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Old 23rd April 2004, 03:18 PM   #1
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Question When do the tubes have a fever?

I am wondering what is a "normal" working temperature for a tube.

My KT88 in Single Ended reach 103 degC in 8 minutes, and maximum 125 degC in 20 minutes. (I wonder if 125 degC is the end of scale of the meter, since the 5U4G also rise to the same 125 degC )

I put the sensor at the top of the valve and then cover with metal paper.

It is really hot, but since I can not compare I don't know if its normal or not.

What is the temperature of your amp?

Cheers,

Manuel
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Old 23rd April 2004, 03:21 PM   #2
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I do recall that my kt88s get hot enough that you'll get a burn by touching them. The easily help heat the room.

I'm not sure of the exact temperature, but it's possible that 100C+ is the right temperature.

Let's see what someone else has to say.
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Old 23rd April 2004, 03:25 PM   #3
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I forgot to mention :

This 125 degC in 20 minutes are measured in a room at 26 degC and no signal at the amp.
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Old 23rd April 2004, 03:41 PM   #4
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Hi,

Quote:
This 125 degC in 20 minutes are measured in a room at 26 degC and no signal at the amp.
That's cool....when you live in Alaska.

Don't worry so much, it's pretty normal.
Your amp is class A so when running a signal as you put it it win't change much anyway.

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Old 23rd April 2004, 03:54 PM   #5
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By the way, is it possible to tell by thermal difference between used tubes and new?
which is probably silly because of unlimited differences in make model and manufacture date. can this method detect premature failure? if you say, set pre-defined limits based upon an average temperature for specific tube models? or will other methods more accurately predict failure?
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Old 23rd April 2004, 03:58 PM   #6
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Hi,

Quote:
By the way, is it possible to tell by thermal difference between used tubes and new ?
There won't be any difference in bulb temperature so, no.

Quote:
or will other methods more accurately predict failure?
It's almost impossible to predict failure from a tube.
What you can measure is emission which is an indiator of cathode health.

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Old 23rd April 2004, 05:37 PM   #7
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Most tubes are rated for a minimum of 180°C at the lowest, 200 for common power tubes and up to 250°C for some sweep tubes. Any more requires special glass, like my Bendix 6384 which is rated for full life (and it's long) at 300°C!

Tim
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Old 23rd April 2004, 08:40 PM   #8
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Yeah I have 5894 transmitter tubes and they are made with a special quartz glass for heat.

Many high power tubes get unbelievably hot. Just look at a magnetron. A power magnetron usually has a heatsink and a forced airflow to help remove great amounts of heat.
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Old 23rd April 2004, 09:06 PM   #9
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Although it isn't designed to go more than...130°C? They have thermal cutouts in the ovens.

I know my 4CX250R's are basically 4X150s (which I have a few as well) with ceramic rather than glass seals, and as such handle higher temps, allowing greater dissipation. They're basically identical on the outside, aside from the white stuff between the metal bits and a better-constructed radiator.

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Old 24th April 2004, 03:42 AM   #10
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Default Right on.

The temp for KT88s and 4cx250bs are 250 degrees C.

For those unfamiliar with the 7034/4x150A, the max temp is 175 degrees C at the base and grid 2 seals and 200 at the plate seal, and 250 at base end of the plate, and junction of the fins.

Take care guys.
Steve
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