On Aug 26, 11:30*am, laura halliday wrote:
Indeed. I have several old ARRL and RSGB Handbooks and a
number of older references (including Single Sideband for the
Radio Amateur).

You can see the fashions change. At one time the cool part to
use was the 7360 beam deflection mixer tube; at other times the
challenge was to see how many 40673 MOSFETs you could use
in a single radio. Many of these designs are difficult to reproduce
now.

What's amazing is that some of the SSB rigs appearing in the 60's in
the pages of QST use not one, not two, but three 7360's :-).

40673's were also used with wild abandon. If someone wants an honest-
to-goodness 40673 they can pay $15 for a NTE221 in a metal can, but
most experimenters would use a BF998, obviously a different package
but same function.

I'll see your BC-455 tuning capacitor and raise you an Eddystone
dial. :-)

You can make nice radios by applying the right technology to
the problem. My recipe for a stable VFO nowadays is a
postage-stamp size surface mount board mummified in bubble
wrap and stuck to the end of a good, solid air variable capacitor.

The brass-screw-in-a-solenoid is something I just tried in my
reproduction of the MMR-40 and I am very impressed with its stability
and mechanical simplicity as a PTO. A similar but not identical
mechanical design is used in the WA6OTP PTO, I haven't tried it yet.

Tim.

On Aug 26, 10:46 am, Tim Shoppa wrote:
....
The brass-screw-in-a-solenoid is something I just tried in my
reproduction of the MMR-40 and I am very impressed with its stability
and mechanical simplicity as a PTO. A similar but not identical
mechanical design is used in the WA6OTP PTO, I haven't tried it yet.

Tim.

Though not actually PTO if it's a brass screw, right?

Cheers,
Tom

On Aug 26, 2:26*pm, K7ITM wrote:
On Aug 26, 10:46 am, Tim Shoppa wrote:
...

The brass-screw-in-a-solenoid is something I just tried in my
reproduction of the MMR-40 and I am very impressed with its stability
and mechanical simplicity as a PTO. A similar but not identical
mechanical design is used in the WA6OTP PTO, I haven't tried it yet.

Tim.

Though not actually PTO if it's a brass screw, right?

Permeability of brass is slightly less than air, which is why the PTO
goes up in frequency as I screw the brass screw in.

It's not an iron-powder slug (which would go down in frequency as it
enters the solenoid) but it's still a PTO.

If you haven't tried them, I highly encourage you look at the PTO in
the MMR-40 and the WA6OTP PTO. Completely 100% homebrewable and I'm
very happy with the results.

Tim.

On Aug 26, 12:05*pm, Tim Shoppa wrote:
On Aug 26, 2:26*pm, K7ITM wrote:

On Aug 26, 10:46 am, Tim Shoppa wrote:
...

The brass-screw-in-a-solenoid is something I just tried in my
reproduction of the MMR-40 and I am very impressed with its stability
and mechanical simplicity as a PTO. A similar but not identical
mechanical design is used in the WA6OTP PTO, I haven't tried it yet.

Tim.

Though not actually PTO if it's a brass screw, right?

Permeability of brass is slightly less than air, which is why the PTO
goes up in frequency as I screw the brass screw in.

It's not an iron-powder slug (which would go down in frequency as it
enters the solenoid) but it's still a PTO.

If you haven't tried them, I highly encourage you look at the PTO in
the MMR-40 and the WA6OTP PTO. Completely 100% homebrewable and I'm
very happy with the results.

Tim.

Well, I beg to differ with you about the (main) reason the frequency
changes. Consider what happens if you have two coils magnetically
coupled and you monitor the inductance of one while you apply a short
across the other one. Then consider what you'll see on the inductance
meter as you change the coupling between the measured coil and the
shorted one. The references I've been able to find so far suggest the
permeability of brass is slightly higher than that of air, but I
suppose it's a function of the composition of the brass; in any event,
I'd bet that the shorted-turn effect is very much larger than the
permeability effect with respect to changing the inductance of the
oscillator coil.

Cheers,
Tom

On Aug 27, 3:58 pm, K7ITM wrote:
On Aug 26, 12:05 pm, Tim Shoppa wrote:



On Aug 26, 2:26 pm, K7ITM wrote:

On Aug 26, 10:46 am, Tim Shoppa wrote:
...

The brass-screw-in-a-solenoid is something I just tried in my
reproduction of the MMR-40 and I am very impressed with its stability
and mechanical simplicity as a PTO. A similar but not identical
mechanical design is used in the WA6OTP PTO, I haven't tried it yet.

Tim.

Though not actually PTO if it's a brass screw, right?

Permeability of brass is slightly less than air, which is why the PTO
goes up in frequency as I screw the brass screw in.

It's not an iron-powder slug (which would go down in frequency as it
enters the solenoid) but it's still a PTO.

If you haven't tried them, I highly encourage you look at the PTO in
the MMR-40 and the WA6OTP PTO. Completely 100% homebrewable and I'm
very happy with the results.

Tim.

Well, I beg to differ with you about the (main) reason the frequency
changes. Consider what happens if you have two coils magnetically
coupled and you monitor the inductance of one while you apply a short
across the other one. Then consider what you'll see on the inductance
meter as you change the coupling between the measured coil and the
shorted one. The references I've been able to find so far suggest the
permeability of brass is slightly higher than that of air, but I
suppose it's a function of the composition of the brass; in any event,
I'd bet that the shorted-turn effect is very much larger than the
permeability effect with respect to changing the inductance of the
oscillator coil.

Cheers,
Tom

I agree, except Air has no permeability, you can't magnetize air. You
can't magnetize brass. What you have is a proximity detector. aka
metal detector.

73
n8zu

On Aug 28, 7:29*am, raypsi wrote:
On Aug 27, 3:58 pm, K7ITM wrote:
I'd bet that the shorted-turn effect is very much larger than the
permeability effect with respect to changing the inductance of the
oscillator coil.

Cheers,
Tom

I agree, except Air has no permeability, you can't magnetize air. You
can't magnetize brass.

I think you're confusing permeability with susceptibility. Even vacuum
has non-zero permeability.

I'm still considering Tom's points. What I know from decades of
experience, is if I put a brass slug into a solenoid, the inductance
goes down by a little bit. "Little" is relative to the effect of a
ferrite slug, which makes inductance go up by a lot in comparison.

The "little" is key for the PTO designs I mention, it allows rather
fine tuning with very simple mechanicals. If I think of it as shorting
turns as it moves in, that is completely out of whack in my head with
the observed changes, because shorting all the turns causes a big
change in inductance, not a small one.

Tim N3QE

On Thu, 28 Aug 2008, Tim Shoppa wrote:

I'm still considering Tom's points. What I know from decades of
experience, is if I put a brass slug into a solenoid, the inductance
goes down by a little bit. "Little" is relative to the effect of a
ferrite slug, which makes inductance go up by a lot in comparison.

Which of course is why you see iron core and ferrite slugs in high
inductance coils, but in small inductance, like you'd see at VHF,
the slugs were brass.

Or, that tuning wand with some sort of ferrite at one end and brass
at the other, so you could tell whether the tuned circuit was too high
or too low.

At that level, brass and ferrite were seen as the same thing, a means
of varying the inductance. The choice was made by how much variation
you needed, and which way, rather than what action was taking place.

Michael VE2BVW

On Aug 28, 6:25*am, Tim Shoppa wrote:
On Aug 28, 7:29*am, raypsi wrote:

On Aug 27, 3:58 pm, K7ITM wrote:
I'd bet that the shorted-turn effect is very much larger than the
permeability effect with respect to changing the inductance of the
oscillator coil.

Cheers,
Tom

I agree, except Air has no permeability, you can't magnetize air. You
can't magnetize brass.

I think you're confusing permeability with susceptibility. Even vacuum
has non-zero permeability.

I'm still considering Tom's points. What I know from decades of
experience, is if I put a brass slug into a solenoid, the inductance
goes down by a little bit. "Little" is relative to the effect of a
ferrite slug, which makes inductance go up by a lot in comparison.

The "little" is key for the PTO designs I mention, it allows rather
fine tuning with very simple mechanicals. If I think of it as shorting
turns as it moves in, that is completely out of whack in my head with
the observed changes, because shorting all the turns causes a big
change in inductance, not a small one.

Tim N3QE

Hi Tim,

OK, a 'speriment for you, and another way of looking at it that will
yield the same result:

Make two "slugs," one solid and one hollow. If it's permeability
that's doing it, the hollow one (with thin shell, of course) will have
considerably less effect. If it's the "shorted turn" effect, both
will be about the same.

Consider that there is no time-varying magnetic field inside a shell
made out of good conductor--really zero if it's a superconductor, but
dropping to practically zero after a few skin depths. So you are
removing a volume of magnetic field when you put a conducting slug in
the coil's field. With less field, there's less energy stored, which
implies lower inductance. The same thing happens when you put a coil
inside a shield-can: the inductance is reduced. A points that seems
to be not well known: the unloaded Q of a helical resonator is less
than the Q of the same coil that's not inside a shield. There are
graphs available to determine the lowering of inductance by a
cylindrical shield around a coil.

Cheers,
Tom

On Aug 28, 9:25 am, Tim Shoppa wrote:


I agree, except Air has no permeability, you can't magnetize air. You
can't magnetize brass.

I think you're confusing permeability with susceptibility. Even vacuum
has non-zero permeability.

The definition I found for:
Permeability (electromagnetism), is the degree of magnetization of a
material in response to a magnetic field

In this case 1 means no permeability.

73

n9zu

raypsi wrote:
On Aug 27, 3:58 pm, K7ITM wrote:
On Aug 26, 12:05 pm, Tim Shoppa wrote:



On Aug 26, 2:26 pm, K7ITM wrote:
On Aug 26, 10:46 am, Tim Shoppa wrote:
...
The brass-screw-in-a-solenoid is something I just tried in my
reproduction of the MMR-40 and I am very impressed with its stability
and mechanical simplicity as a PTO. A similar but not identical
mechanical design is used in the WA6OTP PTO, I haven't tried it yet.
Tim.
Though not actually PTO if it's a brass screw, right?
Permeability of brass is slightly less than air, which is why the PTO
goes up in frequency as I screw the brass screw in.
It's not an iron-powder slug (which would go down in frequency as it
enters the solenoid) but it's still a PTO.
If you haven't tried them, I highly encourage you look at the PTO in
the MMR-40 and the WA6OTP PTO. Completely 100% homebrewable and I'm
very happy with the results.
Tim.
Well, I beg to differ with you about the (main) reason the frequency
changes. Consider what happens if you have two coils magnetically
coupled and you monitor the inductance of one while you apply a short
across the other one. Then consider what you'll see on the inductance
meter as you change the coupling between the measured coil and the
shorted one. The references I've been able to find so far suggest the
permeability of brass is slightly higher than that of air, but I
suppose it's a function of the composition of the brass; in any event,
I'd bet that the shorted-turn effect is very much larger than the
permeability effect with respect to changing the inductance of the
oscillator coil.

Cheers,
Tom

I agree, except Air has no permeability, you can't magnetize air. You
can't magnetize brass. What you have is a proximity detector. aka
metal detector.

73
n8zu
Brass slug acts as a shorted turn, absorbing rf and decreasing
effective inductance. You can tune a circuit with a link coupling coil
of a few turns connected to a low ohm value pot!