odd coil

[KA9EGW]

so today the delivery guy dropped off my new coil, it is from a bc tx, huge, about 4" ID and a foot long, and is edge wound, it is probably a hundred micro Henry or better...and it has two rods running up through the middle of it with a sliding doohickey about 2" diameter and 2" long, on a plastic chain...it slides on the two rods so it cant rotate and even though its hollow like a giant fuse cap the rods go through so you can't stick anything up inside of it.
I understand the coil, its purpose is to inductify, but what's with the doohickey? is that supposed to be a tuning slug?

[KA9EGW]

Pic of coil

[KA2DZT]

Well if it's made from brass which it probably is, it a tuning unit which sort of shorts out some the turns (even though it doesn't make any contact with the coil) when moved into the coil.  This is my first guess but I'm sure someone here knows exactly what it is.

Fred

[Steve - K4HX]

I think you have it Fred. It looks very much like a coil out of a RCA BTA-1MX or BTA-500MX. Those have a brass insert like the one pictured that is used for tuning.

[KA9EGW]

So if I'm using it in an amplifier tank tuned by air or vacuum variable(s) I can remove the insert and its support rods.  Anyone got a need?

[Steve - K4HX]

Yes, you can remove it.

[KA9EGW]

If anyone here has a need for it they can have it for the cost of shipping...

[KA9EGW]

Update: apparently (read: seller said) it's a Johnson 200-411-2

[W7TFO]

That tuning scheme was pretty slick. 

No contacts to burn like a roller. 

No shorted turns eating up power. 

Used a fixed cap for simplicity.

As the brass slug went in, the inductance was lowered.

73DG

[KA9EGW]

Oh, indeed!  But how amenable to covering 160-10?  HI!

[WQ9E]

I agree that is a neat tuning scheme and even when used with switch taps it would provide easy fine tuning of the inductance value.

Many of the old alignment tool sets included a "magic wand" that had a brass slug in one end and a ferrite slug in the other.  Inserting either end into a properly adjusted coil would result in decreased output, an increase indicated alignment needed attention.

[KA9EGW]

I think I may have one of those wands...

[WA2SQQ]

I agree that is a neat tuning scheme and even when used with switch taps it would provide easy fine tuning of the inductance value.

Many of the old alignment tool sets included a "magic wand" that had a brass slug in one end and a ferrite slug in the other.  Inserting either end into a properly adjusted coil would result in decreased output, an increase indicated alignment needed attention.

Yup! I have one and often wondered why more people don't know about these.

[Opcom]

There's a 3CX3000 amp on the www for 6M using 2-3 turns of pipe and a kind of cylinder like that. My guess it is best for single band tuning.