|[AMRadio] Oscillator Filament Voltage change and frequency drift|
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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