[AMRadio] Oscillator Filament Voltage change and frequency drift

[Jim Candela]

Don K4KYV said:
> > 
> > I ran the VF-1 off a CVT.  It too, drifted with
> slight 
> > changes in fil. 
> > voltage.  I think both the VF-1 and Collins PTO's
> use a 6AU6 
> > as oscillator 
> > tube.  My "silent" Sola beats fooling around
> swapping tubes.  
> > I suspect even 
> > a good tube would eventually become drifty with
> age.
> > 
> > Don k4kyv


Don,
   
 I'm wondering about those 6AU6's. Not all of us have
a Sola CVT, so I have another idea. Change the tube
for a 5AU6, and then take the 6.3 vac (usually
unbalanced), and 1/2 wave rectify with a low drop
Schotcky diode (about 0.3 volt forward drop), filter,
and then run through a 3 terminal low drop 5 volt
regulator. I'll do some book searching for some
suitable part numbers. I think it will work with
minimal fuss. Some of the available low drop
regulators (use pnp output transistor)only need about
1/2 volt minimum in/out differential to maintain
regulation, so the trick will be to keep the regulator
Vin at the 60 hz ripple minimum value at > 5.5 volts.
This will require a big filter capacitor in value, but
5,000 mfd at 10 volts is still real small in size.

Jim
JKO