On Tue, 05 Oct 2004 01:36:33 GMT, 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
....snippage

I tested an RCA AA5 I have here (uses battery tubes) and
a modified Hallicrafters S120 (also basically AA5) and neither drifted
very far on turn on. The worst was the RCA! at 1.8khz in the first 5
minutes. The RCA drifted 3khz in the first 5 minutes from cold start.
However once both had a chance to fully warm up (20minutes)
the drift was in the less than 100hz region. Generally most AA5
radios for AM broadcast reperesent the lowest possible cost designs
with the least consideration for circuit performance above a minimal
level.

Tubes experence fairly large initial warm up temperature changes
and the surrounding circuits often do as a result of that. When you
consider that your going from 20c to greater than 50C in the first few
minutes there is no surprize there (transistor VFOs would be hard
pressed for that great a temperature change too!).

The solution is manifold. use components that experience minimal
dimensional or other characteristic change for temperature. Coils
and capacitors are the biggest influence here.

The AA5 you have uses a hartly osc and the common circuit has less
than 4 components in the VFO, namely the tube, variable capactor,
a padder cap (vfixed usually) and the coil (slug tuned). Lets look at
each. In the design the tube has changes at warm up of both
mechanical, things move such as cathode to grid spacing when heated
and electrical its operating point shifts as the tube reaches operting
temperture. The mechanical tuning cap, while likely the most stable
device if heated enough the aluminum plates will deform and posible
change spacing, there may be other forces from the chassis mounting
as that warms too. The padder cap is usually a cheap component
in AA5s and the typical part used has a poor temperature coefficienct.
Lastly the coil, this can also be a big factor as the coiled wire can
mechanically change dimension from heating but, you also have a
powered iron or ferrite tuning slug that also has a temperature
coefficient and the cheaper (older) materials can really be poor
with temperature. I might add that some of the AA5s coils were wax
impregnated and the materials used can also experience dimensional
changes while heating up causing the coild to deform. I may add
that operating voltage changes can influence stability and drift.
Tubes are no worse than transistors, just warmer. What differes is
that there are two sources of voltage sensitive drift in tubes, The
heater(filament) voltage must be stable as it affects tube operating
characteristices such as gain and also the environment due to heating
of the area around the tube. The other votage that must be stable is
the B+ (high votage usually but can be anywhere from 12 to 300v
depending on tube and circuit).

Hopefully you can see that VFO design superficially can be very simple
but has many details that can influence stability. It is possible to
design a tube (or transistor) VFO that is very stable but it requires
good components, temperature compensation and good mechanical
construction.

If you want stability in your AA5... Better cooling most ventilated
very poorly. Other things, put the components away from the tube
[but not too far ] to minimize heating effects. Regulate the
voltages. The latter is harder as the average AA5 uses a series
heater string that has a sometime shakey warmup. Also the AA5
uses a poor supply in the form of a 35w4 half wave rectifier that has
to warm up to work (it's a tube too!) and the lack of good power
supply filtering (add to this the caps are old!!).

As a experiment with tubes:
The Hallicrafters was a basket case when I got it so modiflying it was
a reasonable thing to do (just to make it work) lest the purist
classic radio people protest. First was converting from live chassis
(typical AA5 off the mains) by adding a transformer to supply 120V for
rectifier and 6.3v for the heaters. Rewired the heaters for 6.3V and
change the 50C5 to a 6AQ5 I had on hand (rewire socket). This radio
used a selenium rectifier which was bad so silicon bridge rectifier
was used with new 100uf caps as filter to get clean 150v B+. Since
If selectivity was minimal I added an old 6KHZ mechanical filter and a
second IF amp tube (5899 subsub mini) to fix that and and a bit of
gain. Replace a dozen poor quality and just plain bad caps in various
points including the Local osc section (VFO). I also added a 6AR5
bfo/product detector for ssb operation as the original BFO design was
poor at best. It was a major rebuild with many circuit changes mostly
for fun. However the 6BE6 and the Hartly local osc (VFO) was
retained to keep the tuning dial something near calibrated. A stable
VFO using tubes was straghtforward using good quality parts ( both the
BFO and the LO are LC osc). The result is a stable (after warm up)
general purpose reciever that I use for 3885KHz AM and occasional 75m
SSB listening.


Allison
KB!gmx