Voltage regulator tubes, matching components for thermal drift, solid
(physical) construction, for starts. Shielding not only for noise, but
also to minimize thermal drift. If you didn't leave the whole thing on
all the time, maybe you left the filaments on. If you actually turned
the thing off, be ready to wait an hour or so after power up for things
to settle down.

Collins used different techniques -- a permeability tuned oscillator
among them.

Growing up, a bunch of us were hams, living in a few block radius. One
character was really proud of the VFO he'd built; very solid
construction, quality parts, the whole 9 yards.

He had it sitting on a shelf that was mounted to an outside wall of his
house. We'd go pound on the outside of the wall, and you could hear
the thing wobble like mad.


In article , Robert Casey
wrote:

The local oscillator in "All American 5ive" vacuum tube
AM radios all drift an annoying amount at the upper
end of the AM BC MW band. The oscillator would be
running at about 2MHz, and warm up drift (from cold
start to about an hour being on) is typically 20KHz.
Enough to make that station at 1520 tune itself out.
AM radios used a hartley style oscillator using the
equivalent of a triode with its plate to B+, grid
capacitivitly coupled to the LC osc tank, and cathode
connected to a secondary winding on the LC osc tank.
Usually an air variable cap, and fixed inductor wound
on a cardboard coil form.

VFO's for ham radio work would involve higher frequencies,
and I would think that they not drift anywhere as bad as
the AM radios did. 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?