Great info Jim, I've been confused on the spreadsheet myself trying to convert
an SB-220 to 6m.
What LCR meter do you recommend? I don't have a good one.
Thanks & 73,
Jim WE4S
Get BlueMail for Android
On Dec 19, 2022, 8:16 AM, at 8:16 AM, "jim.thom jim.thom@telus.net"
<jim.thom@telus.net> wrote:
>Date: Sat, 17 Dec 2022 17:15:35 -0500
>From: Steve Bookout <steve@nr4m.com>
>To: amps@contesting.com
>Subject: Re: [Amps] L-PI amp network
>
><Jim,
>
><Thanks a bunch for your input.
>
><I was close.
>
><I had used .65 for the current and a "K" factor of 2.0 as I'm 99% CW.?
><It's been DECADES since I was above 28.1 MHz.? I could have easily
><chosen 1.8.?? I also had '9999' for the choke and .1 for suppressors.?
>I
><also had '5pf' for strays, where you used '3pf'. At least, I wasn't
><totally lost.? I was counting on being able to use GM3SEK's
>calculator.
>
><Having said that, I followed along with your numbers, and I can ALMOST
><get what you got.? I'm looking for where my entered data is different
><from yours.? At one point, I hit the comma instead of a decimal
>period.?
><Boy, did that mess things up.? Had a heck of a time finding the
>problem.
>
><Not sure how you got a Q of 19.? I had entered a Q of 12 and that was
><reflected in the C1/C2 info from the get-go.
>
><How much of a big deal is it going to be if I use 1/4 copper tubing??
><It's what I've got.?? I've got many feet of .015 x 2.00 inch copper,
>but
><even if I split it, it's too thin to self support.? Do you have a good
><source for a suitable copper strap?
>
><I'm lucky as I have a 14 inch metal lathe I use to wind these coils.?
>In
><the past, I've used #4 solid copper (3/16 inch) as I had a bunch of
>it.?
><Makes good solid coils, but it work hardens, making changes a PITA.? I
><DO put the lathe in neutral and turn it by hand...
>
><Steve, NR4M
>
>## You can't use a K factor of 2.0 since that's for class C. You
>can't
>run any high MU GG triode in class C on CW or you will get severe key
>clix
>on CW. It has to draw a small amount of idle current for CW use, like
>10-25 ma. On my hb amps, and also my 4 x L4B's, I can adjust the bias
>V /
>idle current ..on the fly. No point in sucking 180 ma of idle current
>on
>an 8877, when using CW mode. This is why I don't use zener's for bias.
>A
>string of 6A10's or 10A10's is dead simple, and they have a 400 / 600
>amp
>surge rating, you won't blow em up, nor overheat them. A 1N5408 runs
>fairly warm with 1 amp CCS flowing through the string. A spst toggle
>can
>tap the string for CW use. Then tap the other end for required bias
>for
>SSB. I'm not a data expert, but I believe u can reduce the idle
>current
>way down for data / rtty / FT-8 modes.
>
>With increased bias V.... you then require a tiny bit more drive..to
>overcome the extra bias V.
>
>Enter ZERO for all the suppressor parameters.
>The xtra stray C is added on both line 43..and also line 53. I
>guestimated 5 pf for line 43 and 3 pf for line 53. I don't have a
>8877
>tube + socket to test, and measure.
>
>Without a digital LCR meter, you are dead in the water. It's ur number
>one
>tool when building amplifiers, or anything else. Then you can map out
>both
>the tune and load caps in fine increments and make up a cheat sheet.
>Ditto with measuring coil values. I also use it to measure ALL the
>stray
>C. The 8877 is 10 pf from anode to grid. Once in the socket the anode
>to
>grid C will rise a bunch...due to the proximity of the lower anode fins
>to
>the chassis below em. You will also get more stray C from rear and
>side
>cabinet walls..and also the top lid. This is why you have to be
>careful
>when mounting the tube, when amp is used on upper HF bands, like
>17-15-12-10-6m. Using the lcr meter, between anode and chassis, then
>you
>know exactly how much stray C you have..and can enter into the
>spreadsheet
>on line 43. If tube is mounted in the back corner, u will get stray C
>from both the sidewall and also rear wall. In that case, the tube's
>anode
>has to be further away from the side / rear walls. C between anode and
>side / rear wall is inversely proportional to distance. IE: 1"
>spacing is
>X pf. 1/2" spacing will be 4X pf. Same concept as any air variable
>cap.
>On that 3CX-6000A7 6M amp, I had scott keep the anode a full 2.5" away
>from
>the left sidewall, and also 2.5" away from the rear wall. Any amount
>of
>stray C will screw up everything. Good news is, stray C is easily
>measured. Once known, plug it into the software.
>
>Copper strap was obtained locally at my metal supermarket. They have
>3' x
>7' sheets of copper, in thickness from .015" all the way up to .125".
>.032" is ideal, and easily holds it's shape. I had em shear in 3'
>lengths,
>in 3/4" widths..and also 1"..and also 1.5" widths. You can buy copper
>strap from Georgia copper in .032" thickness..and in several different
>widths. https://www.gacopper.com/032-strap.html
>
>They also have it in 12" lengths in 1" widths.
>
>Copper flashing works good too...and is cheap.
>
>The huge advantage of strap coils is...they are easy to snip off, say
>1/4"
>at a time, and repunch the hole with a roper whitney or eq punch tool.
>They come with dies from 3/32" up to 9/32"...in 1/32" increments. 7 x
>dies
>included. You can't drill copper strap, punch it. I use the same
>punch
>to punch 3/16" (6/32") in .125" thick 6061-T6 plates all the time (
>for
>10-32 screws)...and also 5/32" holes for 8-32 screws. Dirt cheap on
>amazon, HF, etc.
>
>When u snip off the end of the strap coil, u simply twirl the entire
>coil
>a few degrees, so the newly punched hole fits it's termination on the
>cap.
>What u are doing when twirling the entire coil is.... you are reducing
>it's
>diameter a tiny bit. That's a bitch to do when using tubing.
>
>If you are going to use tubing colis, download K6STI's 'coil' program.
>It's dead on vs measured coils. It will only work down to a min of 2
>turns. It will do fractional turns, like say 2.75" turns etc. BUT you
>have to know where each end is gonna terminate. On his coil software,
>you
>can change the diam of the coil to anything you want..and also it's
>total
>length. and also factor in the leads on each end of the coil (more
>tubing,
>and usually flattened on the extreme ends). It only works for tubing
>or
>wire coils, NOT strap coils. It's very close though, when using strap
>coils. The strap coils use wider material, but have minimal spacing
>between
>turns, so it comes out a wash.
>
>When I mentioned a Q of 19..that was withOUT the 1st coil !! That was
>just
>a normal PI network. Which is why I don't use a normal PI network on
>15-6m.
>
>1/4" tubing runs damned hot on 15M..if you use a dead cxr for say 10-12
>seconds, let off the footswitch and touch the coil, it will take ur
>finger
>prints off. Ok for cw / ssb..and that's it.
>
>Tubing coils only conduct current on the outside of the tubing, never
>the
>inside (like water inside a pipe). Tubing coils also don't conduct on
>the
>inside of the completed coil. The current bunches up on the outer
>circumference. IE: 1/4" OD tubing has a .785" circumference..and
>only
>1/2 of that handles the actual current.... = .3925"..which is
>miniscule.
> IF strap is used, it will have to be .3925" wide to handle the same
>current. 3/4" wide strap will handle 90% more current vs 1/4" tubing.
>1" wide strap will handle 2.55 X more current vs 1/4" tubing.
>
>Current handling capability decreases as to the square root of the
>ratio of
>the 2 x freqs.
>IE: A coil on 28 mhz will only handle 1/2 the current as the same coil
>material on 7 mhz.(28 / 7 =4. sq rt of 4 = 2) And circumference is
>directly proportional to diameter. 3/8" tubing will handle 50% more
>current on any freq..vs 1/4" tubing, etc.
>
>For 10m, both coils are very small (.8 uh and .91uh) When coils are
>that
>small, that's when strap coils make sense. You can use almost zero
>spacing
>between turns on a strap coil. A tubing coil will require at least it's
>tubing OD for the spacing between turns. (You can see the effect on
>both
>UH....and also the coil Q asap, on the coil software. UNloaded Coil Q
>is
>just the XL of the coil / ESR of the coil. The coil software will
>spit
>out Q, XL, and ESR.
>
>OK, here is the interesting part that folks always forget. XL is
>directly
>proportional to freq...since XL = 2 x PI x F x L
>
>ESR, (effective series resistance) or RF resistance is NOT
>proportional to
>freq. ESR on 28m is only double vs 7 mhz. Meanwhile the XL on 28 mhz
>is
>quadruple vs 7 mhz.
>
>What this means is the coil on 7 mhz might have an UNloaded Q of say
>500 on
>7 mhz. Same coil now has an UNloaded Q of 1000 on 28 mhz. Joe ham
>thinks
>his 28 mhz coil is superb, since it has an UNloaded Q of 1000. In
>actuality, the 28 mhz coil has DOUBLE the ESR vs the 7 mhz coil. With
>the
>same RF current, squared X double the ESR, the 28 mhz coil now has
>TWICE
>the watts dumped into the coil..... vs 7 mhz.
>
>When we talk about loaded Q (total network Q) of a PI net, higher
>loaded
>Q's result in directly proportional higher tank circulating currents.
>Current in the main coil with a Q of 19 is gonna be a lot more vs a
>loaded
>Q of just 11. 19/11 = 73% MORE current with a Q of 19.
>
>When I switch from SSB to say a dead cxr, the average plate current on
>SSB
>( with a lot of both inboard + outboard processing) is exactly 1/2 of
>the
>key down plate current. With a dead cxr, average plate current has now
>DOUBLED..and the amount of watts dumped into the tank coil has now
>QUADRUPLED. It's just I squared x R. In this case it's RF current
>squared x ESR of the coil. Now you can see why so many amplifiers
>burn up
>bandswitches, crack the ceramic, melt the polystyrene on airdux, etc,
>when
>used on FT-8..which appears to be all the rage these days.
>
>The way I deal with heat is..... don't generate as much of it in the
>1st
>place..which implies running the lowest loaded Q as practical. On the
>lower bands, a loaded Q of 7-10 is ample....resulting in more than
>sufficient harmonic suppression. The tube in GG already has 6-7 db of
>2nd
>harmonic suppression, before the PI / L-PI / PI-L / L-PI-L network is
>installed. My ants are not resonant on their even harmonics, and my
>loaded
>40m yagi, due to it's loading scheme, resonates on 24 mhz..and not 21
>mhz.
>
>Ok, after getting the loaded Q as low as practical, next up is the coil
>loss..and the only way to minimize that is to use bigger OD tubing, or
>wider strap. Current handling capability is directly proportional to
>circumference of the coil..which is directly proportional to the tubing
>diameter used. With strap coils, it's proportional to strap width.
>
>The 11m ops have not figured out the L-PI dual coil setup..yet. They
>instead use too high a loaded Q, then try to reduce the loaded Q by
>reducing the plate load Z by overdriving the tube. The problem is, with
>their typ 5-10 amps of plate current, their 3 x turn tubing tank coils
>run
>so hot, that in some cases they will crack the ceramic on both the
>ceramic
>vac tune + load caps.
>
>BTW, you can always take your .015" x 2" wide strap, cut it lengthwise,
>then double up, to double the thickness to .030". That's been done
>several times before, and works good. You can solder the mating edges
>in a
>few places. It's the outer coil layer, that's handling all the
>current.
>
>Jim VE7RF
>_______________________________________________
>Amps mailing list
>Amps@contesting.com
>http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|