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572b Tubes - hard to find quality & supply

Coat?

As in Burlington Coat Factory?

Thoriated tungsten is an alloy. A cathode made with it has no coating.

Depletion of thorium from the surface of the cathode wire reduces emission.

Feel free to argue about how this may come about, but an explanation of the chemistry can be found in more than one authoritative textbook on the subject. Among them "Care and Feeding of Power Grid Tubes".

73
 
Guess if you have crazy line sag

2 , 3 even 4 tube commercially made multi band models don’t pull much current

Even if a little sag on the 120 line , the change is minimal in the filament winding

Almost every cetron 572b tube I have tested over the years has tested near full output. Tested with my tv7 tube tester

Ones that hadn’t failed due to whatever issue occurred .

If anything, it’s wise to make sure to tap primary of transformer to match line voltage you have.

If option is not available, can add series dropping resistors to the filament primary if filament transformer is separate from plate transformer. If not then can add on secondary or can introduce some loss with the leads on secondary side with smaller gauge wire.

This would also add some inrush protection. Inrush on a cold filament is what puts a lot of strain on the filament structure.

I practiced this on every ceramic tube amplifier I made in the past.

on the larger ceramic tubes even states to cycle filament no more than once a day.

variac is another option if line changes often on a day to day basis or time of day. Can also setup so when un keyed is lower and steps up when keyed. I have seen this in broadcast 2 way pa’s.

Running the proper filament voltage on a indirectly heated cathode is even more critical.

Tube specs state percentage you want to stay within for any given tube
 
And folks who power their Black Cat JB2000 from a 120-Volt circuit will be mystified when the power has dropped in half after only four to six months of operation.

At least you can clearly see the yellow-white light from the filaments darken and change color under load in this model.

Same cathode metallurgy. Same issue.

73
 
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nomadradio said:
And folks who power their Black Cat JB2000 from a 120-Volt circuit will be mystified when the power has dropped in half after only four to six months of operation.

At least you can clearly see the yellow-white light from the filaments darken and change color under load in this model.

Same cathode metallurgy. Same issue.

73
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Not a single customer has complained about premature emission depletion on a 572b amplifier I have serviced

And their have literally been hundreds

But not talking about a cb amp like the jb 2000 with a couple 3-500z tubes. An amplifier that puts a larger load on a 120 circuit.

What is the voltage before keyed , is it within spec to begin with. And is the amp being used within spec or beyond .

Is the amp in question on a dedicated circuit , is it a 15 or 20 amp circuit . What is the line voltage to begin with.




If filament stays within tolerance should last many years , and run within specs . Assuming no other issues occur

The quality of the filament wire is the difference between the current China tube and old school cetron . When it comes to emission life.

Have tested hundreds of cetrons out of sb-200’s , fl2100b’s and the dentron clipperton l that test to full output. And most of which were wired for 120 !!

As I test all 572b tubes when an amplifier comes in for repair. Hi pot and check emission output on my tv7 tube tester

If you have a spongy line then yeah 240 would be the way to go

Most do not
 
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Additionally, with a pair of 120 volt primaries wired in parallel, you only have 25% of the internal resistance on the primary winding, than you would with both windings in series.
 
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Hmm. Ohm's Law.

Stepping down from 120 to 6.3 means that the filament voltage will fall one Volt for every 19 Volts of line cord/outlet voltage drop.

Stepping down from 240 means the line would have to drop from 240 to 200 Volts to cause the same droop in filament voltage.

On the other hand, a drop of (say) 10 Volts on the AC power out of 240 will cause the filament to droop just more than a quarter of a Volt. The ratio between wall-outlet voltage and the filament voltage is now 38 to one. And since the current drain from the same wattage draw is half what it was at 120 Volts, you'll half the voltage drop in the same size wires leading back to the breaker box.

I call that a four-to-one advantage of a 240 hookup over a 120-Volt outlet.

How important it is? YMMV.

But the arithmetic of the proposition is clear enough.

73
 
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nomadradio said:
Hmm. Ohm's Law.

Stepping down from 120 to 6.3 means that the filament voltage will fall one Volt for every 19 Volts of line cord/outlet voltage drop.

Stepping down from 240 means the line would have to drop from 240 to 200 Volts to cause the same droop in filament voltage.

On the other hand, a drop of (say) 10 Volts on the AC power out of 240 will cause the filament to droop just more than a quarter of a Volt. The ratio between wall-outlet voltage and the filament voltage is now 38 to one. And since the current drain from the same wattage draw is half what it was at 120 Volts, you'll half the voltage drop in the same size wires leading back to the breaker box.

I call that a four-to-one advantage of a 240 hookup over a 120-Volt outlet.

How important it is? YMMV.

But the arithmetic of the proposition is clear enough.

73
Click to expand...
Sometimes the simplest solution is not always the correct answer. Especially if it overlooks any of the variables. In this case it just depends on where the greater resistance is. Is it within the line wiring between the amplifier and the panel, or is it in the pair of primary windings on the plate transformer?

In the case of smaller amplifiers that have the option to run on 120 or 240 volts, the higher internal resistance is almost always going to be in the pair of primary windings, because they have relatively wimpy transformers.

Even with double the line current on the paralleled primaries, the internal resistance is only 25% and that can reduce B+ voltage drop, particularly when on a 20 amp circuit with a short run to the panel. This is not uncommon for smaller amplifiers like an FL-2100.

PS: While I've referenced plate voltage in this example, it also applies to filament voltage, as many of these smaller amplifiers share a common transformer for both.

Incidentally, this also means that an amplifier with a separate filament transformer, will have better filament voltage regulation. This is because the B+ load, does not cause additional voltage drop within the primary of the filament transformer, as they do when both secondaries share a common primary.
 
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nomadradio said:
Coat?

As in Burlington Coat Factory?

Thoriated tungsten is an alloy. A cathode made with it has no coating.

Depletion of thorium from the surface of the cathode wire reduces emission.

Feel free to argue about how this may come about, but an explanation of the chemistry can be found in more than one authoritative textbook on the subject. Among them "Care and Feeding of Power Grid Tubes".

73
Click to expand...
Does it make any difference if we refer to the thorium on the surface as a layer, or coating? Is that not just nitpicking? The point was today's tubes simply do not have as much thorium on the surface of the tungsten wire.

Gone are the days where you could accidentally overload a tube and damage its thorium layer, while still having the ability to rejuvenate it at a higher filament voltage, in order to boil a fresh layer of thorium out to the surface.
 
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Shockwave said:
Does it make any difference if we refer to the thorium on the surface as a layer, or coating? Is that not just nitpicking? The point was today's tubes simply do not have as much thorium on the surface of the tungsten wire.

Gone are the days where you could accidentally overload a tube and damage its thorium layer, while still having the ability to rejuvenate it at a higher filament voltage, in order to boil a fresh layer of thorium out to the surface.
Click to expand...
Problem I hear from many is they do not have a 240 receptacle / circuit in their shack. My 120 circuits here at my house don’t drop but maybe a volt when loaded down by a Ameritron 811h at full rated output of 800 watts pep.

Most modern homes have 20 amp 120 circuits which have #12 romex .

If you have cruddy line voltage coming in , the circuit is loaded down with other stuff , improper gauge wire for the receptacle or a smaller ampacity circuit then you may be forced to switch over to 240.

This is what makes the vdmos so appealing to many of my customers . The ability to squeeze a bunch off of a 120 circuit.

Putting in a 240 circuit is not cheap if cannot be done by oneself . Many do not want to bear that expense

Even on 240 you want to tap the primaries correctly and add correction if needed internally
 
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kb1sel said:
All I use is for my customers amplifiers are penta labs 3-500z and 572b tubes

Yet to have an issue with one of their tubes , they also have fantastic customer service

You can take that to the bank as I work on a LOT of amps that use these tubes

YouTube Amprepairguy
Click to expand...

So im still waiting for these 572B quad set of tubes to arrive, what wattage cab I expect to get out of new tubes?
 

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