4CX250B pig in a poke turns up dead.

[nomadradio]

Ebay. You pays your money and you takes your chance.

The listing said "no way to test".

The 4CX250B tube on the left has a problem we call the "blown fuse". Note that pin 1 of the tube on the right has a thin strip of vacuum-deposited metal that connects the pin to the metal ring around the base. This connects that ring to the tube's screen grid. A special ring-shaped capacitor is built into sockets for this tube. Using the ring for this connection reduces the inductance of the circuit path. A big deal at 150 or 250 MHz.

But this link has been popped from the tube on the left. Most-common cause we have seen is leaving off the tube's parasitic-suppression choke. The wattmeter tends to show just a bit more power without it. But the tube will oscillate and produce voltages high enough to arc from the plate to the screen grid. The surge current from the arc will vaporize the thin metal layer and leave this "blown fuse" spot where the metal had been.

And yeah, this tube was toast. Just one more thing to keep your eye peeled for if you consider buying one of this type tube used.

We don't see this as much as we used to, when there were nearby builders using this tube. An easy check to make before risking your amplifier plugging in a "mystery" tube. At least this customer had the sense to have the tube checked before rolling the dice.

73

 

[Captain Kilowatt]

Personally I would never buy a used 4CX250B. The grid will only handle 2 watts and will be destroyed with any over load. The are a good tube but fragile in that respect. I used them in FM broadcast service as IPA's between the exciter and the 4CX5000 PA tube. Just my opinion mind you.

 

[Shockwave]

Fair Radio Sales use to sell the 4X150A NOS tube in a can for $40 each. Later versions carrying the 7034 JAN stamp, have the same 250 watt dissipation as the 4CX250B and I've had excellent results using them as economical replacements.

 

[Shockwave]

Ebay. You pays your money and you takes your chance.

The listing said "no way to test".

The 4CX250B tube on the left has a problem we call the "blown fuse". Note that pin 1 of the tube on the right has a thin strip of vacuum-deposited metal that connects the pin to the metal ring around the base. This connects that ring to the tube's screen grid. A special ring-shaped capacitor is built into sockets for this tube. Using the ring for this connection reduces the inductance of the circuit path. A big deal at 150 or 250 MHz.

But this link has been popped from the tube on the left. Most-common cause we have seen is leaving off the tube's parasitic-suppression choke. The wattmeter tends to show just a bit more power without it. But the tube will oscillate and produce voltages high enough to arc from the plate to the screen grid. The surge current from the arc will vaporize the thin metal layer and leave this "blown fuse" spot where the metal had been.

And yeah, this tube was toast. Just one more thing to keep your eye peeled for if you consider buying one of this type tube used.

We don't see this as much as we used to, when there were nearby builders using this tube. An easy check to make before risking your amplifier plugging in a "mystery" tube. At least this customer had the sense to have the tube checked before rolling the dice.

73

I suspect the fragile grid #2 suffered permanent damage as a result of this internal arc but, I would be curious to see if that tube could still produce decent output when switching the connection from pin 1, to the low inductance grid ring.

 

[nomadradio]

It might still work. Didn't put it in the tester.

The other, unseen side effect of this is for the screen grid to get hit hard enough to vaporize the gold electroplated surface of the grid wires.

Splatters condensed gold droplets onto the oxide surface of the cathode. Tends to poison it and reduce available cathode current. That gold layer is there to prevent the screen grid from going into secondary emission. Just another risk if enough of the gold gets sputtered away from the grid wires.

If you had a bullet-proof amplifier that would shrug off a tube failure, there's little to lose trying this tube.

The Pride doesn't qualify on that count. Risk counts as a cost, and the potential value of this tube didn't justify trying it. More to lose than to gain.

73

 

[Shockwave]

That gold layer is there to prevent the screen grid from going into secondary emission. Just another risk if enough of the gold gets sputtered away from the grid wires.

If you had a bullet-proof amplifier that would shrug off a tube failure, there's little to lose trying this tube.

The Pride doesn't qualify on that count. Risk counts as a cost, and the potential value of this tube didn't justify trying it. More to lose than to gain.

73

When tetrodes get near the end of their life, that secondary emission on the screen grid can be problematic. Not that these tubes had any problem producing full output, just that the screen voltage would rise up when keyed too long and draw way too much current, until the tube burned up. Lower impedance screen supplies (heavier resistive loading) or shunt regulation allows many more hours of use from these tubes. Of course, this doesn't help with the damage caused by an internal arc, as in the case of this tube.

 

[Shockwave]

Adding a "glitch" type resistor in front of the tube socket pin 1, RF bypass cap and inline with the screen voltage, would have most likely saved this tube. Common screen glitch resistor values used on tubes like the 4CX250B, 4X150A or 8122 are between 47 and 100 ohms with a wattage low enough to burn open in any sustained arc or short. 1 watt works good, no more than 2 watts.

 

[Crusher]

I have a bunch of old sockets out of Harris transmitter. They used 1/4W or 1/2w 47ohm I believe on 250B type. I’d have to dig out. Lol, somewhere. Depending on screen voltage, I learned a 5K ohm 40W works good on screen. But it does pull a lot of current. Usually around 65mA ish. Works nicely on the GU74B. I laugh when I see guys using 5W 6.8K or something similar. You know those suckers get hot.

 

[Shockwave]

I have a bunch of old sockets out of Harris transmitter. They used 1/4W or 1/2w 47ohm I believe on 250B type. I’d have to dig out. Lol, somewhere. Depending on screen voltage, I learned a 5K ohm 40W works good on screen. But it does pull a lot of current. Usually around 65mA ish. Works nicely on the GU74B. I laugh when I see guys using 5W 6.8K or something similar. You know those suckers get hot.

That does sound right. The 47 ohm would only drop half the voltage and could be half the wattage as a result. In hindsight, my wattage suggestions are on the high side. With the cost of these tubes today, you are better off making that resistor small enough to be the weak link for certain. Even if it costs an otherwise unnecessary repair to change a resistor, it's much better than matching up another set of tubes.