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Author Topic: 8295A  (Read 6478 times)
AE6PR
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Posts: 14




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« on: April 21, 2016, 08:58:56 PM »

I found a 8295A in with some other tubes at a surplus outlet. It had a tag  marked high grid current on it. I have been running it on filament voltage for about 25 hours total. Can I install this tube into the HT33 for testing? I do not want to damage the amp. I do have a variac so I can scale it back for tests. Any thoughts?
Thanks
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K6AER
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« Reply #1 on: April 21, 2016, 09:21:59 PM »

The 8295A is a ceramic tube and as a result you can run it on filament heat forever but it will do nothing to improve the tube performance or prevent flashover. Being Ceramic the seals are probably good. Check the anode, screens and grid for shorts with a ohm meter. If open then use the tubes. Note the grid current and anode current for wattage out.
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AE6PR
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« Reply #2 on: April 22, 2016, 07:55:10 PM »

I put the tube in the HT33. I had two small arcs somewhere inside the chassis most likely at the selector switch for the meter although the tube ran on idle current for 20 min. The arcs concerned me as the PL172 did not arc when put in service. Should I try to run it up with RF or consider the 8295A junk status? The amp is more important than risking it for a tube.
Thanks
« Last Edit: April 23, 2016, 07:26:48 AM by AE6PR » Logged
KD6VXI
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Posts: 177




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« Reply #3 on: April 23, 2016, 08:21:05 AM »

Once again,  the educator needs an education.

Being a cold cathode tube,  heating it will activate the getter material inside.   

The arc sounds you hear can / could be flash over inside the tube.

But,  it could also be a cap breaking down (I had bypass caps do this on the cold end of a plate choke,  among other areas)  or something else.

Have access to a hi pot tester?   That can tell you the condition of the tubes internals,  re alignment and gas.

Or,  go back to letting it sit,  heater on only,  for another day and try again.

Also,  a glitch resistor and fuse in the HV would be a good idea.  10 ohms at 20 watts or better,  and a piece of #30 wire.   

--Shane
KD6VXI

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K6AER
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« Reply #4 on: April 23, 2016, 08:46:30 AM »

Shane,

The tube has a indirectly heated  cathode. It is at operational temperature in 180 seconds. Nothing is gained by heating it over time. Only running the tube with RF being generated will getter the tube anode. The anode has to get hot not the cathode. The filament only gets the tube components hot enough for emissions to flow. This providing the tube has gettering material. I could not find any information about the tubes manufacturing on gettereing. Most power tubes are gettered with DC wattage during manufacturing.

You can find out more about gettering of tubes at this tutorial from 1950 at: http://www.tubebooks.org/file_downloads/Getter_Material.pdf

Temperatures on the anode for gettering needs to be over 600 degrees F. The tube filament is 6 volts at 8 amps. at 50 watts there is not enough energy to get the tube anode to the required temperature.

At Alpha we had to burn in the Chinese 4CX1500B's in order for them to perform well on 6 meters. Generally several power cycles would bring the tubes to full power. Even new tubes are not without production problems.
« Last Edit: April 23, 2016, 09:30:24 AM by K6AER » Logged
AE6PR
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« Reply #5 on: April 23, 2016, 05:45:04 PM »

Thanks everyone-turns out it has a grid to cathode short(neg bias with no hv). Oh well, I am out only 20 bucks.
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N3AJB
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« Reply #6 on: April 26, 2016, 08:08:16 AM »

Hi Shane,

One last ditch approach to revive tubes with shorted elements is to connect 120 vac across the shorted elements.  The current surge is supposed to burn up the short.   I read of that approach used to restore 3-500s with shorted elements. 

If you want to try this, you might attach jumpers with very small alligator clips to the grid pin and cathode pin with the other ends of the jumpers connected to 120vac through an ON/OFF switch.  I would turn the switch on and off as quick as possible. Then check the tube. If you don't blow a fuse, you might have burned out the short.   If you have a variac you might start with  lower voltage.

Of course, you could burned up parts of the elements but it might be worth the try.  Good Luck

JON N3AJB
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AE6PR
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« Reply #7 on: April 26, 2016, 08:58:58 AM »

I can try it but the short does not show up until the filament heats up. cold it is open. Any thoughts?
Thanks
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AE6PR
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« Reply #8 on: May 02, 2016, 08:53:49 PM »

Update-I was reading in the owners manual for the HT33A and there was a paragraph on the PL172. The manual stated that negative grid current readings on the meter do not indicate a defective tube. Readings can go negative up to the edge of the scale. I put the 8295A back in and have been running it for 10 minutes on idle current at a time. It has not tripped anything nor is making any audible arcing noise. My only question is how damaging to the amp can a flashover be at rated rf output? The amp is stock.
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K6AER
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« Reply #9 on: May 02, 2016, 09:24:04 PM »

RF is not the question.

It is the discharge of the high voltage and the low impedance discharge of the capacitors. If the flash over takes place in the tube the tubes internal components can be welded together or sharp points may result on internal critical surfaces leading to future arc over points.

With hundreds of joules of energy discharging, series components may get fried. Dropping current resistors may blow. Caps may weld them selves internally. Unprotected current meters may blow open.

If you are putting unknown tubes in service it is best to put a series metal film resistor  with the anode feed. Something like a 10 watt 50 ohm will work nicely. That way if you have a tube short the resister gets blown and saves most everything else.
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W3RSW
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« Reply #10 on: May 03, 2016, 04:48:59 AM »

The 8295 is a cold ANODE ceramic pentode. Note that forced air over an operating, finned anode will not let the anode get over 200 to 250 deg. C.  That is not nearly enough temperature to getter enclosed gasses.

By design the getter material is on the sleeve of the indirectly heated oxide cathode. Running just the filament voltage inside the cathode sleeve will getter this tube.  Don't however run without a cooling blower running regardless whether plate voltage is applied. Filament heating alone still has to be air cooled.

Please refer to more modern literature describing ceramic oxide cathode tubes.
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Rick, W3RSW
KM1H
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« Reply #11 on: May 07, 2016, 07:12:17 PM »

Rick has the only correct answer for that tube.

While the Eimac version has a good history, regettering after years of non use is always a good idea. Some indirectly heated tubes, especially some RCA, can take 24 hours or more of filament only time.

Since an external Variac also affects the filament voltage in that amp that is a great way to destroy the tube.

Carl
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