[Amps] L-PI amp network
Steve Bookout
steve at nr4m.com
Tue Dec 20 17:25:13 EST 2022
Hello Jim,
Wow! What a treasure trove of good info!
Thank you for sharing it with me.
I appreciate the detail in which you EXPLAIN things. Although answers
may be enough, I really want to know 'why' it is, what it is.
I do have an AAD meter and my AIM 4170, which is one of my mainstays.
I hadn't thought about the clicks. I know that some of the amps 4
decades ago were using a 22 volt zener for CW bias, and I think thats
what the Eimac tube info shows. I can use something a bit less than
that and call it 'good'. I can just tweak it for a low value of
quiescent plate current.
I think I'm going to attempt to make some strap. As I've said before,
I've got beaucoup 2 inch wide copper. Once I can get the folding
started, I can get it really flat with the 6 inch vise on my Bridgeport.
73 de Steve, NR4M
On 12/19/2022 8:16 AM, jim.thom jim.thom at telus.net wrote:
> Date: Sat, 17 Dec 2022 17:15:35 -0500
> From: Steve Bookout <steve at nr4m.com>
> To: amps at 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
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