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Author Topic: HF AMP  (Read 3565 times)
KB0KYT
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Posts: 27




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« on: August 16, 2012, 02:02:13 PM »

I want to build a hf amp i have parts and some 4cx250s and stuff but id like some help. Im near omaha nebraska im in plattsmouth nebraska. If anyone is close or in omaha and can help me that be great.please let me know. you can call me at 1 402 298 4064
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KZ1X
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Posts: 3229




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« Reply #1 on: August 16, 2012, 08:02:37 PM »

Just in case you don't get any bites... sounds like there are some great, active clubs in your area...

http://www.bellevuearc.org/

http://k0kkv.org/


http://www.aksarbenarc.org/main/


This eHam.net thread seems like it's relevant:

https://www.eham.net/ehamforum/smf/index.php?topic=77221.0

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K7MH
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Posts: 347




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« Reply #2 on: August 17, 2012, 07:57:50 AM »

There should be some articles out there about building an amp like that.

If you happen to already have many of the parts or can get then at a very reasonable price you can do ok with an amp project.
If you have to chase all the parts down and buy them at retail and pay for shipping from different companies then building your own can be a pricey project, but a fun one none the less.
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K8AXW
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« Reply #3 on: August 17, 2012, 08:58:32 AM »

KYT:  If you build your amplifer from an existing article, set of plans, etc., I reallycan't understand what you would need help with. 

I mean no offense, but perhaps you should consider building a few things less ambitious first.

Starting small teaches parts layout; parts substitution, parts acquisition, etc.  It's also becoming more difficult to scrounge parts for a linear these days.  Buying parts from the major vendors (like MFJ, RF Parts) will turn into a very expensive build as 7MH pointed out.
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KE3WD
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« Reply #4 on: August 17, 2012, 09:51:15 AM »

KYT:  If you build your amplifer from an existing article, set of plans, etc., I reallycan't understand what you would need help with. 

I mean no offense, but perhaps you should consider building a few things less ambitious first.

Starting small teaches parts layout; parts substitution, parts acquisition, etc.  It's also becoming more difficult to scrounge parts for a linear these days.  Buying parts from the major vendors (like MFJ, RF Parts) will turn into a very expensive build as 7MH pointed out.

This is sound advice. 


These forums are full of guys asking how to wire a mic to a radio (!) and the same guys then seem to think that they can save money buy building a linear amp from scratch, doesn't make sense. 


73
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KB0KYT
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« Reply #5 on: August 17, 2012, 11:22:36 AM »

i have parts for it but i wanted a lil guidance you know a lil input on it   i cant find plans for a single tube amp with a 4cx250  i already have a power supply  its built from a old pageing transmitter that had a 4cx250 in it so all my voltages are there
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W8JI
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« Reply #6 on: August 17, 2012, 02:15:25 PM »

i have parts for it but i wanted a lil guidance you know a lil input on it   i cant find plans for a single tube amp with a 4cx250  i already have a power supply  its built from a old pageing transmitter that had a 4cx250 in it so all my voltages are there

No, all your voltages are probably NOT there unless you want class C and lower power.

First, there are many poorly designed amplifier projects using tetrodes. 4CX250's are tetrodes, and tetrodes are fussy to build properly.

You need a very stiff regulated screen supply for the 250B, and regulated bias. Otherwise, you could run it grounded grid. VK3ZL built a number of cathode driven 250 amplifiers. You need about 2000-2300 volts anode, and I doubt the pager ran that. It also probably ran class C bias and screen voltages, and no regulation.

I'm not saying you cannot do it, but using a 250B is a whole lot worse than using an 811A or 572B tube, and this is your very first project. 

 



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W9GB
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Posts: 2659




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« Reply #7 on: August 17, 2012, 02:23:49 PM »

Quote
I have parts for it, but I want some guidance or input.
I can't find plans for a single tube amp with a 4cx250  
I already have a power supply  
It is built from a old paging transmitter that had a 4CX250 in it,
so all my voltages are there.
==I see Tom, W8JI answered first ==

So, you want to take parts from a commercial VHF (~ 151 MHz) paging transmitter and
build a Multi-band HF amplifier (3-30 MHz).

This tube would be best served in a 6 meter or 2 meter amplifier design.

World Radio Labs (WRL) in Council Bluffs, IA produced many ... Using this exact tube.
You need to READ ARRL or W6SAI Radio Handbooks from 1950s and 1960s.

September 1958, QST magazine AND featured in 1959 ARRL Handbook
A Compact 650 Watt Amplifier
http://www.qro.it/amp/schemi/pdf/59hb205.pdf

For Tetrode final tubes, like the 4CX250 ....
I prefer usage of modern Control Board for proper operation (and protection) of the tube.
Better than 1958 ARRL build ... uses modern components and protection
http://www.ifwtech.co.uk/g3sek/boards/tetrode/tetrode-1.htm

Jerry - K7LFE amplifier build
http://home.comcast.net/~k7rld/4cx250bamplifier.htm

Earlier eHam.net question about using 4CX250 tube
http://www.eham.net/ehamforum/smf/index.php?topic=625.0;wap2

Bob Grinnell at Surplus Sales of Nebraska (just north of Qwest Center and new baseball field) in Omaha likely has surplus items to complete your build.

Lastly, LEARN how to use your computer and Internet.  
The answers provided to you, we're obtained with 5 minutes of time and
solely using Google Search Engine for web links.
« Last Edit: August 17, 2012, 02:33:04 PM by W9GB » Logged
WB2WIK
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Posts: 20666




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« Reply #8 on: August 17, 2012, 02:48:32 PM »

i have parts for it but i wanted a lil guidance you know a lil input on it   i cant find plans for a single tube amp with a 4cx250  i already have a power supply  its built from a old pageing transmitter that had a 4cx250 in it so all my voltages are there

I think what Tom wrote is probably correct.  You probably have lower than optimum plate voltage, the correct heater voltage, and unregulated screen and grid bias voltages because a pager transmitter is normally Class C and run at much less than "max" power for the tube -- because the transmitters were normally unattended and expected to run for years without anybody looking at them.

For an amateur linear amp, you'd probably want to change everything except the heater (filament) supply. Smiley
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WB2WIK
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Posts: 20666




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« Reply #9 on: August 17, 2012, 02:51:58 PM »


i have parts for it but i wanted a lil guidance you know a lil input on it   i cant find plans for a single tube amp with a 4cx250  i already have a power supply  its built from a old pageing transmitter that had a 4cx250 in it so all my voltages are there

I think what Tom wrote is probably correct.  You probably have lower than optimum plate voltage, the correct heater voltage, and unregulated screen and grid bias voltages because a pager transmitter is normally Class C and run at much less than "max" power for the tube -- because the transmitters were normally unattended and expected to run for years without anybody looking at them.

For an amateur linear amp, you'd probably want to change everything except the heater (filament) supply. Smiley

If you're serious about an HF "linear" amp using these tubes, you might want to look at the Collins KWS-1 design.  It used two of them (well, 4X150As, really -- but same requirements) and actually had pretty good IMD performance as I understand it.  Never tried to measure one, but that was the word at the time.  The entire KWS-1 manual including all schematics are on line at the CCA website.
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ZL1BBW
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Posts: 413




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« Reply #10 on: August 20, 2012, 12:16:11 PM »

I have built a linear using 4cx250 Valves, used the G2DAF circuit, not the greatest performer I ever had.  If you have a power supply, run it up and find out what it reall will give you.   For simple first time amp, get a 3-500 and use that, much easier to work with, and much less likely to go pffft.  Also there are so many ccts for 3-500 GG amps, that you could fill a house with them.
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ex MN Radio Officer, Portishead Radio GKA, BT Radio Amateur Morse Tester.  Licensed as G3YCP ZL1DAB, now taken over my father (sk) call as ZL1BBW.
ZL1BBW
Member

Posts: 413




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« Reply #11 on: August 20, 2012, 12:19:25 PM »

PS.  Make the single band to start with, much easier until you get it working, then you can add in the band switching etc, later.
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ex MN Radio Officer, Portishead Radio GKA, BT Radio Amateur Morse Tester.  Licensed as G3YCP ZL1DAB, now taken over my father (sk) call as ZL1BBW.
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