Pi-L Output Network Calculator

[WA1GFZ]

Carl,
Thr L pi does produce a transient peak in the attenuation well above the operating frequency so you just need to keep it away from any harmonics of the operating frquency.  Be careful with a strap off the C1 cap any L could produce a VHF series resonant circuit. Better to do it with two inductors with a common point at C1.
I think a pi will work at 1kohm load with 40 pf at the tube plate.

[K1JJ]

Frank,

I did try a straight pi-network already and found it had too much C1. (stray, tube and min C1)   C1 wanted to be all the way out. It's a tiny 30pf cap with a min of about 5pf IIRC.

When I disconnected C1 and tuned L1, the power came up quite a bit.

I'll try the tap and see if I can get away with it using a fat copper strap.

T

[DMOD]

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

Hi Carl,

I assume this spreadsheet is for a pi filter before the final L, where the C1 is tuning, C2 is loading, and the L helps extend the impedance matching?

Thanks

Phil - AC0OB

[DMOD]

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

Hi Carl,

I assume this spreadsheet is for a pi filter before the final L (L2), where the C1 is tuning, L1 is the pi inductance, C2 is loading, and the L2 helps extend the impedance matching?

Thanks

Phil - AC0OB

[Gito]

Hi

Look it this way,what we are thinking is The tube as a source ,than according the pi section theory C1 is the resonating capacitor,

how do we load the tube first set the loading capacitor maybe to 300pf ,make a dip by C 1 ,we got a certain plate current.if it's to low open C2 ,than re dip by C1 the plate current,do it again till the plate current dip is the plate current the tube needs.
If we can't get the plate current dip as needed shorten the L a little ,than use the tuning procedure again ,till we get the plate current dip as needed.

So this has a 3 variable.

After calculating the values of  the transmitter and knowing the output impedance of the tube ( R1.) than the reactance of C1 is   XC1 = R1 : Q
Q =12 ,knowing XC1 we can calculate the value of C1.

So looking at this the C1 of the tube is fixed ,to have the operating condition that we want.

Now If we We think the other way ,

now we used the 50 ohm  output as the source and the tube impedance (R 1) as the load ,now C2 become the resonance C  and C1 the load C .
 Since C1 is fixed, than to know the circuit is resonant  we used C2 to dip ,right?. if we find the plate current is to low, shorten the L than re dip C2.

A pi section works both ways.

Gito.n

[WA1GFZ]

Gito has a point. Put a 1k resistor from the plate to ground and then drive the 50 ohm output with a generator. Monitor the resistor with a scope for a peak. make sure you use a low c probe to not add C to the plate circuit.
I've done that trick with a sweep generator.

[K1JJ]

Tnx for the info, Gito.

Yes, Frank, that's the first thing I did, was to run the MFJ-259B backwards into the 50 ohm pi-network output. I was able to find the proper settings.  Though, the Q was too high with C1 in there and I found with less C the power out was better. Thus, I'm gonnna try the L - pi network next by tapping C1 off the coil.

Unfortunately, I keyed the amp into the MFJ-259B the other day and now its on it's way back to MFJ for repair of the PIN diodes... sigh.

So, I'll use the FT-1000D and swr meter instead.  I'll hook up the scope and 1K load as you suggested.

I have the amp apart on the bench now and added a few things including a handwound single layer cathode choke. I think that commercial choke had too much inter-C anyway. 

It also needs a good metal cabinet and shielding since I'm taking out the station audio gear badly with RF during testing. This 28mhz stuff is a different animal compared to 40M and below.

T

[W2VW]

Hoe long did the MFJ fly before it landed.

[WA1GFZ]

Tom is so mean to his toys...Toy story 4, Tom's toys revolt
When you had the mfj connected did you simulate the tube operating z with a resistor between the plate and ground? Running without the resistor the Q would seem very high.
Do you know which pin diode is used in the mfj? The ERB amps are loaded with pin diodes and there are a couple 3379 low frequency pins near the BB amp module.

[K1JJ]

Dave,

I must admit the MFJ took it like a man. A little screaming and burnt flesh, but other than that, he would have made Mr. Jue proud.


Frank, I did use a resistor across the tube.  In fact tonight I used a 1200 ohm globar and ran the backwards 50 ohm pi network test tonight. I found by tapping C1 one turn into the coil I get a good range of tuning for both C1 and C2.  I then tried a 700 ohm globar and saw a similar tuning range adjustment capability.  I was using the FT-1000D swr meter, not a scope probe. I measured my probe and found the capacitance was too high to use.   I think this may work FB when I fire it up tmw.

The input tuning was off too, so put in a 35 ohm resistor and got that matching 1:1 using a mini pi-network at the cathode.

Other than RF in the audio, I'm hoping for some good results tmw.

T

[Chuck...K1KW]

Hi Folks,

Just read this thread and here is the simple solution that I use with excellent results. 

Ready?.....

Use a Q of 20 or 25 or what ever it winds up being so that you have a reasonable tuning range with your C1 cap.  Take a turn off the Pi net inductor.  I've run a Pi Network on 2 meters (8877)with a Q of almost 60.  The inductor was a 1 1/2" wide copper strap about 4" long.  Effficiency was the same as my 8877 amplifier on HF.  Same deal with my 8877 amp on 6 meters.  I used a neutralizing cap in both cases for C1.

Yes, there is very slightly more loss due to the higher tank current but at a Q of 25 the current is only twice what it is with Q of 12.  No biggie.  Plus, you will get much better harmonic supression. 

On HF many years ago I experimented with Q in the pi net and could not measure any change in efficiency with Q's up to 30.  The Inductor did get warmer requiring larger wire.

So....take a turn or two off the inductor and make it from larger tubing to keep the loss low.  I use 1/2' soft copper tubing for tank coils on 10.

Chuck

[K1JJ]

Yo Chuck -

Tnx for the info.  Yes, I can see how that would work.  Let the Q be what it is and use a proportionally larger material in L1 to compensate for the increased current.

I remember you talking about a test you ran with your 8877 amp using the plug-in coils. IIRC, you put in the 75M coil, took an output power measurement and then retuned the amp on 75M using the 40M coil. The results were very similar, though the Q was much higher with the 40M coil. Both coils were made of copper tubing, which helps, of course.

For this monoband 10M amp, I am using 3/8" copper tubing, about 3 turns, 3" diameter, 5" long.  I figger the inductance is about .5 - .6uh the way it is stretched out.   

I should have some new results tomorrow. As you suggested, I'll be willing to try reducing L1 if need be - for a higher Q.


T

[Chuck...K1KW]

Hi Tom,

I've seen that guidance for a Q of 12 over the years in all the handbooks and other references.  They also warn against higher Q's but in reality the increased loss is minimal and easily dealt with by going to larger diameter conductor which will eliminate the added loss.  For a single ended linear amplifier, a higher Q in the tank will help with IMD also.  Eimac mentions this in one of their publications on the use of their power tubes in linear service.  As I recall, Q's up to 25 helped but with diminishing returns after that. Q's under 10 saw a clear increase in IMD.

The tank coil in my 10M amp is 2 turns of 1/2" tubing about 2 1/2" in diameter, 4 " long.  It's about .3 uH.   I can just get the amp to resonate with the 20M coil (.65 uH) on 10M and there is no measureable change it efficiency.

Chuck

[Gito]

hi

Attaching three different setup of an amplifier with different phase angle,that has a relation with the Q value of the circuit.with a high intrinsic Q of the coil( inductive reactanse/coil resistance).


Gito.n

[WA1GFZ]

So if Chuck is running .3uh then as I suspected you are way too high at .7uh.
The tapped inductor works just be careful where the second resonant peak lands or you will generate harmonics.

[K1JJ]

I spent some time on the 10M amp today.  Finally got the thing working stable and putting out some soup.

The problems were parasitics, low power output and C1 not tuning well.  I smoked a bank of parasitic resistors.

Here's what I found:

1) I originally had a 350 pf plate coupling cap. I thought that was plenty of capacitance because it's about 17 ohms at 29mhz.  But when I replaced it with a .005 ufd, the amp started tuning much better. Maybe the 350 pf was resonanting with some lead length and causing the parasitic.  A .005uf cap shows about 2 ohms at 29 mhz.  

2)  As Chuck suggested, I decreased L1  from 3 turns 2 turns.  This made a big difference and let me use the standard pi-network with C1.  Now C1 resonates at about 20pf and gives a nice tuning range.  Obviously I had too much inductance if I wanted to have a tunable C1.

3)  The original C1 vacuum cap was gassy and glowed blue inside when in operation. This caused power to drop way off when it ignited. I replaced it with another one and it runs FB.


At this point all seems to work FB except I still have some 1-2kc acoustical audio feedback when I run the amp. Not sure where it is coming from. I can disconect the mike and it goes away even with the audio turned up.  There is also a slight bit of RF getting into the audio when I run the amp more than 1/3 power.

I don't know about the acoustical audio problem, but when the rig gets shielded it should help the RF feedback.  Right now it's sitting on top of a rack cabinet with no top or sides.

I may be on 10M tmw trying it out. I'm limping along at about 1/3 power due to the remaining feedback problems , so be nice!  

Thanks to Frank/GFZ  and Dave/VW for their email suggestions and the guys here on the board who posted info.

T

[WA1GFZ]

You might want to shield it before you go to high power. There is a lot of radiated energy coming off an open rig like that.
The nice thing about a low z tube is you can tolerate a lot more C1. Just need to have a coil that can handle the current.
You might try a snubber off the cathode with say 100 ohms in series with 50 pf door knob. This will load any high Q VHF signals.

[K1JJ]

You might want to shield it before you go to high power. There is a lot of radiated energy coming off an open rig like that.
The nice thing about a low z tube is you can tolerate a lot more C1. Just need to have a coil that can handle the current.
You might try a snubber off the cathode with say 100 ohms in series with 50 pf door knob. This will load any high Q VHF signals.

Yep, I expect to take a trip down to the scrap yard in Wallingford next week to pick up some aluminum sheet to build a cabinet.

As you suggested before, I added a 1K in series with a .01uf cap from the cathode to ground.  Next time it's apart I'll add the 100 ohm and 50pf.

After a long carrier, I felt the parts inside and see L1 is room temp warm, the plate choke is cool as well as other parts in the tank.  The problems with the Bird meter went away once I got the tank tuning right. Must have been some VHF parasitics before.  Even the security system stopped talking back.

All quiet on the western front now.  10M is not an easy band to master with homebrew gear.

T

[WA1GFZ]

The trick is to find the right snubber value to deQ the feedback that causes oscillations. Might even try lower cap values

[Gito]

Hi

From the RSGB book

Instability in a phone transmitter can often be caused, particularly when working on the higher bands,by the existence of RF fieids or current in the vicinity of the high gain a.f amplifiers.the first voltage-amplifying stage being the most susceptible.Rectification of the modulated rf voltage takes place .and resulting a.f voltage is fed back into the transmitter trough the modulator.At some frequency the phase will probably be such as to produce positive feed back,and the whole system will oscilate at an audio frequency.
Short,direct and screen wiring between modulator,transmitter and power circuits ,in conjunction with the liberal use of r.f chokes and bypass capacitors of 0.001 - 0.01 uf at each end of the power line is generally sufficient to effect a cure.
 The effect of an RF choke choke may be achieved very simply by slipping a small ferrite bead over the grid lead of a valve as close as possible to the grid pin on the valve holder.


Some audio first stage AF amplifier use an RF choke and a bypass C at the control grid of the tube

Gito.N

[K1JJ]

Thanks for the post, Gito.  That sounds exactly what's happening - the stray RF is getting rectified in the first stage after the mic and then producing positive audio feedback.

Next week I will build a metal cabinet for the amplifier and if that's not enough, I will start heavily bypassing the in and out control leads.  A few ferrites will be used too.

I'm confident I can tame this thang...


Question:  Twice I burned up a few parasitic resistors, but only one out of a parallel bunch of five.   I have five 100 ohm 1 watt resistors in parallel across a 3" long copper strap loop. (20 ohms)  Only one of the resistors chars, while the others stay cool. What is causing this? Do they all get hot but one breaks down faster and drops in value producing a runaway situation? 

Right now I have five in there - one is charcol while the other four look OK and continue to operate like this.   Wish I had a 20W, 20 ohm  glowbar.


T

[K1JJ]

Update:

I added some ferrites to the microphone cable and audio preamp leads.  The audio rectification feedback is almost gone completely. I can run the power up and run 100% modulation now.

The problem still remains with the security system not liking 29 mhz.  It has a voice synthesizer and says someone is trying to break into the "playroom" where ever that is....   The new cabinet shielding should fix that next week.

10M was quiet today so just did some testing into the antenna and went black out.

T

[W2VW]

Update:


10M was quiet today so just did some testing into the antenna and went black out.

T

No it wasn't. Chuck and I had a nice conversation via backscatter.

Chuck worked a few Euros. So did I.

Pete CWA was working them one after another......

Maybe you were listening on the wrong antenna : )

[K1JJ]

Are you saying I got sh@@t in my ears and can't hear squat?  

Cool on the Euros. Can't wait to join in and give Pascal a few 59's.

Naw, today I got up late and tuned in at about 1PM, plus the antennas are fixed "Armstrong" west now.  At 14 degrees F out there tonight, I think they will stay there for a while....

T

[K5IIA]

Don't stick your tounge on the tower when you go out there.