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Author Topic: Feng Shui  (Read 8204 times)
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W9ZSL
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« on: September 10, 2015, 10:55:39 PM »

Yes!  It exists!  I just spelled it wrong!  It's Chinese/Japanese and means "arrangement".  Like this maybe?  Chance got a couple of matching Cardwells and a GE coil, and THAT is Feng Shui.  We knew it all along.  Old Ham saying.  That looks right.

Question,  The caps are a perfect match so the two shafts are exactly on the same plane.  Do you recommend mounting these on ceramic standoffs?

Dang.  I think I got it!


* Feng Shui.JPG (784.57 KB, 2048x1536 - viewed 514 times.)
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K1JJ
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« Reply #1 on: September 10, 2015, 11:18:19 PM »

Vely pletty!

No standoffs required. For a pi-network where neither cap needs to float, a direct mount from capacitor frame to chassis is preferred - the lowest inductance path.  Unless there is an arcing problem due to the design of the cap being closer to chassis than the spacing, (and there should not be) just mount them both on chassis as is.

BTW, you DO want to use ceramic insulated couplers on both shafts, because even though they are grounded, you will still feel RF on the shaft or knob screw at times. (esp on a big rig on the higher bands)

T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

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« Reply #2 on: September 10, 2015, 11:43:39 PM »

I have more couplers than Trump has excuses!
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W9ZSL
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« Reply #3 on: September 10, 2015, 11:45:26 PM »

I can finally start violating a virgin chassis.   Cheesy Grin Shocked

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W9ZSL
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« Reply #4 on: September 11, 2015, 12:11:43 AM »

Tom, as I recall the ARRL discovered that effect at W1AW and without the insulated coupling to the knob, anyone touching the control became part of the circuit!  It actually set off a whole series of effects because "Just a Tweak" by a human meant the human screwed up the resonance.  The insulated coupler solved that.  The W1AW transmitters became extremely unstable because of the higher voltages.  Along comes the op to stabilize the system and he becomes part of the system.  There's a lesson in there somewhere.  I'll be danged.  Sounds like quantum physics! Not my problem.
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w4bfs
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« Reply #5 on: September 11, 2015, 09:45:10 AM »

just looking at that roller inductor .... I belive that is from a TCS tx .... if so the military used that at 40W level 18 mHz max freq .... I don't know how high in freq you are planning on but a single 813 plate modulated will go to around 300W and at 8 to 10 kOhm modulated impedance ... roller could arc between turns on higher bands ....I would consider something beefier there
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It would from many blunders free us.         Robert Burns
K1JJ
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« Reply #6 on: September 11, 2015, 01:11:00 PM »

just looking at that roller inductor .... I belive that is from a TCS tx .... if so the military used that at 40W level 18 mHz max freq .... I don't know how high in freq you are planning on but a single 813 plate modulated will go to around 300W and at 8 to 10 kOhm modulated impedance ... roller could arc between turns on higher bands ....I would consider something beefier there

I agree.

I've burned up a few of those smaller rollers when running a few hunfred watts in linear. In plate modulated service with an 813, it will be hopeless.  The contactor will develop hot spots quickly.

A good service job for that roller is in the amplifier input tuning circuit if you can fit it in. The 813 grid is a high impedance input, so make a reverse pi-network with two additional small variable caps. 50 ohms in and ~4K out into the 813. I use several of these rollers for just that job. Perfect 1:1 swr for the driver every time.

Here's the best roller inductor you can get for the plate tank -  below. I use two of these myself - one in a 1500w 8877 linear amplifier.  It uses a robust double wheel for the contactor and heavy copper bar strap for the coil . This one even comes with a turns counter and a strapping ceramic shaft coupler..

On eBay now. A little pricy but well worth it in the end. Get it before someone else does:

http://www.ebay.com/itm/Vintage-Large-Variable-Roller-Inductor-with-Counter-and-Handle-/371404019185?hash=item56796795f1

Remember never to tune the roller when RF power is going through it. The big roller will probably handle it, but the small one will burn its contact wheel and inductor quickly. Un-key the rig, tune it and then key the rig.


Otherwise, go with a heavy band switch and strapping coil.
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
w4bfs
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« Reply #7 on: September 11, 2015, 01:31:18 PM »


... go with a heavy band switch and strapping coil.


yes .... or a plug in coil per band of heavy wire / copper tubing ..... no unintentional resonances
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Beefus

O would some power the gift give us
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It would from many blunders free us.         Robert Burns
W9ZSL
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« Reply #8 on: September 11, 2015, 02:42:47 PM »

Actually the roller is the same one used in the single 813 Amp in the '57 handbook.
It was used in the BC-375. There is a separate coil for 10M.  I'm doing a more conventional layout.  Input eventually will be around 325 watts max but I'm going to start with lower power.  The BC-375 ran at about 1KV which is actually recommended for AM with this particular schematic.  Gotta start somewhere. I checked out the one suggested.  Expensive for sure.


* '57813C.jpg (123.6 KB, 672x512 - viewed 472 times.)

* '57 813A.jpg (267.25 KB, 1136x1200 - viewed 504 times.)

* '57813B.jpg (80.85 KB, 576x416 - viewed 467 times.)
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w4bfs
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« Reply #9 on: September 11, 2015, 03:50:50 PM »

yes .... that .07 inch spec on the plate cap sez a 1200 Volt or so limit on plate mod ... that 813 should work a long time at that level
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Beefus

O would some power the gift give us
to see ourselves as others see us.
It would from many blunders free us.         Robert Burns
W9ZSL
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« Reply #10 on: September 11, 2015, 07:18:21 PM »

That's what I'm thinking Beans.  One of my plate xfmrs will give me both 3200 and 2560 VCT.  However, the other iron is 4600 and 3750 VCT.  That one is reserved for the RF. I'll need to build the lower-voltage supply regardless.  I could easily use one supply for both the RF 813 and the 811A modulator to start and work my way up.  That is where I considered using the ART-13 mod iron.  The result would be around 200 watts in.  That coil would handle that fine.

I agree about using a rotary inductor.  Among other things it would really mess up the front panel if I decided to replace it.  I would rather go with fixed coils.  I have an Air Dux 2006T; 2 1/2" diameter, 6 TPI using #12.  I could use that for 40 and 75 then go with copper tubing for 20 and up.  Getting the right inductance for each band is another story.  

As for my plate cap, the spacing is .1" rated at 4.4KV.

It still looks mighty purdy regardless!
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« Reply #11 on: September 12, 2015, 12:01:00 AM »

I have more couplers than Trump has excuses!
I have more excuses that Mr. Trump has couplers!
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W9ZSL
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« Reply #12 on: September 12, 2015, 02:26:35 AM »

I can't use that coil.  The only way it would work is if I cut a big hole in the chassis so it can be lowered about an inch.  Just kidding!  Actually if the caps are fastened to the chassis, their shafts would be about an inch LOWER than the coil's. I always liked the looks of the Collins rigs post WW2.  They were always symmetrical with the front panel layout. They had Feng Shui!  All that aside, I have a better use for the coil.

I think individual plug in or screw in coils makes sense.  I have a crystal for 3885 and my dipole is cut for 3885 so I can start there.  Besides, like you said, the coil won't handle much over 1150 VDC.  That gives me more leeway for the tank circuit.  I only have the 75 meter dipole up 20' with a center support for the Balun.  It's incredibly close to 1:1 SWR.  I plan on raising the center to 30' for an inverted "V".

Make the Amp 75M mono-band for starters with room for expansion and modification.  

Quick, easy and dirty.   Cool
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