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Old 11th Oct 2005, 3:12 pm   #1
Steve G4WCS
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Default KT66 excessive current

A friend has just had to replace the KT66 valves in his quad amp.

His quandry is- and I dont know the answer- why do dieing power valves take excessive current. theroretically as the cathodes go low emission the anode current should decrease ?
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Old 11th Oct 2005, 6:06 pm   #2
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Default Re: Interesting question- answers on postcard please

hello Steve,
Usual cause of power valves/rectifiers/television line timebase amplifiers drawing heavy current is a tiny quantity of gas within the envelope.
Caution is needed due to the fact that this is often caused by incorrect biasing or faulty [leaking] coupling capacitors. The gas is often liberated from the anode that can glow red under a fault condition. I would suggest your friend checks these points to avoid rapid demise of his new KT66's. Regards. JOHN.
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Old 11th Oct 2005, 8:14 pm   #3
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Default Re: KT66 excessive current

I have changed the thread title to make it meaningful. Please use meaningful titles in future or your messages will be deleted.
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Old 11th Oct 2005, 8:52 pm   #4
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Default Re: KT66 excessive current

Quote:
Originally Posted by steve.collier2
why do dieing power valves take excessive current. theroretically as the cathodes go low emission the anode current should decrease ?
Output valves don't just fail by going low emission. Because they are power valves, they are more prone to being overdriven due to faulty biasing, leaky capacitors etc. A lot of output valves 'die' because they have developed grid current or leakage between electrodes. The UL41 is famous (infamous)? for this. Overdriving due to faulty bias or leaky grid capacitors causes excess current. This leads to greater dissipation of heat from other electrodes and, as has been said, can even make the anode glow. If the screen grid passes too much current, this can also glow. This causes gasses to be given off, impairing the vacuum. Sometimes metalic deposits can be left after the valve cools down and these deposit themselves on the inside of the glass and between the pins causing leakage. Add all that to the impaired vacuum and the poor old valve starts drawing very heavy current (sometimes with a nice purple glow). Even if the faults are repaired, the valve is too far gone to be serviceable. Emmision could be perfect however.


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Last edited by Sideband; 11th Oct 2005 at 9:05 pm.
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Old 12th Oct 2005, 8:37 am   #5
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Default Re: KT66 excessive current

Quote:
Originally Posted by Paul Stenning
I have changed the thread title to make it meaningful. Please use meaningful titles in future or your messages will be deleted.
I simply do not understand your point hear Paul. Richard has repeated exactly what I said in my thread. I think I will draw my membership to a close. Regards. JOHN.
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Old 12th Oct 2005, 9:47 am   #6
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Unhappy Re: KT66 excessive current

I must admit I was a bit surprised by Pauls response, I wanted to ask the question generically of power valves and not just the KT66.
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Old 12th Oct 2005, 10:59 pm   #7
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Default Re: KT66 excessive current

The original title was "Interesting question- answers on postcard please" which doesn't give any indication as to what the thread is about.

My new title was clearly not great either, bit it was somewhere near!
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Old 13th Oct 2005, 7:42 am   #8
Steve G4WCS
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Smile Re: KT66 excessive current

fair do's

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Old 13th Oct 2005, 8:53 am   #9
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Default Re: KT66 excessive current

My toys [EF50's] have been put back in my pram......
Regards. JOHN.
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Old 13th Oct 2005, 11:05 pm   #10
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Default Re: KT66 excessive current

Thanks both. I have exchanged email correspondence with John too.

I'll close this thread now, as it's unlikely to get back on topic. Please feel free to start a new thread on the original subject.
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