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Author Topic: Roller inductor or band switch  (Read 5503 times)
KB1SEL
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Posts: 18




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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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KB1SEL
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« Reply #11 on: November 15, 2012, 12:27:43 PM »

I had a 4k ultra and a 8k ultra. The 4k ultra had a edge wound johnson roller inductor for the l1 coil. And the henry 8k ultra had a tube type custom roller made by henry for the l1 coil. The 4k ultra had 2 sets of wiper contacts which made contact on the outside of the coil. This made cleaning the dust between them and the ribbon  fairly easy. But the henry 8k ultra was a little bit more difficult. To get at the actual roller to clean off any dust or grime which stuck to the roller shaft you had to take the whole roller assembly out.

I never experienced any failures with the 4k ultra roller. To go from one band to another did take some time like n3qe pointed out.

I used to own a Rockwell Collins hf 80 series amplifier which used a  double pi l tank circuit configuration. It had massive pancake roller inductors. The shafts were gold plated and so were the rollers. But still after a good amount of band changes the gold plating started to come off. And even with a quality  filter over the air inlet dust would still accumulate on the silver plate flat stock and the roller shafts.

What really caused problems with any amplifier I had with a roller inductor is when the dust and grime would get stuck between the actual roller wheel and the shaft it rode on.  When this would happen then the roller wheel would have a less then optimal electrical connection to the shaft and arcing usually occur which would cause pitting. The problem would only get worse and worse until cleaned or fixed. Same goes for  the connection between the roller wheel and the actual inductor surface that it rode on.

I dont think I would ever buy another amplifier for ham with a roller inductor if I planned on changing bands often

It also helps to have the ability to get replacement parts if their is a failure of the roller assembly or any part of it.

Have any of you tried to use variable capacitors for c1 and c2 and a fixed coil for c1 with multiple vacuum relays to progressively shorten the electrical length of the l1 inductor. Or maybe a multi band box with a setup like this but with fixed caps that are switched in for each band in parallel with the c1 and c2 caps so only a fine tune adjustment of the c1 and c2 caps would be needed for each band.
« Last Edit: November 15, 2012, 12:41:43 PM by KB1SEL » Logged
WB2WIK
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« Reply #12 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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KB1SEL
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« Reply #13 on: November 15, 2012, 02:49:14 PM »

Hi Steve, I think the majority of the dust was from the fan mounted to the front wall in the 4k ultra. It would draw air in from the vented top cover.

The 8k ultra pressurized the entire rf compartment and had holes in the rf deck floor  under each inductor and around the base of the tube. This allowed cooling for the l1 and l2 coils and everything else in the tank circuit. The chimney extended from the top of the tube to the rf deck cover.

The rf deck cover only had a hole big enough for the exhaust from the tube chimney. I like this method of cooling over the conventional way with the chimney at the base of the tube.

Since with the chimney at the base of the the hot air can re circulate into the blower depending on where the inlet is. And it also adds unnecessary heat to the rest of the components in the same compartment.

The guys at henry are great they are really nice.
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KB1SEL
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« Reply #14 on: November 15, 2012, 04:33:27 PM »

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
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