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Old 19th September 2009, 06:56 AM   #1
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Default Standby voltages for tube circuits.

This has been commented on , on and off over the years.
Cathodes get stripped if they have HT on them before the heater comes up to the right temperature etc. This reduces tube life and`performance suffers.

Some tube circuits are put into standby by having a partial heater voltage on them ( 60 % of nominal voltage ? ) and no HT voltage on the anodes.

Is this a safe condition for the heaters ? Power up will bring up the heater voltage to nominal value and I guess HT comes up instantly ( due to a relay or other switching schemes ).

What would be considered the best way to put the circuit in standy without damaging the heater or the cathode coating and ensuring normal tube life and performance.
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Old 19th September 2009, 07:11 AM   #2
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Remember the "insta-on" tube TV's of the 60's and 70's? When off, they put a diode in series so effectively the tubes were seeing about 50% of the AC peak voltage through their heaters.

Didin't seem to hurt them - the last "insta-on" TV used by my family lived 34 years and only one tube changed (the video out, IIRC) in that time

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Old 19th September 2009, 08:03 AM   #3
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Originally Posted by ashok View Post
Cathodes get stripped if they have HT on them before the heater comes up to the right temperature etc. This reduces tube life and`performance suffers.
Really? I've read info that describes the idea of cathode stripping as a bit of an old wives tale. I can accept out of bias conditions as a possible failure mode due to bias circuits not being up to speed when HT is applied and the tube going into a run-away failure, but cathode stripping - has anyone actually seen this in real life?
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Old 19th September 2009, 08:07 AM   #4
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"By the 1950s, lifetime was a predominant concern of tube designers and industrial users, and if switch-on surges would significantly reduce lifetime then this would be reflected in the literature of the time, which it is not. There could conceivably be some benefit with directly-heated filamentary tubes, whose filaments are more mechanically fragile. The high-end audio world is full of mystical beliefs, and very probably this is one of them." http://www.john-a-harper.com/FilamentHeating/index.html
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Old 19th September 2009, 08:43 AM   #5
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On the other hand Tommy Flowers, the BPO engineer, found that the best way to avoid valve failures in large installations like the Colossus computers was to leave them on permanently, so power-up failure must have been something of an issue in the 1940s.
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Old 19th September 2009, 08:55 AM   #6
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or power off?
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Old 19th September 2009, 09:34 AM   #7
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"........or power off? ....."

Power on failure is related to failure due to inrush current. The switch on current is much larger than the operating current as the heater resistance is very low due to the low starting temperature. Additionally the heating is worst at the weakest points on the heater coil and that eventually breaks down. Just like filament lamps. Switch off doesn't have such problems.
However cyclic heating and cooling of the heater would also cause metal fatigue and cause eventual breakdown. Both these must be why Tommy Flowers wanted to keep the tube circuits permanently on.

Maybe strong physical movements when the heater is still hot ( after switch off ) could also cause failure ?

But then the focus of this thread is on what is the best method to ensure long tube life. For standby status , reduced heater voltage and no HT is good enough ....so far ! On " power up " I guess the heater voltage can be switched up to full instantly and let the HT rise a bit slower .
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Old 19th September 2009, 11:41 AM   #8
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On the other other hand [sic], if you leave heaters on for long periods with no HT you run the risk of "cathode poisoning" where non-radiating layers develop in the cathode coating. They had to develop special valves for computers (where some valves could be idle for long periods) to overcome this problem. There are no free lunches : what a horrible world!
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Old 19th September 2009, 03:53 PM   #9
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........"cathode poisoning"............

Right, that jogs my deteriorating memory! Thanks.
So applying a low HT supply will keep the tube at a strange operating point. It will keep dissipation down but is this odd operating point OK ? Does it overcome "cathode poisoning" ?

Or do we say that heater on ( partial voltage ) and no HT is out of the question if it's going to be that way for several hours ?
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Old 19th September 2009, 04:25 PM   #10
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I'm afraid you've reached the limits of my knowledge on this subject. I don't know how long the valves have to be idle for the poisoning to start happening. It's probably more than a few hours.
Personally I use valve rectifiers in the HT supply and switch amplifiers off when I'm not using them. The valves can be used after a solid-state supply as slow switch-on devices.
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