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what is a 'pulse rated' tube?

E

exces3

Member
Sep 4, 2012
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OK.
a thread about Russian tubes was getting too long.
.......................
what is a pulse rated tube?

My understanding is.. a pulse tube has
10X or so as much cathode current emission as a 'normal' tube.

--> useable as a normal tube as long
as you heed normal (CW) limits
on the grid, screen and plate dissapation.

or not,
please enlighten me

3333333s
 

Basically pulse rated tubes are tubes that are rated for radar service where the tube is required to provide very brief pulses of RF that far exceed the normal ratings of the tube. They can do this by providing those pulses for only a fraction of a second at a time. They generally also have a much higher peak high voltage rating. For example a normal 4CX250B can produce maybe 250-300 watts in normal SSB service before it's ratings are exceeded while a pulse rated tube can deliver pulses of a couple of thousand watts in very brief pulses.
 
Pulse rated tubes are highly dependant on a vacuum that is not contaminated and a cathode that has not been poisoned by operation with a poor vacuum. Just about every Russian surplus tube will suffer from both of these problems and is why they can't be used in pulse service anymore and are only worth a fraction of their original cost. I'd much rather have a 3CPX1500A7 than a GS-35B for these reasons.
 
Captain KW.

how can a tube have a pulse-rating for plate voltage
that is significantly different than for DC?

a tube, or anything else...
....arcs,......... or it don't

something does not add up.
or there is more to the story.
please discuss.

73
 
exces3 said:
Captain KW.

how can a tube have a pulse-rating for plate voltage
that is significantly different than for DC?

a tube, or anything else...
....arcs,......... or it don't

something does not add up.
or there is more to the story.
please discuss.

73
Click to expand...

See Shockwave's post above regarding quality of the vacuum. A tube's normal max DC rati.g is not necessarily the point at which it will arc. 3-500s typically will highpot out at 10kv but you would never run tbem at that.
 
The rated DC plate voltage on a quality tube should be significantly lower than the point where an internal arc would occur. At 4000 DC plate volts, the peak plate voltage can approach 8000 volts once the RF output combines with the plate voltage. This is why a good 3-500Z will highpot test at 10,000 volts. The 3CPX1500A7 has a 4000 volt continuous DC plate voltage and can hold back 15,000 volts in pulsed service! That's nearly 4 times the continuous DC rating and what we should expect a similar tube with a fantastic vacuum to provide.

Things look much worse when you start looking at the Russian surplus tubes. For example a brand new GS-35B that hasn't spent 25 years under a less than perfect vacuum is rated at 3000 DC volts continuous and can only handle double that under pulse service. To me it sounds like we have just about zero headroom here because once we apply RF on top of the DC we are already at or close to the maximum 6000 volts the tube can holdback.
 
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Shock and Captain...
...................
is there an issue beyond...
..
applied DC plus instantaneous RF(at the plate), shall not exceed xxKV...
or not.
please discuss.
73s
 
exces3 said:
Shock and Captain...
...................
is there an issue beyond...
..
applied DC plus instantaneous RF(at the plate), shall not exceed xxKV...
or not.
please discuss.
73s
Click to expand...

If a rope has a breaking strength of 1000 pounds is there any other reason not to exceed that? Same idea. Bad things will happen and just about as fast.

Is there a reason for your asking? Besides good engineering practices what else is there to consider? Please discuss.
 
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if a tube can withstand a plate-to-screen voltage of 20KV
for one microsecond.

why is the long-term(DC) rating listed as much less than that?
 
Oh, and I forgot to mention that nearly double the plate voltage once RF combines on the plate assumes the amp is tuned fairly well. It can get much worse if the tank circuit is not tuned right or the load impedance rises much above 50 ohms. That's when you're more likely to get an early 4th of July.
 
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I think the root of his question goes beyond the headroom needed to prevent arcing. When we highpot test a tube, we often hold an abnormally high voltage on the plate longer than pulse duty with the goal of it not arcing. That's because the current is limited to a few microamps where even if it did arc, it shouldn't do damage.

When we combine voltage with current we get power. With power comes dissipation. How much heat can the parts within the tube handle? Assuming the vacuum is good enough to handle the increased voltage, the next limiting factor you run head into is the dissipation of the anode and fragile grid or screen. Hold them in a condition of extreme voltage with significant current flow and parts other than the filament will start to glow inside the tube.
 
The radiator on a 3cpx1500 is different then a regular 3cx1500a7 or yu-158. The radiator is actually positioned higher up around the anode then a conventional 3cx1500a7 or yu-158 tube. This is what allows the increase in voltage on the anode.

All 3 tubes have the same grid dissipation rating so one is no better then the other. The only advantage is that of the yu-158 which are select pulls which meet or exceed certain test parameters when tested at eimac off the line.

But does not mean it is capable of any more output then the others when run in linear service.
 
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