:

I looked at a few tube VFO schematics, and
I don't see anything that different from the AM radio
hartley osc circuit. So how did they avoid drift, or were
you expected to leave your VFO on all the time?

Several ways:

1) Better components - Drift of the kind being discussed is mostly due to
thermal effects. Capacitors, inductors and even resistors change value when
heated, and the component selection makes a *big* difference in stability. For
example, a variable capacitor with aluminum plates is inherently more affected
by temperature than one of similar construction with brass plates.

2) Better design - Reducing heat reduces thermal drift. High C is usually less
drifty than low C. A high gain tube that is loosely coupled to the tank circuit
is usually more stable than a low gain tube tightly coupled to the tank
circuit. There's lots more, of course.

3) "Weakest link" - As sources of drift are corrected, sources which were once
negligible become dominant. Often a design will go through several revisions as
sources of drift are identified and corrected.

4) Compensation - When all else is done, the use of thermal compensating caps
can reduce drift to very low levels.

Remember too that "stable" is user-defined. A rig that drifts 300 Hz on each
transmission might be considered "very stable" on AM or FM, barely acceptable
on CW, and useless on SSB or FSK/PSK

73 de Jim, N2EY