The AM Forum

The layout of my new afterburner has some opened area in the tank area. I wonder if it is worth changing over to a pi-l network. The only negative thing I see is extra switch decks and higher voltage on C2. I'm using a 7500 volt loading cap. The positive is you end up using lower C values for both caps and better second harmonic attenuation.
Anybody ever try it?

Do people have problems with harmonics these days (besides John Edwards)?

Extra space? Put another tube in there!

Brett

I have enough valve for me

I have heard the Tron lambast the 32V3 pie L many times as a harmonic generator.
My 32V3 is still stock in the final department, mostly because I am too lazy to rip the L out of it.

I am not sure the pie L is not more trouble than its worth

Brett

Waste of effort and good parts, Franz.  Your coaxial antennas will attenuate the second harmonic. (or an antenna tuner for those who use them)  Leave some room in there for the mice.

Every amp I've ever built has used a standard Pi.  When is the last time anyone got a 2nd harmonic citation?

Adding an L to a Pi is like the guy who follows his dog around the backyard with a pooper-scopper... ;D



T

A Pi L is simply a Pi Net with an extra L tacked on; its still a lowpass filter. It will attenuate ALL harmonics a bit better than a straight Pi. Anyone claiming that it increases harmonics best calculate the as designed values first before making comments.

A Pi L is used on most commercial ham amps these days and the latest FCC rules have increased the harmonic attenuation for all and that means homebrew also.  Its easy to do this with a toroid even at 2500W +. The key is to use the proper transfer impedance between sections in the calcs.

Carl
KM1H

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A Pi L is simply a Pi Net with an extra L tacked on; its still a lowpass filter. It will attenuate ALL harmonics a bit better than a straight Pi. Anyone claiming that it increases harmonics best calculate the as designed values first before making comments.

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Yes, I always thought the Tron was full of BS...

Brett

Really?  Wasn't aware. What do you figure a good pi-network and a good Pi-L both with a Q of 12 should do, not counting the antenna attn - for harmonic suppression?

Does the FCC measurement include the antenna too or just the rig's output?

Based upon these new rules, do you feel a Pi-L is needed on a big amplifier these days?

T

I was looking at 250 to 300 ohms at C2. I noticed the network changes to lower C1-C2 value and higher L1 value.The nice thing is my tank coil is too big as it sits but good if I change to a Pi - L. A Pi-L will give you about 15 dB more attenuation and it appears to drop the Q of the first section which reduces circulating current. L2 does not have to be a lot of inductance. I actually have a nice ceramic form with silver plated tubing that I could stuff in. I'm thinking L2 does not need to be as heavy conductor as L1. I'm using 3/8 plated with 10 mils of silver for L1. I was thinking 1/8 for L2.
Now a question does this network help with the flywheel effect and IMD???
Another question how about the Q on the input. A better Q on the input help things also. Maybe I need to drive the simulator.
My band switch can be modified to have 4 wafers 2 in parallel for each function.
I modified each wafer to hold double contacts so 4 contacts in parallel for each connection. I think it is a model 85 with 3 supports.

Cute  ;D

Tom,
I figure you get around 35 dB attenuation with a pi network. True an antenna tuner strips off some of the harmonics. A coax fed antenna will just reflect the harmonic energy back at the rig. But, in the case of my LPDA it may just get radiated with gain.
A Pi-L I need a G2 padder for C2 but if I go pi-L a coil sits in the same place and don't need adittional C.

Thats a good transfer value Frank.  The Alphas, Ameritrons, etc that run 2500W out use #12 wire using either a hunk of B&W stock or a toroid. The RF heating is low at least on CW/SSB, my 3 holer Alpha 76CA was a Caribbean contest amp run hard and put away wet for decades but the tank circuit was never overheated.   What tube for the final? YC-156?

Charlie wants 43 dB down these days below 30 mHz. Old construction is grandfathered, HB or commercial.

And the extra L should have no or minimal effect on IMD.

Carl

4CX3000A GG. I built it back around '85. I'm rebuilding it and changing caps to vacuum variables.
It has a bit more gain than a 4-1000A in the same mode so a bit over 100 watts will drive it to 1500 watts out with 4KV on the top hat. A YC would drop right in and the tank circuit could handle it.

Nice tube. I still have a few 4CX5000A parts left over from when I built a few to order. Thats when pulls were cheap and sockets somewhat reasonable in surplus.

Its more a CB tube these days. ;)

Carl

5 is a good shack heater. I almost went that way until I found my 3s made more power than the two 5 pulls I had. They were pretty beat. A couple of of my 3s are close to new.   

Do like my Globe Chief does.    It has two antenna connections, one for the PI only and one with the additional loading coil.    If you can't load without the coil, switch over to the other connection.   I just leave it connected to the connection with the coil and jumper the coil out with a short clip lead when it is not needed.

They where REAL common when I lived in Houston, both for amateur and CBers.

The CBers like them because strapped as a triode, they make a nice dummy load... Pretty much indestructable.  3Kw drive, 18-22Kw out.

I know the oil and gas industry used them for something, but I never really found out what for.  We had a BIG induction heater at DuPont / Conoco in Ponca City that used the 4CX5K.  I worked nights, and it was used by dayshift ops....  Never found out what it was for.

Shaker tables where BIG users of the 4X5 as well.  I had a source for the 4X5 at one time for 150 dollar pulls, 150 dollar sockets, and pole pigs a dollar a pound..  This was in Baytown, Tx.  Wish they where still around :(

--Shane

my tubes also came out of a Ling shaker table. The tubes were yanked by the guy who sold them to me when it was being traded in for a solid state amp.

I started with 4-1000A shaker tables at National where they had a row of them for the military products group. After I left there I got them at a local 5KW AM station. The CE made a few bucks and the RF tubes didnt get recycled into the modulator until they were near dead. It broke my heart when the station got sold and went with an outside service company who ran everything until it was worthless.

The 5K's came from Lings at Sanders Associates where I spent 9 years and always had a steady supply including sockets and chimneys ;D Sanders was huge in military work and is now known as BAE-Nashua and still the largest industrial employer in NH.

I havent been able to latch on to any 3CPX 800's, 1500's or YC-156's locally tho. Havent really looked that hard yet.

Carl

Crap I had connections at Sanders once. Yes I've seen the bank of 4 4-1000A rigs. Never saw the 4CX5K. I did see one with real big old buzzard glass external anode jugs.
CTR surplus and Murphy's surplus have 800s

Sanders had tables at most facilities in the area. Canal St had them in the mill building at the street on the lower level as well as the old "annex" in the rear which did a bit of its own shaking when they were running.  Those old mill buildings were shaking from looms and those long runs of wide belt driven machinery since they were built.

Carl

I bet Bob Samson could have snagged a few for me.
Tom Vu remember the ARN84 TACAN radar an lapunt. They put a shaker on the third floor for the qual program. I was out on the back fire escape one day taking a break and noticed the bricks on the corner of the building were about to fall off the outside wall. The vibration was going to take the building down. I gave jumping Jackass Lopez a call and told him to take a look. They figured it more efficient than taking stuff to the ground floor chamber test area. 

I thanked him many times ;D

Carl

ow that hurts...now I really feel stupid.

When I first went on the prowl and asked around in the various engineeering groups Bob's name came up. He was referred to as a gruff SOB who guarded his area like a Marine.

Bovine effulent!, he was a pussycat if you didnt come on like most know it all engineers.

Carl

Yup, but if you were real people Bob was a great guy. We did a fair amount of testing up there. I met him in '75 I learned a lot working with him.

& Bovine coprates, urk.
 :D

I was there 69 to 79 or 80....time flys and it appears longer than I recently thought. Gotta find an old resume  ???

Carl