Valiant VFO Resistor

[Glen Zook]

The consensus is that the resistor should be inside the VFO box.  This is because the temperature compensating was done by Johnson with the resistor inside.  As such, it actually takes much longer for the VFO to stabilize if the resistor is not inside.  There have been several amateur radio operators who have researched this (by experimentation) and have decided that it is best to leave the resistor inside the VFO compartment.

Now the original resistor definitely does not have enough power dissipation and therefore definitely needs to have a higher rating.

Like the Heath VF-1 and DX-100 series VFOs, the VFO in the Ranger, Valiant, etc. can be stabilized more by a few changes.  The first is to change the tube from a 6AU6 to a 6AH6.  This is a direct replacement and the only thing that might be required is to "touch up" the trimmer capacitors (only) because the inter electrode capacitance of the tubes are slightly different.  This single "modification" makes "light years" difference in the stability.

The second is to change the 0A2 voltage regulator to an 0B2.  This drops the screen voltage from 150 volts to 108 volts which definitely helps stabilize the oscillator.

The third thing is to modify the B+ to the oscillator by regulating it down from the 300+ to 150 volts.  Using the 0A2 removed from the screen circuit is a good place to use that regulator tube.

Doing these 3 things will make a noticeable improvement in the stabilization of the VFO frequency.

Glen, K9STH

Website:  http://k9sth.com


--- On Tue, 11/18/08, hudlerb1 at netzero.com <hudlerb1 at NETZERO.COM> wrote:

> From: hudlerb1 at netzero.com <hudlerb1 at NETZERO.COM>

There has been some controversey over whether this 18K VR dropping resistor should be inside or outside the VFO box. 
 
One thought, if it is out of the box, it keeps the VFO from heating as much, thus less drift.
 
Another is if it is under the chassis, it never warms completely over a long time, thus drifts slower but never reaches operating temp.
 
5 or 10 watt resistor?
 
I'm getting back into my rig to finish the mods, calibrate again, and get it back on 160 very soon.


      

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