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Single 4CX250B base amp for the masses!

Master Chief

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Apr 5, 2005
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My favorite amp of all time is the Pride DX-300. While not perfect, it is a great little amp. Complete documentation, including schematics, are available here:
http://www.cbtricks.com/Amp/pride/dx300/index.htm

So why not build one from scratch, fixing some of the problems of the Pride, and learn something in the process. The Pride has a reputation of being the "driver" of choice for the much larger amps out there, especially when converted to mono band operation (according to Nomad Radio). This makes it an excellent candidate for those who only need a little power, and those looking to build a nice amp for use as a driver.

This proposed amp should be simple and inexpensive to build also.

So if someone is willing to be "teacher", I'm willing to be builder/investor, complete with photo documentation.

I'm serious as a heart-attack here! I want to build this amp!

If I can get the moderators to give me mod privileges in this section, then I'll keep the thread trimmed down to only necessary information.

Let's get started!
 
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I look at base amps as two sections:

POWER SUPPLY DECK
RF DECK

These can be built on the same chassis, but for me, I'd like to build them on separate chassis in order to use a single power supply for "other" amp projects. So let's start by building the power supply first! I'll list my wants, and we'll start the discussion. Once we've come to a conclusion, I'll go buy the parts and the fun begins!

===============================================
Power Supply requirements:

1. Provide enough power to operate 2 x 4CX250B for future projects.

2. Operate from 120V single phase primary.

3. Designed for AM operation! (Not a stressed out SSB power supply)
===============================================
Discussion:

Although the initial project will be based on a single 4CX250B, I would like to have a power supply capable of operating two in the future. A wise man once said to build the best power supply you can afford. Its cheaper than building TWO!

A single 4CX250B can be run off of 120V easily. This power is readily available to the majority of the people who want to build this amp. While there is no argument that 240v is better, the requirement for this amp is 120v primary.

The main education is the design of the transformer! I can order one with 120/240 taps. What other taps are recommended? Since the this tube is a tetrode, we also need a screen supply capable of handling two tubes. Should we use a completely different transformer for the screen supply?

The rest of the components will be based on the requirements of the power supply.

Its probably good to discuss the advantages and disadvantages of voltage doublers too. We could use an education on that also!

So what say you?

BTW, I'd like to get into some of the theory behind the component decisions also. Can we have a complete discussion?
 
Master Chief said:
My favorite amp of all time is the Pride DX-300. While not perfect, it is a great little amp. Complete documentation, including schematics, are available here:
http://www.cbtricks.com/Amp/pride/dx300/index.htm

So why not build one from scratch, fixing some of the problems of the Pride, and learn something in the process. The Pride has a reputation of being the "driver" of choice for the much larger amps out there, especially when converted to mono band operation (according to Nomad Radio). This makes it an excellent candidate for those who only need a little power, and those looking to build a nice amp for use as a driver.

This proposed amp should be simple and inexpensive to build also.

So if someone is willing to be "teacher", I'm willing to be builder/investor, complete with photo documentation.

I'm serious as a heart-attack here! I want to build this amp!

If I can get the moderators to give me mod privileges in this section, then I'll keep the thread trimmed down to only necessary information.

Let's get started!


3 transformers.

1 for filaments. You use a small resistor on this one to control the filament voltage. This serves two things.. 1, it limits inrush. 2. It keeps you from pushing the fils to a higher amount of emissions than they are designed to. One of the Pride problems is they push the envelope of filament voltage.

1 for anode supply. This is the one that you want to be able to go to 110 / 220. You want +/- 5 and 10 percent taps as well. I would honestly get a transformer capable of about 3 times the voltage you need, if you will be ordering it, since the taps from pwdahl are free.

1 for screen and grid. Since you can electronically regulate these, a +/- 350 or so AC volt CT xformer will work here. Then, you use zeners to electronically regulate the bias.

The 250b and similiar tubes draw neg screen current. Remember this when designing the screen supply.

You need to figure out what you want to be the drive level for the tube, then get your grid resistor out, and THEN design your xmission line xformer. Too many people just design for 50 ohms, not taking into account that YOU can SET your drive level on a tetrode, since they require NOTHING to drive. Just enough signal to overcome circuit losses and provide negligible voltage on the grid. Want more drive, change your turns ratio and the grid term resistor.

Bill Orr is your friend in this project.

Making one monoband is pretty easy. Makes a lot of things go easier on you, and you have the DX300 / KW-1 schematics to rape, Nomads, mine, and others notes on cbtricks about it, etc.

I have up to 4 tube schematics, and am working on a commercial 2 tuber for MC Turner (if ya remember him) right now. Interesting amp, has a switch on the back to do grid driven or cathode driven. Stupid design, but interesting. The 250 can't handle anything but being grid driven. TOO small grid / screen.

I like all my tubes to have handles now. 100 times power multiplication, and you ARE the big man when you have NO drivers to power, AND sag your 200A service on carrier... AND have a plate current meter that PINS.. And is rated at 5A on the scale :)

You should talk to psycho. He knows all about linear and nonlinear amps. Just ask him.. He'll tell you :)


--Toll_Free
 
Toll_Free said:
3 transformers.

1 for filaments, 1 for anode supply, 1 for screen and grid

OK, what do I order, and this goes for all three...... What primary, secondary, current, etc? I need specifics. How does one read what Eimac publishes for specs and then decide on what parameters the transformers should be? And explain the 3x voltage thing for the anode supply.

Toll_Free said:
You need to figure out what you want to be the drive level for the tube
A Cobra 2000 peaking at about 28 watts.

Toll_Free said:
then get your grid resistor out, and THEN design your xmission line xformer. Too many people just design for 50 ohms, not taking into account that YOU can SET your drive level on a tetrode, since they require NOTHING to drive. Just enough signal to overcome circuit losses and provide negligible voltage on the grid. Want more drive, change your turns ratio and the grid term resistor.
But this is part of the RF deck, no?

Toll_Free said:
You should talk to psycho. He knows all about linear and nonlinear amps. Just ask him.. He'll tell you :)
Sarcasm, I like it!
 
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Master Chief said:
Toll_Free said:
3 transformers.

1 for filaments, 1 for anode supply, 1 for screen and grid

OK, what do I order, and this goes for all three...... What primary, secondary, current, etc? I need specifics. How does one read what Eimac publishes for specs and then decide on what parameters the transformers should be? And explain the 3x voltage thing for the anode supply.

Toll_Free said:
You need to figure out what you want to be the drive level for the tube
A Cobra 2000 peaking at about 28 watts.

Toll_Free said:
then get your grid resistor out, and THEN design your xmission line xformer. Too many people just design for 50 ohms, not taking into account that YOU can SET your drive level on a tetrode, since they require NOTHING to drive. Just enough signal to overcome circuit losses and provide negligible voltage on the grid. Want more drive, change your turns ratio and the grid term resistor.
But this is part of the RF deck, no?

Toll_Free said:
You should talk to psycho. He knows all about linear and nonlinear amps. Just ask him.. He'll tell you :)
Sarcasm, I like it!

:) If you like sarcasm, you'll love this next one..

Bill Orr's Radio Handbook. It explains everything you want to know. Couple good chapters on building amps... Power supplies, power factor, cap input vs choke input (cap input suited for SSB, high peak, low avg power amps).. Choke input doesn't do as well with huge plate current swings, but puts out a HELL of a lot more REGULATED power with constant current type draws (AM / FM / RTTY / Etc).

To figure out what you need, take a look at the spec sheets first. Filament spec = 6 volts, +/- .3 volts. You want to run the tube as cold as possible, so you would want a 6.3 volt xformer, with +/- 5 and 10 percent taps on the input if possible. Then, when you figure out what your unloaded voltage is (since we don't know the xformer winding ratio, resistance, etc, we can't do this mathmatically), you can design the resistor for dropping fils V to the acceptable level. You want taps, so you can customize the incoming voltage at a later date, etc. It takes 2.6A to light a single tube. Take that figure LITERALLY AND FIGURATIVELY. If you build, design and work around what you want, vs what you want in the future, you can be assured of long life. A double tube amp should have a fils xformer that SATURATES at exactly what the tubes want, ie... 2.6X2=5.12A.. Get a 5.25A Xformer, and you have no inrush. If the core is saturated, it can't deliver more current, can it. Incidentally, this is what Heath did in the SB220, and is the reason you DON'T need ANY soft-start on the fils of a SB220.

Now, do you want it class AB all the time, or switchable bias? When you decide what you want to build it "as", then you can order the bias xformer. Actually, you can order it now, then just design your bias supplies around it. You need +/- 350 volts AC.. Actually, you NEED DC, but if you get the AC output, then electronically regulate the DC to the same thing, your regulation (static and dynamic) goes WAY up. So, +/- 350VAC is the order of the day. How much does a pair of tubes need current wise? Well, the screen is allowed a max of 12 watts, and you want a pair of tubes. 25 watts is MAX.. How much current do you need for 25 watts at 350 volts? .1 amp seems like too much to me :) Eimac rates it as 5 mils.. A pair of them is what then? :) OK, since G1 (the grid) is 2 watts dissipation, if we get a transformer that will do enough for G2, our xformer situation is fixed. .15A @ +/-350VAC will be enough for G1 and G2. Center tap xformer is a must, to get + and - referenced to the chassis. Look at a DX300

I'm busy. This is enough to get you started on the power supply. Incidentally, RF Parts used to stock xformers big enough for a pair of tubes for the low voltage side of the Pride. They also have one that comes stock in a viking... I've raped them before, as well.


--Toll_Free
 
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i hope you get this off the ground MC,
me and a buddy in australia will be working together on a 2x250b in the future, he bought it off ebay as a 3 band hf amp,
we need to rework the input and output matching into a single band 11mtr amp and intend to use it as a learning curve for something a little bigger,

i will be watching this project with great interest and i hope you can get others onboard,
im hoping you have better luck than the solidstate idea,

good luck on this one :D
 
seedkey said:
How about a microwave oven transformer for the high voltage?

Something tells me I'll be doing the teaching, then someone else will come here, start telling me I'm wrong, and I'll disappear again.

Anywho, you <<can>> use microwave xformers for the power supply, but they generally have two problems... Little iron (meaning, they might be OK for SSB, but unless you get a 3kw nuke, they will suck for any constant carrier mode) means they saturate quickly. And, they generally are wound with smallish wire on the secondary, meaning they take more turns. More turns = more resistance = more voltage sag = more IMD = get the picture?

I've used a "inverter" power supply from a Panasonic. Kind of a halfway bastardized thing between solid state and old school... PWM of the xformer ... Means more output (peak) for a given input.


--Toll_Free
 
bob85 said:
i hope you get this off the ground MC,
me and a buddy in australia will be working together on a 2x250b in the future, he bought it off ebay as a 3 band hf amp,
we need to rework the input and output matching into a single band 11mtr amp and intend to use it as a learning curve for something a little bigger,

i will be watching this project with great interest and i hope you can get others onboard,
im hoping you have better luck than the solidstate idea,

good luck on this one :D

Simple. Gut the input. If your going to use a real radio (100 watts), put 100 watts worth of swamping R on the grid, and feed it with a coupling cap. Instantaneous wideband input.

Output, easy, too. Just remove all the extra crap, keep 1/2 more turn on the output than the 10 meter selection uses, and you should be REALLY close.


Otherwise, watch out... I heard grumblings about solid state stuff.... Toll is designing again fewl... I'll MODULATE my Messenger.

;)

--Toll_Free
 
lol@tf,
seems like this modulator thing is gaining pace,
more and more people are having a go,
motor really has it going on with his 5lb globeking,
me been a duck i did it by modulating the pa in my old 4lb globe,

are you thinking of modulating the m4v? i like it when you think BIG,

hey im ready to receive your transmissions toll. about 7.5kc rx audio bandwidth in my ar144 more if i need it,
talk to me WIDEBOY lol,
be interesting to see how that goes with the m4v,

good luck.

sorry MC back to the 250b
 
bob85 said:
lol@tf,
seems like this modulator thing is gaining pace,
more and more people are having a go,
motor really has it going on with his 5lb globeking,
me been a duck i did it by modulating the pa in my old 4lb globe,

are you thinking of modulating the m4v? i like it when you think BIG,

hey im ready to receive your transmissions toll. about 7.5kc rx audio bandwidth in my ar144 more if i need it,
talk to me WIDEBOY lol,
be interesting to see how that goes with the m4v,

good luck.

sorry MC back to the 250b

Actually, the M4v is ancient history. We did that in the 80s, called it a plate modulated viking (but, it was a 16X454 box built by Messenger) and had everyone on 6 fooled for a year... Until we got the valient.

You can do it a multitude of ways.. AM shunt regulator, modulate bias, etc. As I said, been there, done that. Easily accomplished.

Bob Carver explained in the 70s how to get tube sound from solid state. It's easy, it's impedance. You guys figure it out from there :)


--Toll_Free
 
I don't have much to add at this point, TF seems to have pointed you in the right direction electrically.

For me, the electrical part of it is just "Drill"; you need a certain amount of voltage and current applied to the correct points and things fall into place.

I never start a project until I have the enclosure(s) for the RF deck and power supply in hand so that I can visualize where thing are going to go. The CX tubes must have forced air cooling so, the RF deck has to be constructed so that pressurized air can be directed through the tubes and chimneys.

This is not a blow-by-blow description, I can't be there to hold your hand; It is expected that your have the radio electrical knowledge and the mechanical apptitudue to start and finish this project.

1. Get all of your big parts (transformers, tube sockets, blowers, tuners etc.).

2. Get an enclosure and physically lay the project out.

3. If necessary, seal the seams to make it air-tight.

4. Cut your holes and mount your components.

5. Make the electrical connections in stages; Power cord, fuse assemblies, on-off switches (step-start if you like).

6. Next construct your minor supplies (relay switching, screen and bias supplies.

7. Mount / wire your antenna switch and control relays;

8. Run coax inside the RF deck for input (tuning) and from tank circuit to coax jacks.

9. After verifying that your antenna switch is working and that all your small supplies are working, wire the tube socket.

10. Construct your MAIN high voltage supply for this amp (hopefully you already have a test supply and a variac to test this thing on low voltage first).

11. Install the componets for the HV portion of the amp, the Plate choke, the DC Blocking capacitor, the tank circuit.

12. Dip the Tank coil for resonance at the frequency or frequencies if multi-band.

13. Make all your connections and triple check them then, bring the amp up slow with low B+ volts and RF drive.

14. Look, listen and smell for problems like burning parts.

15. I mount meters in all my equipment to monitor operating status, if all is OK, crank it up a little more till you reach your desired level.

That's a general outline of how to do it, but nothing is set in concrete, what ever works for the individual will probably be alright.

.
 
I have the enclosures. I'm very aware of having to layout the components. BUT, what components do you buy?

Let me ask this SIMPLE question and lets get ONE part right now; the large HV TRANSFORMER.

I need a transformer that will power two 4CX250B tubes. I want 120/240 primaries. What should the secondary voltage be, at what current, and why? 3500V? 4800V? 2400V @ 1.5 amps?

There have been discussion on the "weight" of the iron also. Remember, I need a power supply that can handle AM use, not a lower duty cycle SSB version.

Why would someone order a 60lb transformer over a 27lb transformer. I'm looking for the education and open discussion of ideas, not where to drill the holes and what bolts to use to mount it.

You pick a tube and you spec out a transformer for the power supply; how do you choose?

Just the first transformer please, no need to discuss what blower is needed at this point.

Also, I own a number of Orr's "Radio Handbook". Which one of his publications are you referring to?!
 
4CX250B data sheet
The tube requires no more than 2000 volts on the plate so you need a transformer that has an AC secondary of 1350-1400 when using a capacitor type filter. The plate current would be around 500 mA for a pair of tubes when properly run but for AM service I suggest a current rating of 1 amp for the transformer.The screen supply should be around 350 volts so you need a transformer with an AC secondary of around 250 volts.I suggest you do not tap off the plate supply with dropping resistors for the screen supply. It can be done but is not a good idea for AM or SSB service where the plate current is rather dynamic as it will cause fluctions in the screen voltage. I suggest you start looking at places like RFparts or Surpluss Sales of Nebraska unless you want to pay top dollar from Peter Dahl for the iron you need. Do you really need dual voltage primary BTW? And what about the 5 and 10% taps?? Are they really necessary as well? Maybe a bit nice to have if being picky about setting voltages but the amp won't mind a bit of leeway.Generally big amateur amps and comercial stuff have a series of taps to compensate for local line voltage variations but they are far from necessary for homebrew projects.
 
QRN said:
4CX250B data sheet
The tube requires no more than 2000 volts on the plate so you need a transformer that has an AC secondary of 1350-1400 when using a capacitor type filter.


...Very true. RF Parts has a "stock" xformer.

BUT, the 250b's will take more voltage. I run them to about 2800, 3000 no load. They put out lots more suds, but you have to be more careful tuning them.



The plate current would be around 500 mA for a pair of tubes when properly run but for AM service I suggest a current rating of 1 amp for the transformer.



...Properly run? In the CB service? Not highly likely.

Spec your xformer for .5A CCS, or near 1 amp ICAS. And I look at that as a minimum.



The screen supply should be around 350 volts so you need a transformer with an AC secondary of around 250 volts.


...This I don't agree with. You lose regulation using a transformer that has a lower secondary rating than the voltage level you wish to get.

...IE, do you use a 12 VAC secondary for a 13 volt power supply?

...You want the best you can get, when you homebrew something. You usually spend more than you would purchasing new or used equipment, so it's either to get something better than is commercially available, or because you get a hard on watching your plate current meter peg the 5A scale :)


I suggest you do not tap off the plate supply with dropping resistors for the screen supply. It can be done but is not a good idea for AM or SSB service where the plate current is rather dynamic as it will cause fluctions in the screen voltage.


...Agreed.


I suggest you start looking at places like RFparts or Surpluss Sales of Nebraska unless you want to pay top dollar from Peter Dahl for the iron you need.


...Peter Dahl has some transformers spec'ed out exactly for what he wants, for amps I and a couple others in San Diego built in the 80s. Think Mudshack, if you look in their catalog. That store didn't do the amps, but they where the front :)


Do you really need dual voltage primary BTW? And what about the 5 and 10% taps?? Are they really necessary as well?


...No, but they are free. When you order a xformer from Dahl, why NOT get freebies? You are paying top dollar, and getting the best your money can purchase.. Why NOT get something better?

...And yes, they are necessary....

Maybe a bit nice to have if being picky about setting voltages but the amp won't mind a bit of leeway.Generally big amateur amps and comercial stuff have a series of taps to compensate for local line voltage variations but they are far from necessary for homebrew projects.



...I don't see how something that is free isn't necessary or wanted for homebrew use.

To each his own.

--Toll_Free
 
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