Single 4CX250B base amp for the masses!

[543_Dallas]

543, Quoted from the manual on cbtricks.

" Class
AB operation of the tube permits high peak powers with low distortion products and harmonics. "

The pi network will be a big factor on harmonic suppression. A pi-L would help even more if you're concerned about that.

The IMD (splatter) levels are probably going to be way better than most as long as your screen and bias supply is stable and you don't expect more than 300 watts out of each 250b.

I've saw 800 watts from one with 2500 on the plate but it's dirty. That's why I moved on. It takes a bucket full of the things to make any power...then your total plate impedance drops with each additional toob and the harmonic suppression of the pi network suffers.

As a comparison a 2879 has horrible IMD performance unless you run them at 60 watts pep. The spec sheet shows this and I believe this is why older 100 watt ham rigs used a pair of them when one would do the job.

If I can't put +40 on your S meter and be completely gone when you change the channel I'm not happy. The locals can't figure out how that's possible. One guy lives a couple of miles away and thinks his receiver is just really good.

 

[kopcicle]

A pi-L would help even more if you're concerned about that.

MN2700B. There are a few reasons why I use the Drake tuner. Pi-L is just one.

 

[7oh7]

I took the Schematic and edited it to a 10 meter mono band.
I have removed the watt meter from the schematic, In hopes that someone would help me add 3 individual meters to monitor,

1) Plate Volts
2) Bias Volts
3) Plate Current (Original owners manual has spec'd out the processes)

I would prefer 3 individual meters instead of one meter with a switch to read all 3.
Any and all help would be appreciated.
Thanks.

I don't mean to insult anyone by changing the "look" (aesthetics) of the Pride DX-300.
I'm Just looking to expand my knowledge.

 

[kopcicle]

https://www.w8ji.com/metering_amplifier.htm

 

[Shadetree Mechanic]

I took the Schematic and edited it to a 10 meter mono band.
I have removed the watt meter from the schematic, In hopes that someone would help me add 3 individual meters to monitor,

1) Plate Volts
2) Bias Volts
3) Plate Current (Original owners manual has spec'd out the processes)

I would prefer 3 individual meters instead of one meter with a switch to read all 3.
Any and all help would be appreciated.
Thanks.

I don't mean to insult anyone by changing the "look" (aesthetics) of the Pride DX-300.
I'm Just looking to expand my knowledge.

If you took out the original meter then I am lost. That would be my first clue. Not that I would be much help anyway. I didn't even know this amp existed until I saw this. Trying to follow along and learning a lot. Don't pay any attention to me and please proceed. Just glad to be here.

 

[nomadradio]

I tend to adapt analog meters with a "bottom up" approach.

First thing to know is the combination of the meter's coil resistance, and the full-scale sensitivity. The current needed to drive it to read full scale. Frequently you'll see some fine print at the bottom of a meter's face with the letters "fs" followed by the current rating. Or you can put it in line with a multimeter, and drive it with a series resistor from a variable low-voltage supply until it reads at the top of the scale, reading the current from the multimeter.

The combination of coil resistance and fs current plug right into Ohm's law, to tell you the voltage you would see across the coil when it's reading full scale.

This number is now the minimum voltage drop your shunt resistor must produce to measure circuit current.

For example, a meter with a 1000-ohm coil has a full-scale sensitivity of 1 mA. This keeps the math pretty simple, since 1 millamp through 1000 ohms produces a drop of 1 Volt.

If you want to measure an amplifier's plate current, you would put a shunt resistor in series with the final tube's plate supply. Safest place is in the negative side of the high-voltage, between the HV rectifiers/filters and ground. Not the only place, but this is a simplified example.

Again, to keep the math simple, let's measure 1 Amp of plate current full scale on this meter. To get 1 Volt to drive the meter, at 1 Amp of plate current will call for a shunt resistor in parallel with the meter of one ohm.

A real-world example using a hamfest flea-market meter will be different, but that's the basic method.

And yes, Tom's method in the link above is the comprehensive approach.

73

 

[7oh7]

Thank you Nomad.
This not something I will be dedicating all my time on at "this moment". But I will be doing this on my next Pride DX-300. Like I said, I was given a bunch of parts. I am going to have to go over some of these boards. I have 2 Input/Relay boards that are missing some of the components like the 9:1 transformer and the 200 mh choke for the bias supply. I also have to go over 2 Low voltage boards and 1 set of the Rectifier/High Voltage boards. I'll be getting the Franken-Pride up and running then I'll be doing the meter add ons to it as the box(enclosure) is in poor shape and in need of decal and repainting which i don't plan on doing.
Although, I was at the local ham fest this weekend and purchased a rack mount panel with 5 Motorola meters with the markings of FS=50 UA.

I'll be posting pics as time goes on.
Thanks again.

 

[nomadradio]

RF Parts has the 9-to-1 transformer. You still have to wire the three windings in series, but it's the right part.

If all you care about is 11 meters, a 56 uH choke works fine in place of the tiny 200 uH part. Makes it easier to get one with a higher current rating, less likely to go poof if there's a surge from the tube.

73

 

[7oh7]

Thanks Nomad. Here's pics of the Franken-Pride (Second Pride that I'll be putting together) main boards.

 

[nomadradio]

Wow, that looks like our original "prototype" version of the low-voltage board from 2004 or so.

It came with a 1k 5-Watt bias control. We added a pair of 2.2k 2-Watt resistors piggybacked onto it, one from each end to the center lug. This was intended as a safety measure. If a tube surge blew out the resistance element in the bias control, the fixed resistors would nearly always survive. You would lose the ability to adjust the bias, but at least the tube would have negative bias applied to it. Without those two resistors, a blown bias pot ran the risk of removing the critical negative voltage from the tube's grid circuit altogether. Not good for the tube, or other stuff in the power supply.

Since those days, we adopted a "protection" diode. Kinda like the reverse-polarity protection diode on the power input of a 12-Volt mobile radio.

Since any surge voltage shooting out of the tube's grid will have a positive polarity, placing a 6-Amp rectifier from the relay board's bias input to ground will protect the bias control by shunting the surge voltage directly to ground.

Blown bias controls are scourge I have mostly designed out of the bias boards we sell now. The two "piggyback" resistors are now installed on the low-voltage boards made since about 2005 or 2006, along with the protection diode.

Putting it on the relay board, wired to the "cold" side of the little toroid grid coil also protects the tiny grid choke. That's what's at the center of this closeup, showing our version of the relay board.

Good luck with the two high-voltage boards in the pics. Can't read date codes on the six filter caps, but if you see a tiny bulge in the rubber end cap at the positive ends of them, it's a sign that failure is not far down the road.

73

 

[7oh7]

Nomad,
Funny how you have described what I am getting on my bench this afternoon. Although, I already have the tube out of the Pride that has gone CRITICAL!

Symptoms are as follows,

• Soon after the box is turned on, Instantaneous over heating of the Tube to the point of "Chernobyl". (Pictures of the tube attached)
• Relay Pulsing (Not chattering)
• When taking the box out of tune a bit, the relay doesn't pulse anymore, but watts are fluctuating all over the place.
• At one point, the box ran away on me. In which the relay stayed engaged and pulsing watts were read on the meter in excess of 100 watts RMS. With no drive going to the box.
• I did note that the bias control felt crispy as i spun it!

At that point I turned the box off and walked away from it. Now that one pride is leaving the bench, I'll have room for another. (FrankenPride no longer a priority)

Keep in mind that this Pride has been modified and has been running flawlessly for over 4 years, that I know of. (Owner said he has had it for 6 years without an issue) It has been modified to use the 4cx400 tube. (that's one of the reasons I asked about the differences between the 2 tubes in a previous post)

Note,

• the blisters in the metal, under the fins, above the ceramic.
• the discoloration between the tube base and the anode fins.
• the second picture shows how hot the socket has gotten. The dark marks next to the pins, is the residue of the tube socket starting to melt to the tube base.

 

[7oh7]

Update FYI.
Just as you spec'd out Nomad.

• Bias VR went out.
• Bias VR was of a wrong value (2.5k)
  
• 2 2.2k 2 watt resistors were never installed on the Bias VR.
  
• 1 diode of the Bias diode string went out.
• Resistors 48 and 49 blew open like a fuse ( didn't burn )
• 2 50 ohm resistors were used for parasitic suppressors burned out.

Because of the Bias VR failing and being that the resistors were not installed on the Bias VR the box went critical. Just as you mentioned Nomad. I have made the repairs to the box and installed the original tube but unfortunately the original tube is very weak.
I replaced the tube and the box is now preforming as it should.

Thanks again Nomad and 543.
Kopcicle thanks for the write up on the metering circuit.
I should be starting on the FrankenPride soon.

 

[kopcicle]

.

 

[nomadradio]

That has to be one rough, tough tube to get that hot before it finally croaks.

Never seems to matter how much it's rated for, someone will find a way to exceed it.

Still, a bigger blower for a bigger tube sure sounds wise. Not much room in there to improvise.

73

 

[7oh7]

Lil video I took when things went bad, really bad.
Enjoy,
Keep in mind its a repair and not a total rebuild from the bottom up.
I'm not in the business of cleaning cigarette tar out of an amp.