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Author Topic: I want to biuld 3-500 watt amp using cx250 or 150  (Read 3681 times)
KC9VZB
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Posts: 45




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« on: January 19, 2013, 06:58:56 PM »

I started building legal limit amp based on 400 tubes.  I have most of the parts,and the cabinet is running out of room.30 amps for filaments seams insane. The hv transformer scares me just a little.Being new to hobby,I,m thinking it would be smarter to start smaller,something that would fit on shelf.I have searched arrl and got a copy of bill orrs 15th and 19th book.the closest I come is to use a design for higher power and use smaller tubes.question is does can anyone steer me to a how-to build amp based on the 150 or 250 tubes I have several of both.
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G3RZP
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« Reply #1 on: January 20, 2013, 01:01:02 AM »

I presume you mean the 4CX250B/7203 and not 4CX250F/7204- and by '400', you mean 4-400?

There's a design in the 1960 ARRL Handbook for a pair of 4X250, which are close enough for these purposes to a 4CX250B. There's a pair of 4CX250B in a design in Bill Orr's Radio Handbook, 22nd edition.

The 4CX250B is not terribly linear, and needs well regulated shunt screen supply - better than VR tubes or a zener, despite published designs. The 4-400 can be run grounded grid without a screen supply: the 4CX250 family cannot.

Have a look at http://ahars.com.au/documents/eimac01.pdf.
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W1VT
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« Reply #2 on: January 20, 2013, 09:20:40 AM »

http://www.arrl.org/hf-tube-amplifiers

ARRL members can download articles from the HF Tube Amplifier page, which includes classics from the ARRL Handbook.

Zack Lau W1VT
« Last Edit: January 20, 2013, 09:24:07 AM by W1VT » Logged
KC9VZB
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Posts: 45




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« Reply #3 on: January 20, 2013, 02:57:13 PM »

Thanks Zak,I didn't see that one earlier,theres a 650 watt that looks doable from 1959 arrl.                                       
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K8AXW
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« Reply #4 on: January 20, 2013, 09:00:10 PM »

VZB:  30A for filaments isn't a big deal.  An amplifier that uses 1200-1500V will kill you just as dead as an amp running 3,000V.

Maybe it's time to step back, take a deep breath and reevaluate.... like using a bigger box, then continue with your present project rather than start over...... since you have "most of the parts."

My first homebrew amp was a pair of 813s in GG.  I did a beautiful job on it but never even tried it because of no parts for the power supply.  I have no idea what ever happened to it.  That part of my mind is totally blank.

The second one came from an ARRL Handbook and uses a pair of 3-500Z tubes.  I also built the 3,000V power supply from the same book. 

It works great and is a joy to use.  If you have a few minutes you can read about it here:

 http://www.arrl.org/news/build-your-dream

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KB1GMX
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« Reply #5 on: January 21, 2013, 04:19:29 PM »

Generally your better with 3-400 or or 4-400over the 4cx150/250 series.

One the 4cx250 will make a lot of power but dirty amp if the screen (G2) is not hard regulated
and that is an annoying complication to the power supply.  That voltage is around 320V and is best
not derived from the 2000V supply.  The bias for G1 (about -50V) should be fairly stiff as well.

Two, the 4cx250 is best used in a grid driven amp with not more than 2W power grid driven
(good for QRP radios).  Also never drive the grid to any current (for linear operation) as its
not designed to take more than a few watts.  For Class C (CW, FM, or plate modulated AM)
there is a very low limit on permissible grid current (6mA).

NOTE: grid driven means dealing with either  swamping the input for stability or if a tuned
network is used neutralization.

Three, air!  You need a good blower to force air through the anode fins, and the right socket
and chimney.  there is a lot of back pressure so If the bower isn't loud, its too small.
Also use the correct air system socket as that helps.

Plate voltage is 2000V your can squint and maybe have it float to 2200 but you risk arc over.

Plate current will be high at least 250Ma for one tube.  That means you need a very good
transformer.

One tube is good for 290-325W.  For 500W you will need two.  For more than one two tube
the bias components need to be duplicated for each tube or use matched tubes.   The
Screen(G2) bias supply will need to be more robust as well.

An amp with that tube is harder and required more metering and complexity.  Done right
that is a fine tube with a potential for long life if not abused.


Allison
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G3RZP
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« Reply #6 on: January 21, 2013, 11:12:45 PM »

Adding to what Allison says, a quiet blower that gives enough air is BIG!  A Marconi tx using 16 of them in a distributed amplifier had a quiet blower. It was run on 3 phase, had an impeller about 2 feet wide and 4 feet in diameter and ran around 1500 rpm.

Low noise means large size and low speed.

The basic 4X150/250 design is about as old I am - 65 years.
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KC9VZB
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« Reply #7 on: January 23, 2013, 06:05:41 PM »

very inspiring story k8awx thanks.As cost is a big issue,I've learned I enjoy the process perhaps more than the end.Sounds like I should stick with the 4-400 amp.And some patience .But for most hamming I do I only need a little more power.Solid state is probably the way to go.And of course, a better antenna.During recent CQ contest the station in Cayman  Ils,couldn't make out my call sign.Amp no good in 100 watt contest.
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K8AXW
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« Reply #8 on: January 23, 2013, 10:19:25 PM »

VZB:  Thank you..... if you're still going to build I'd suggest you stick with a tube amp.  Solid state amps present their own set of pitfalls and are less forgiving of mistakes in design, construction and setup.  It should be much easier to scrounge parts for a tube amp than a SS amp.

Good luck.  It's a fun trip.
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ZENKI
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Posts: 906




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« Reply #9 on: January 25, 2013, 05:17:33 AM »

Just build a Single 3-500 amplifier. This tube makes the best beginners amplifier. You can leave the space in the box for another 3-500, but thats not necessary most of the time.
You can use all the supported parts from Harbach for the SB220 and Drake L4B. All the modules are build and ready to go. You can also buy many of the parts from Ameritron for the AL80 amp.
Its also easy to find  spare SB220 transformer on Ebay.
If you contact Lou from King 6 meter conversion, he might have many of the parts for sale from a gutted 6 meter converted SB220.

Why I like the 3-500 is that its instant on. 3 minutes can be an eternity when you trying to work the DX.  This makes you less like want to have a solid state amplifier.
I have got all sorts of homebrew amplifers with every tube imaginable and my most used amp has 3-500's in it


I started building legal limit amp based on 400 tubes.  I have most of the parts,and the cabinet is running out of room.30 amps for filaments seams insane. The hv transformer scares me just a little.Being new to hobby,I,m thinking it would be smarter to start smaller,something that would fit on shelf.I have searched arrl and got a copy of bill orrs 15th and 19th book.the closest I come is to use a design for higher power and use smaller tubes.question is does can anyone steer me to a how-to build amp based on the 150 or 250 tubes I have several of both.
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G3RZP
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Posts: 4317




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« Reply #10 on: January 25, 2013, 07:56:55 AM »

If you have 4-400s and you don't have any 3-500Z, just use a 4-400 with screen and grid grounded. Not quite as good, but nearly so.
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ZENKI
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Posts: 906




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« Reply #11 on: February 01, 2013, 05:06:13 PM »

Same goes for the Philips QBE 3.5/750., same socket as well.

If you have 4-400s and you don't have any 3-500Z, just use a 4-400 with screen and grid grounded. Not quite as good, but nearly so.
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