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Author Topic: Roller inductor or band switch  (Read 10369 times)
KB1SEL
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Posts: 9




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« on: November 14, 2012, 08:14:31 PM »

Why so some people still use roller inductors for the l1 coil. I would think that they would present to many problems.
Having to clean them, re-lubricate them, Stray inductance self resonance issues etc etc.
« Last Edit: November 14, 2012, 08:19:53 PM by KB1SEL » Logged
K8AXW
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« Reply #1 on: November 14, 2012, 08:56:23 PM »

Infinite inductance increments vs. "chunks" of inductance when using a switch.  The maintenance is minimal.
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KB1SEL
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« Reply #2 on: November 15, 2012, 05:22:30 AM »

I guess its good to have if you plan on staying on one band or a band change every once and a while. Or if you for some reason need to adjust the q of the circuit often.

But for a multi band amp which will see band changes often I dont know.

It sure is a lot easier to stick an adjustable coil in then to build a tank circuit with fixed coils and a band switch
« Last Edit: November 15, 2012, 05:25:50 AM by KB1SEL » Logged
NO9E
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« Reply #3 on: November 15, 2012, 05:49:48 AM »

I used to have MFJ-962C with taps and now have TT 238 with roller. 962C was much quicker to use but once developed a nasty sparc in its switch. Solved by putting some glue where it sparced. 238 never complains.
Ignacy, NO9E
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KC4MOP
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« Reply #4 on: November 15, 2012, 06:45:09 AM »

Infinite inductance increments vs. "chunks" of inductance when using a switch.  The maintenance is minimal.
I love my Dentron 3KA tuner...the taps are so fast. Make sure these are heavy duty switches.

Fred
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KB1SEL
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« Reply #5 on: November 15, 2012, 07:38:25 AM »

Thank you for all of the comments. A roller inductor in a antenna tuner is one thing but im talking more about its use in a multiband amplifier.
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N3QE
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« Reply #6 on: November 15, 2012, 08:43:08 AM »

Thank you for all of the comments. A roller inductor in a antenna tuner is one thing but im talking more about its use in a multiband amplifier.

Non-amateur (e.g. military or commercial) HF amps used roller inductors for a long long time. But perhaps these are for situations where the transmitter hardly ever does band changes, and the tank may of course have to accomodate the non-ham bands.

Many of these roller inductors require a lot of hand-cranking to do a band change. Some of the fancy-pants military/commercial amps would auto-tune with motors to do the inductor rolling :-)

Amateur amps that have to band change easily, I feel that the bandswitch is the right solution.
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K8AXW
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« Reply #7 on: November 15, 2012, 09:20:51 AM »

SEL:  I didn't understand your question as written.  I apologize.  For an amateur amplifier the tapped coil with switch is the standard practice.

As for roller inductance for an antenna tuner, then I stick with my initial observation. 

One additional note:  I know a guy who built a homebrew linear that uses a roller inductor in his input circuit.  I think this is a great innovation.  Two variable caps and a roller inductor instead of 9 caps (which quite often needs to be experimented with to settle on a final configuration) and bandswitch linkage with the tank coil taps make for a really cool solution.
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WB2WIK
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« Reply #8 on: November 15, 2012, 10:22:35 AM »

One advantage of the roller inductor is more continuous-coverage, e.g., 3-30 MHz or whatever.  That requires some design considerations not always found in bandswitched amplifiers.

The Henry 4K Ultra is a great example of this implementation.

Many Henry amps used an edge-wound roller inductor as the main "plate tuning" element, with doorknob capacitors switched in to set the various band segments, along with a conventional air variable for the loading cap (also with bandswitched doorknob padders for the lower bands).  In some of those designs (I had a few), there wasn't any conventional plate tank "bandswitch" at all: The front panel bandswitch drove a chain and sprocket system to switch the "input" networks, and a camshaft to activate large spring-loaded lever switches which engaged the various fixed capacitors.

A bit complex, but it actually works fine and I've never had any of these fail, at least not from that switching system.

Even the very old (1950s vintage) Johnson Thunderbolt used a roller inductor in the plate tank circuit.  Since its input tuning was adjustable from the front panel (and it had the option of "untuned/swamped" input as well, for those with more drive power available), it covered 3.5 through 30 MHz continuously with about the same efficiency anywhere in that near decade of spectrum.

Not an unusual approach.  I've never had to service one of the roller inductors, even when used daily.
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N6AJR
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« Reply #9 on: November 15, 2012, 11:17:05 AM »

You can be more precise with a roller inductor, but it is slower and more tedious to use.  tune up time is something you want to avoid in a linear amp. most amps I own are either automatic tune or no tune. the few manual amps I have a 7 position switch for the different bands on the inductor and the capacitor are used for most of the tuning from there.  Look at a picture of a Kenwood TL 922 and you will see an easy to tune and good looking manual tune amp.  I also prefer a switched inductor on a tuner as opposed to a roller inductor.  it gets real old real fast to spin up fro 6 or 7 to 122. it takes awhile as you turn, the pile gets bigger and bigger on the rare DX.
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KG6YV
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« Reply #10 on: November 15, 2012, 11:27:02 AM »

I love my Henry 2KD Classic with it's roller inductor.  Tuning/loading is as fast with this amplifier as my AL-1500 with its bandswitch. 

6 of one 1/2 dozen of another...

Greg
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WB2WIK
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« Reply #11 on: November 15, 2012, 02:29:16 PM »

I'm surprised you had a "dust" problem with the Henry amps.  Must have been a fairly dusty environment, I'd think -- I had a 4K Ultra for years and the air flow went from under the chassis up through the tube socket and chimney and out the top of the tube, not much flowing around in the rest of the upper chassis.

In years, I never took off the top cover and used the amp almost every day. Smiley

When I sold it and was packing it for shipment, I did finally remove the cover and even removed the tube to ship it separately.  It wasn't dirty or dusty inside.

My 3K Premier used an edge-wound roller inductor and had a little problem after our big earthquake in 1994, when the whole amp fell to the floor (equivalent of being dropped from a height of about three feet).  It still worked after being set back up, but it tuned funny and the roller felt funny, so I opened it up: The roller had wedged itself between turns of the inductor and was scraping the silver plating.

Henry replaced the inductor for free while I waited, even though the amp was about 3 years out of warranty.  That was nice of them.  Never had another problem.
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WB2WIK
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« Reply #12 on: November 16, 2012, 09:32:43 AM »

A big negative for the 3k ultra and the 8k ultra is the fact that the l1 and l2 roller inductors were made by Henry Radio. So if they end up having a serious failure then your in trouble. Unless you want to spend big bucks to have new ones custom made.

Its also a common for the roller wheel on the l2 to have problems.

Im getting a little off topic. This has been a great topic with a lot of very informative replys

Thanks everyone


The roller in my 3K Premier (vintage 1986, I had the first one ever made) was also fabricated by Henry.  However, they had them in stock.  They still might, I don't know.  The amp shop is still there, even though they're not cranking out tube amps anymore: They do still service them.
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KB1SEL
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« Reply #13 on: November 18, 2012, 07:02:55 PM »

They dont have any more roller inductor parts unfortunately.
« Last Edit: November 18, 2012, 07:05:09 PM by KB1SEL » Logged
WN7T
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Posts: 15




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« Reply #14 on: November 25, 2012, 01:10:51 PM »

If you are building your own amplifier from scratch, I recommend the use of a HEAVY DUTY roller inductor as the main tank inductance for a pi-network output. I have had one in use for 30 years in several amplifier generations. There is almost no maintenance associated with the use of a well designed roller inductor, they are easy to rebuild/re-lube when necessary, and give you the ability to tune your amplifier very efficiently without the worries of "did I find the right point to put the band tap on the coil?/was that popping noise my band switch arcing over again because I miss-tuned the amp?".

My favorite roller inductors are the big edge wound Johnson (Multitronics) 222 series. They are easy to recognize on eBay and appear on a regular basis there for auction. If you plan on running 160 meters, make sure you find a 19 uH version.  Newly manufactured versions from Multitronics Corp. cost a bloody fortune, so stick to eBay or hamfests to find one.

I strongly recommend you consider the use of a heavy gage 2" or 3" diameter 3 or 4 turn copper tubing pre-coil in front of the roller inductor if you wish to load efficiently on the upper bands (10 meters).

Also, make sure to use a tuned pi input network with a Q of around 3 for each band of operation. You could indeed use a small roller inductor as part of the input network. By creating a faux tube impedance (using a fixed "non-inductive" resistor in place of the amplifier tube) with value based on the impedance charts for your particular tube at a given plate voltage, and using an antenna analyzer set to 50 ohms on the amp output (with no plate supply voltage turned on!!!), you can easily map the tuning coordinates for the roller inductor, plate tuning and load capacitors. Things get a bit more complicated if you decide to implement a pi-L tank circuit, but the roller inductance range is smaller.

I have a few pictures up on QRZ of my roller inductor RF deck implementation, if you care to look.

73
Paul WN7T
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