[R-390] re: Call for measurements - 100dB carrier reading

[David Wise]

A 10 ohm 1-turn pot is 10 ohms per turn.
A 100-ohm 10-turn pot is 10 ohms per turn.

Okay, it's not "no better", but it's not 10 times better.
If the 1-turn is 270 degrees, the 10-turn is only 33% better.
It spends its life in the bottom turn, the other
nine might as well be fixed.

If you've tried both and seen a drastic difference, it
must be attributed to the wire size and build quality,
not the number of turns.

I started this discussion not to supplant the
time-honored ten-turn pot (I already conceded that
it relieves the symptom), but to get into the
design problem that makes it necessary.  It's
like a crooked wall with a jack bracing it.
I'd like to know how it could have been built
straight in the first place.

I like mental candy bars,
Dave Wise

> -----Original Message-----
> From: r-390-bounces at mailman.qth.net
> [mailto:r-390-bounces at mailman.qth.net]On Behalf Of Jim M.
> Sent: Thursday, August 30, 2007 2:41 PM
> To: r-390 at mailman.qth.net
> Subject: Re: [R-390] re: Call for measurements - 100dB carrier reading
> 
> 
> Also, a 100-ohm 10-turn
> is no better than a 10-ohm 1-turn.
> ------------------------------------
> ???
> A 10 turn pot (such as the one I identified in previous 
> email) will allow 
> much finer adjustment of zero, and I believe would be more 
> stable.  I have 
> used that replacement in several 390a's now and the meter 
> zero is always 
> dead on zero regardless, once I set it.  The 10-turn pot 
> provides good 
> vernier control as compared to a single turn.  It's wattage 
> is more than 
> adequate.  And it mounts easily.  And it is sealed.  And the 
> shunt resistor 
> stays across it.  AND THE METER NEVER DRIFTS.  Me thinks you 
> guys are making 
> the problem much harder than it needs to be in terms of the 
> meter zero 
> adjustment.  Not to criticize, but I think this subject has 
> turned into a 
> "mental candy bar" as they say in engineering, sometimes 
> called "analysis 
> paralysis."
> 
> 
> ----- Original Message ----- 
> From: "David Wise" <David_Wise at Phoenix.com>
> To: <r-390 at mailman.qth.net>
> Sent: Thursday, August 30, 2007 1:23 PM
> Subject: RE: [R-390] re: Call for measurements - 100dB carrier reading
> 
> 
> Bob Camp already asserted that wattage was not
> an issue.  I'll fill in the detail.  13mA * 67.5mV
> is less than 1mW.  Going beyond worst-case,
> 13mA**2 * 100 ohms is still only 17mW.  Unless
> you were being deliberately perverse or have
> access to nanotechnology, you couldn't make a pot
> that can't take this.  They picked a big one so
> techs wouldn't need a magnifying glass, that's all.
> 
> A 10-turn cermet would be fine electrically, but
> aren't they pcb-mount trimmers or something?
> There would be mechanical challenges and
> it would look odd.  Also, a 100-ohm 10-turn
> is no better than a 10-ohm 1-turn.
> 
> I haven't read Chuck's ER article.  If he was
> just changing R537, he was perpetuating the
> same old foolishness.  The lower the shunt,
> the more likely that tube variations will
> tip you past the point where zero is not
> achievable.  Lame!  That's what drove me
> to look for something better.
> 
> Regards,
> Dave Wise
> 
> > -----Original Message-----
> > From: r-390-bounces at mailman.qth.net
> > [mailto:r-390-bounces at mailman.qth.net]On Behalf Of Perry Sandeen
> > Sent: Wednesday, August 29, 2007 10:14 PM
> > To: r-390 at mailman.qth.net
> > Subject: [R-390] re: Call for measurements - 100dB carrier reading
> >
> >
> > Gents,
> >
> > over the years the 100 ohm pot was replaced 10 turn pots but
> > still using a 2 watt unit.
> >
> > IIRC, Chuck Felton in his ER article on R390 upgrades shunted
> > the post with a low value 1/2 watt
> > resistor.
> >
> > The question becomes: Could one use one of the multi-turn
> > cermet type pots instead? They are cheap
> > and plentiful.  Multi-turn 2W pots are in the $20 range now.
> > I do concede the point that is won't
> > look "original".
> >
> > Had anyone tried this?
> >
> > Regards,
> >
> > Perrier
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