[Amps] L-PI amp network

Sun Dec 18 17:10:38 EST 2022

Jeff, and others,

Not a problem.   I already have an idea to make the connections of the 
coil, snug, but still moveable.   I plan on making my 'best guess' and 
then have enough slack in the mounting to either compress, or lengthen 
the coil.  And, it's not like I'm making this with all fixed value 
components.

When I get it all 'right', the plan is to silver braze my custom end 
fittings to the coil.  To that end, I've picked up 4 oz of 'jewelry 
silver' wire, for the brazing.  I may even silver plate the coils.  
Before anyone chimes in attempting to chide me for being an ignorant 
fool, I DO understand what 'skin depth' is and it's relation to 
frequency.  I AM DOING IT ONLY BECAUSE I WANT TO DO THE PLATING and it 
looks nice for a while.  (40 years ago, I plated a plate coil GOLD using 
some solution used for plating edge connectors on PC boards.)

I'm sure this will eventually work out fine.

Steve, NR4M

On 12/18/2022 2:48 PM, Jeff Blaine wrote:
> Steve,
>
> I realize some guys may be able to get the physical build to match 
> these calculations and have it be dead on.  But in my limited 
> experience, there is still a lot of manual tweaking that may be needed 
> once you get the amp making power.  My advice would be to implement 
> some ability to adjust in the build.  For example, on a coil, don't 
> make it out of something that is impossible to adjust spacing on.  And 
> for the attachment points make sure you can move your connect point a 
> bit if that turns out to be needed.
>
> I would guess you will have a pretty easy time with the 8877 as the 
> Cout in GG is pretty minimal and for a single band build, getting it 
> dialed in with good efficiency will not be too bad. But 10m is still 
> more of a lift than say 20m.
>
> Looking forward to seeing your work once it's done.  I bet it will be 
> lovely.  Good luck OM.
>
> 73/jeff/ac0c
> alpha-charlie-zero-charlie
> www.ac0c.com
>
>
> On 12/17/2022 4:18 PM, Steve Bookout wrote:
>> ...And, I was planning on using about a 1 7/8 - 2 inch dia for the coil.
>>
>> Steve
>>
>> On 12/17/2022 12:55 PM, jim.thom jim.thom at telus.net wrote:
>>> Date: Tue, 13 Dec 2022 17:36:44 -0500
>>> From: Steve Bookout <steve at nr4m.com>
>>> To: amps at contesting.com
>>> Subject: [Amps] L-PI amp network
>>>
>>>
>>>   I cycled all of the unknown tubes thru
>>> this amp.? They all were similar, taking 22 to 27 watts of drive for 
>>> the
>>> same 1500 watts out;? 550 ma to 600ma @.030 ma grid.
>>>
>>> That was the background for my real question for the group.
>>>
>>> Based upon GM3SEK's PI/PI-L calculator, I'm working with a plate load
>>> impedance of ~3000 ohms.? I know from my own experience, and that of
>>> others, that it's a pain to get the plate inductor 'right' so that it
>>> all works and tunes, as it should and make power with reasonable
>>> efficiency.
>>>
>>> I have never used an L-Pi, but I basically understand the idea.
>>>
>>> IS THERE A RECOMMENDED TOOL OUT THERE, WHICH WILL HELP ME FIGURE OUT MY
>>> TWO "L's"?
>>>
>>> Using GM3SEK's calculator, I see there is a line, 'Lead inductance
>>> (total from tube to tank, but excluding suppressor)', which I have
>>> played with.? Adding my 'L' inductance there, say '.6uh', does reduce
>>> the inductance associated with PI, and it does raise the value of 
>>> the C1
>>> capacitor. ? ( I am using a 100 pf /15 KV vacuum variable and right now
>>> as I write this, I don't remember it's minimum C, but it's in the range
>>> of what I think you should see; maybe 5 pf?)? Using this, I was able to
>>> come up with about 15 pf, vs 4 pf in a standard PI configuration.
>>>
>>> Suggestions on if this would be an accurate way to figure this out? ?
>>> Or, another/better way? ? And, what should I be looking for as maybe a
>>> ratio of the 'L' vs the 'L' in the PI network. ? As I recall, I think
>>> I'm looking at about 1.7 uh.
>>>
>>> I would appreciate any constructive comments, or even precautionary 
>>> ones.
>>>
>>> 73 de Steve, NR4M
>>>
>>> ## Steve, I have been using the GM3SEK spreadsheet for a long time now,
>>> since it 1st came out.  It's what I use to design the L-PI 
>>> networks.  It
>>> only works on tubes that DON'T require a suppressor.
>>>
>>> ## assume 1.5 kw out..and a tank eff of 60% on 10m.  1500/.60 = 2.5 
>>> kw DC
>>> input.
>>> 2500w / 4000vdc =  625 ma required plate current.   (28.6 mhz)
>>>
>>> ## Use 1.8 for the 'K'.   4000 / (1.8 x .625) =  3555 ohms plate 
>>> load Z.
>>> With an overall network Q of 19 , line 39 (input q = 17.2 output q = 
>>> 1.8)
>>> Enter 0  for all the parasitic lps/rps on lines 46+47.
>>> Enter 10 pf on line 42
>>> Enter 5 pf on line 43
>>> Enter 3 pf on line 53
>>> Enter 50 uh on line 51
>>> Enter 0 on line 48
>>> Enter 0 on line 52
>>>
>>> Here's what u get.
>>> C1 = 9.6 pf
>>> L =  1.26 uh
>>> C2 =   198.5 pf
>>>
>>> An overall  Q of 19 sucks.   11.825 amps of current through the tank
>>> coil..which is a bunch.
>>> I would use 3/8" OD tubing...or 1/2" to 5/8" wide strap coil. With 
>>> tubing
>>> flattened at each end, then enough width to punch a hole big enough 
>>> for a
>>> 1/4-20 brass screw.   DON'T use  SS  hardware.   It will turn black on
>>> you..and ditto with plane jane ferrous steel machine screws.
>>>
>>> ##  Ok, let's use the L-PI config, which is really a C-L-C-L-C
>>> We will use the tube's anode to grid C (+ strays)  plus the 1st 
>>> coil, to
>>> form a step down L network..... stepped down low enough, that a 
>>> normal PI
>>> net can be designed around, with a lot lower loaded q.
>>>
>>> Enter .8 uh on line 48.   Lower the overall Q on line 39  to 11.
>>> Ideally, you want this 1st coil to be a little less than the main coil.
>>> Yes, u have to keep juggling with the overall q on line 39, each time u
>>> increment the 1st coil on line 39.   Start with .1 uh..and keep 
>>> increasing,
>>> while at the same time decreasing the overall q on line 39.
>>>
>>> Ok, here is what we get.   Note, the overall network Q on line 39 is 
>>> = to
>>> the (input Q + output Q).
>>>
>>> Overall network Q of 11   (28.6 mhz)
>>>
>>> C1 =   12.7 pf     ( input Q of  9.5)
>>> L =   .91 uh
>>> C2 =   171.6 pf.    (output Q of 1.5)
>>>
>>> Note the 1st coil's value (.8uh)  is slightly less than the main coil.
>>> Again, use  3/8" tubing or  1/2" to 5/8" wide strap coils for BOTH 
>>> coils.
>>> Yes, use ur 100 pf vac cap. You won't have any issues, and harmonic
>>> suppression is typ higher when a vac cap is used.  Air caps have the 
>>> issue
>>> with the rotor wiper contact.  I'm assuming ur vac cap goes down to 
>>> 5 pf.
>>>
>>> Harmonic suppression with the L-PI is superb.  With the lower Q,  
>>> you won't
>>> have to constantly be re-tuning  when u qsy.
>>>
>>> Jim  VE7RF
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