Pi-L Output Network Calculator

[K1JJ]

10M monoband amp update:

Amp is running.

At first I thought it had VHF parasitics.  The Bird wattmeter read full scale in both for/ref into a dummy load.  But Dave/VW suggested that a dummyload will absorb power in VHF so it didn't make sense.   This should only happen with a monband antenna.

Anyway, turned out my Bird peak reading wattmeter board had RF getting into the 12V leads.  I tried an old Midland CB swr meter and the swr was flat... :-)

Frank/GFZ gave me some good ideas to prevent parasitics before I knew the wattmeter was the problem. I'm adding all these mods today and hope to test further.

Dave, you were right as usual  - the C1 variable was too much C and I am modifying to eliminate it and go with a tuned L instead.

Might have it running on the air early next week.  I don't hear much AM activity today so far.

T

[W1FVB]

a web based one, that might be useful:
http://home.sandiego.edu/~ekim/e194rfs01/jwmatcher/matcher2.html

Yes!   Just what I was looking for.  That link has calculations for both the Pi-L and the Pi, in addition to 14 more networks.  They are all figured out at the same time... cool.

This should be added to the AMfone website archives. Best I've seen.

It shows that the standard pi-network is best for a tube with big plate shunt C.   For similar conditions, it showed C1 needing 56pf for the Pi and only 9pf for the Pi-L.   In testing, I found this to be true and the Pi was superior to matching this tube with big shunt C.   Total C1, (tube plate shunt + variable capacitor) network requirement needs to be as large as possible to give adjustment range.

I'm all set now.

Thanks for posting that, OM.

T

You're welcome!

[KM1H]

What dont you like about Excel Tom?  There are also free clones.

Carl

[w3jn]

The link to the online calculator under discussion has been added to the Transmitters section of the Handbook forum here.

[Gito]

Hi

Following what I wrote before,I attached a picture to explain what I wrote earlier

The C1 in the picture is the plate tube capacitance,As T wrote before the C of the plate value is what the Pi Section needed( C resonance} .omiting the reactance of the coupling capacitor since it  have a low reactance,than the equivalent circuit is C1=56 pf .L =.64uH and C2 =246pf.

Say that C1 and L has the right value,so C2 must have a 246 pf value

If C2 has a value higher  than 246 pf like 300 pf ,the Amp is out of resonance
also if the value is lower than 246 pf the Amp is also out of resonance.
Only if the C is 246 pf is the Amp resonant ,and there's a dip in the plate current.

The point is when we know the value of C1 and the value of L (after calculating } ,than we can find a dip by turning the C2 .

Than fine tuning to have the rated plate current is by changing the value of the L and dipping again with C2, (C1 is fixed = the plate tube capacitance


gito.n

[W1FVB]

The link to the online calculator under discussion has been added to the Transmitters section of the Handbook forum here.

It is all so possible to save the complete page, so people with a dial up and postal pigeon backed up internet don't have stay online when using the calculator

73 Frits

[K1JJ]

Thanks for adding that calculator, John.     It is a good one cuz it took several days to find a working web-based Pi-L.   Tnx to W1FVB.

Carl:  Yes, I have Excel here and use it for number analysis sometimes. But I prefer the simple web-based calculators. The last Excel one I used for the Pi-L had frozen cells. I didn't have time to get into and have no patience in lieu of these simple web-based ones. You know what I mean... :-)


Gito: OK on C2 tuning the circuit. I guess the problem is if we want to use the amplifier across 1 mhz of the 10M band, it will require some more adjustment than what the C2 alone will provide. The reason is that loading of an amplifer is important to get it as clean as possible. Once C1 and L1 are resonant, C2 can be increased to provide a better IMD figure.

What I have decided to do is to vary the inductance of L1 thru a tunable panel-mounted slug. This way total C1 is just the tube's internal capacitance and outside stray. That is close to about 45 pf which appears to be the maximum I want to use to get a Q=12 or so.  Otherwise with more C1 the Q soars and the tank circulating currents are too high for good efficiency.

Anyway, I will run some tests today with the slug. I'm not sure if I need a ferrite rod ot a steel rod will do- or if it needs to float vs: grounded to the chassis . The coil is 3" X 3" and the slug will be only about 1/2" diameter with the ability to pull all the way in or out.

Any ideas?

T

[w3jn]

A brass rod will DECREASE the inductance, if you overshot your mark on the coil.

[KC2ZFA]

this bring to mind a nuclear reactor's moderator rods 

I'm curious, which tube(s) have 45 pF output capacitance ?

[K1JJ]

this bring to mind a nuclear reactor's moderator rods 

I'm curious, which tube(s) have 45 pF output capacitance ?

I figger 12 * 3.75pf is about 45 pf.... 

T

[K5IIA]

gota have headroom

[K1JJ]

OK OK... so it's not my amp. 

How 'bout this one?   

T

[Gito]

Hi

A pi section network,has 3 variable that's  C1 (resonance capacitor) L and C2 the loading capacitor,usually  we make it to 2 variable circuit with it's L fixed.

These three components makes the circuit resonance,usually tuning is done by opening,closing the C2 (load capacitance} then  after that dipping the plate current by the C1 (resonance) ,this tuning is done ,till the plate current at resonance is what the Tube needs.

We cannot only make the C1 and L resonance than after that  changing the setting the C2 ,since these 3 components depends on each other.

So the C2 setting depends of the setting of C1 and L.to make it resonance.

In Your case C1 (the tube and stray capacitance )is fixed,but C1 value is enough to make these pi section Q.

Since the C resonance is fixed ,we used the other variable component,L and C2
 You are right by making the L variable,but after that ,we must still re tune C2 for a dip in the plate current.

Gito.N


 

[KM1H]

Carl:  Yes, I have Excel here and use it for number analysis sometimes. But I prefer the simple web-based calculators. The last Excel one I used for the Pi-L had frozen cells. I didn't have time to get into and have no patience in lieu of these simple web-based ones. You know what I mean... :-)

I dont find those useful for amp output tanks and prefer the GM3SEK spreadsheet which is real easy the second time thru it. It allows you to make all sorts of changes and see the results. There are several other good links and lots of other info on Ians site.

http://www.ifwtech.co.uk/g3sek/

Carl

[WA1GFZ]

Tom you could make this configuration work if you put a little inductance ahead of C1.
This maks a L-pi network. The tube Z is transformed up to the value you need to get c1 to resonate. Then the pi section transforms the new value back down to 50 ohms.
Look at the old 813 single band amps. This trick was used for the 10 meter single band amp.  I did the same thing in my old 4-1000A linear.

[K1JJ]

Frank,

I might give it a try.  It wud mechanically easier to use than the tuned L1.

If the normal L1 is about .7uh and C1+tube+stray total is about 50pf, what value wud you estimate I shud try for this first coil between the plate and C1?


BTW, right now during testing I'm getting RF into the FT-1000D and audio gear. I don't have the linear shielded yet. Have you had experience with unshielded  vs: shielded tests on 28mhz like this?    I expect to build a cabinet for it later on... no Plexi this time..

T

[WA1GFZ]

Tom did a quick simulation
plate Load 2K 50pf

first inductor .1uh
C1 30pf
main inductor .4uh
C2 load 300pf

Looks like the first L wants to be about 1/4 main inductor. If you go too high the second harmonic comes way up.

[WA1GFZ]

Simulated 40pf at the tube and inductor goes up
.1uh, .5uh
c1=30pf
c2=300pf

[K1JJ]

I will try those values tmw, Frank. It wud be nice to get a C1 variable working again.

Tnx, OM.

T

[W2VW]

If you are going the extra coil route you could try simply tapping the existing coil.

If not the slug on a lead screw tunes a lot like having a C1. Just pretend.

I'm still not sure why you insist on paralleling all those 50C5s.

[Gito]

Hi

Attaching a picture comparing a Pi-section and a pi-l program,works on 30 mHz. it's just an example .an amplifier works on 30 mHz ,with an output of 18 watt and peak rf voltage and the anode is 200 volt .
Compare the C1 value in the Pi network and the Pi-l net work,
also the harmonic reduction of the Pi network.I think it's high enough

Gito.n

[The Slab Bacon]

I'm still not sure why you insist on paralleling all those 50C5s.

'cause he wants it to be like a "Dave Made" He wants to be the boss with
all of that hot sauce"    

[WA1GFZ]

Tom,
Running L pi generates a sharp peak around the fourth harmonic or so. This can be adjusted away from operating frequency. If you run 1K ohm load then a pi should work. I see about .5 uh inductor with C1=30pf, C2 300 pf. (I used 40pf for tube)So I think your initial inductor value was too high. I would try smaller value and see if the Q gets too high. I would only use L pi if your Q is too high but be careful to check your harmonics as it will creat a sharp peak that may line up with a harmonic if not careful.
The values I gave you yesterday is a good place to start.
Tune up at the low end of the band and check efficiency then tune up at the high and again check efficiency. If it works better at the low end you can add L. if it works better at the high end you can reduce L

[KM1H]

I have yet to find a tube that doesnt work well if you simply center tap the 10M coil with C1 as it gives you an automatic 4X step-up in C requirement.

There is no need for fancy footwork and calculations, just pick the load value and K factor, forget about tube C and build what the program spits out.

Carl

[K1JJ]

Thanks for all the suggestions.

I have the amp back apart and will try a few.

Carl, your idea of simply tapping the the main L1 coil with C1 is essentially an L -pi network, right?   Think I will try that since it is easiest to set up and adjust - plus I will have C1 variable again.

Will try it later tonight.

T