[AMRadio] Oscillator Filament Voltage change and frequency drift

Jim candela jcandela at prodigy.net
Sun May 14 00:24:36 EDT 2006

Ok, Here is my first stab at this. The idea here is that the VFO uses 6.3
volts AC, and one side is grounded. This makes it tough to regulate unless
you use a half wave voltage doubler. Trying to use simple half wave
rectification and regulate has in the past failed because the rectifier
dropped nearly 1 volt, and the linear regulator needed at least 2 volts
in/out differential to regulate. So: 6.3 X 1.414 - .9 = 8 volts. On the
surface this will barely work with minimum 2 volt drop across the regulator,
but the input voltage will sag down considerably between the 60 hz half
cycles depending on load current, and input capacitor value. Therefore the
output will be pulsating at a 60 hz rate as the regulator keeps going in and
then out of regulation. This is a good idea that don't work.

In the circuit shown below, the diode drops only .3 volt at 300ma (6AU6
filament), and the regulator only needs about 0.2 volts in/out differential
to regulate at 300 ma. So 6.3 x 1.414 - .3 = 8.6 volts. This should work
since the regulator only needs .2 volts head so long as the input to the
regulator when dipping during the half cycle when the diode is reverse
biased can only decay to 6.5 volts.

I have a question for you mathematicians out there. What is the minimum
value of the input capacitor required in order to maintain regulation of the
LM2941CT? I picked 4700 uf @ 10 volts as a SWAG because it was as big as it
gets before size, and cost become factors. Assume the load current is a 6AU6
filament. The *AU6 tubes don't come in a 5 volt version, just 3, 4, 6, 12.

All these parts are in either Mouser or Digikey, and the major cost will be



1n5817 has forward voltage of about 0.3 volts @ 300 ma forward current:

LM2941CT low dropout adjustable regulator has about 0.2 volt minimum in/out
differential at 300 ma:

LM2941CT Visual:

-----Original Message-----
From: Rev. Don Sanders [mailto:innatehealing at bigplanet.com]
Sent: Friday, May 12, 2006 12:09 PM
To: jcandela at prodigy.net
Subject: Re: [AMRadio] Oscillator Filament Voltage change and frequency

Jim, I would be interested in what you find for low drop diodes and
regulators. I currently double the 6.3 vac and use a 6 volt regulator (7806)
to feed the vfo in TX and the LO in RX here. It works well but is a bit more
complicated. Without the regulator I could not get the SX111, SX100 and
NC300 to stabilize and any shift of the 120 vac would cause 50 to 200 cps
freq drift.

Healthfully yours,
                          DON W4BWS
----- Original Message -----
From: "Jim Candela" <>
To: "Discussion of AM Radio" <amradio at mailman.qth.net>
Sent: Friday, May 12, 2006 9:27 AM
Subject: RE: [AMRadio] Oscillator Filament Voltage change and frequency

> 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
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