[AMRadio] Oscillator Filament Voltage change and frequency drift

Brett gazdzinski brett.gazdzinski at verizonbusiness.com
Fri May 12 09:03:29 EDT 2006

I did lots of experiments with VFO/LO circuits when building
the homebrew receiver.
I found the most important thing in my circuits was the coil.
The best setup I came up with was large B+W coil stock.

Any slug tuned system I tried sucked, as did coils wound on
any type of form.
I tried ceramic forms, slug tuned or not, plastic, bakelite,
I tried using various npo caps to offset things, etc.
I tried various types and sizes of coil wire, close wound, space wound,
more or less inductance, hot wire wound on cold forms, glue to hold
the turns, etc...

The B+W coil LO seems to be REAL stable after a 2 minute warm up,
less then 500 Hz drift, no regulated filament voltage, although
the B+ has a VR tube to regulate it.

I also found using octal tubes as VFO/LO tubes is tough, they 
are large and change a lot as they heat up. There is a wide range
between octal tubes as far as how they drift, and how much.


> Recently, I have seen VF-1's go for big bux amongst the 
> collectors.  I 
> wouldn't throw one away nowadays.
> One problem I discovered with my VF-1 was thermal drift.  I 
> took it apart 
> and noticed the construction; the slug tuned coils are 
> mounted on one wall 
> of the oscillator subchassis, and the slugs and their 
> bushings are mounted 
> on the opposite wall.  So as the temperature varies, the 
> whole compartment 
> expands and contracts, and moves the slugs in and out of the 
> coils.  No 
> wonder the things drift!  Then I noticed the insides of a 
> DX-100, which has 
> a nearly identical VFO.  The coils and slugs are mounted on 
> the same piece 
> of chassis panel.
> So I remounted the coils in mine to the same side of the 
> subassembly.  It 
> was easy - just drill a couple of  mounting holes on either 
> side of the 
> original coil slug bushings (with the slugs removed during 
> the process).  I 
> had to slightly re-route some of the wiring, but it was easy to hook 
> everything back up, and it worked FB once reassembled.  It 
> didn't stop the 
> drift entirely, but reduced it by at least 90%.  It was nice 
> and stable on 
> 160/80, but was still drifty on the 40m. range.
> 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
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