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Old 17th Oct 2018, 12:10 pm   #1
GrimJosef
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Default KT66 storage precautions

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Originally Posted by barrymagrec View Post
I have a few NOS GEC KT66s as well as a new matched pair that were supplied by Quad some years ago ... I am keeping them as a nest egg ...
My experience with M-OV KT66s (can be branded GEC or Osram or, more rarely, Marconi) is that they can be prone to outgassing in storage. This is not good news. If the gas level becomes high enough and the valves are then plugged into an amp with high grid-leak resistance the resulting grid current can drive them into catastrophic thermal runaway before the getter gets hot enough to start taking the gas level down.

My advice to anyone who owns any of these valves would be to run them for 2-3 hours, perhaps once a year, in an autobias circuit which gets them properly hot and which holds their grids very firmly to zero volts. The downside is that they won't be able to be sold as 'unused'. The upside is that they stand a better chance of working reliably if/when they're finally needed.

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Last edited by AC/HL; 18th Oct 2018 at 11:10 am. Reason: Split from a wanted thread
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Old 17th Oct 2018, 2:29 pm   #2
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Default Re: Wanted: Clear glass GEC KT66s

Back in the day one would pop down to the local radio shop and buy a KT66, just a KT66 any make.
 
Old 17th Oct 2018, 3:39 pm   #3
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Default Re: Wanted: Clear glass GEC KT66s

For a long time if it said KT66 on it then, whatever the brand, it would have come from M-OV. The nearest equivalents from other manufacturers would have been some type of 6L6 which would not have been exactly the same (heater current, for example). Towards the end of the valve era re-branders (Teonex, Bentley etc) took to buying small straight-sided 6L6s and printing KT66 on the outside. They weren't KT66s though and if you have any now you may well find that when pushed hard they fail before a 'real' KT66 would have done.

Cheers,

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Old 17th Oct 2018, 5:42 pm   #4
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Default Re: Wanted: Clear glass GEC KT66s

This is probably old news to some correspondents but I found it fascinating looking up the M-OV date and factory codes. Many M-O valves are marked with letters and numbers that can be decoded from the table in the attachment. For example I have a pair of Marconi KT66 marked TE Z which turn out to be Manufactured by MO Valve Co. Hammersmith in May 1962. The other marked one is a grey Osram KT66 marked MD 10. The MD puts it at April 1956 and I am led to believe that around this date KT66 manufacture moved from the factory at Dover to Hammersmith so I suppose this valve could have come from either. I could not find any decode for the numericals prior to 1969. Hope this is of interest. Cheers, Jerry
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Old 17th Oct 2018, 6:32 pm   #5
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Default Re: Wanted: Clear glass GEC KT66s

I should add a credit for the basis of the above info to Jeff West and Bryan Lockey at the Marstran.com bulletin board. Bryan worked for GEC at Gateshead and Hammersmith from 1955-61.
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Old 17th Oct 2018, 7:54 pm   #6
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Default Re: Wanted: Clear glass GEC KT66s

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Originally Posted by GrimJosef View Post
My advice to anyone who owns any of these valves would be to run them for 2-3 hours, perhaps once a year, in an autobias circuit which gets them properly hot and which holds their grids very firmly to zero volts.
Just out of curiosity would it be possible to achieve the same result by running the valves with heaters only - i.e. without HT applied, maybe by say removing the rectifier in a Quad II?
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Old 17th Oct 2018, 7:59 pm   #7
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Default Re: Wanted: Clear glass GEC KT66s

Running valves without any cathode current risks cathode poisoning.
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Old 17th Oct 2018, 8:33 pm   #8
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Default Re: Wanted: Clear glass GEC KT66s

Thanks, I did wonder. I have a pair of Quad II's with GEC KT66's which are not used too frequently.
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Old 17th Oct 2018, 8:42 pm   #9
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Default Re: Wanted: Clear glass GEC KT66s

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Running valves without any cathode current risks cathode poisoning.
Maybe if you did it for thousands of hours [as happened to the unused UHF-tuner valves in a number of late-sixties dual-standard TVs that were sold into the market before BBC2 reached the area] but I wouldn't be worried about cathode-poisoning if you only ran the valves for a few tens of hours.

I supplied some 4X1500A-type RF pulse-duty amps that ran with continuous heater-power-but-0.001% duty-cycle anode current. Never had a problem with them delivering full output when 'Red' forces intruded....
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Old 17th Oct 2018, 10:35 pm   #10
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Default Re: Wanted: Clear glass GEC KT66s

Quote:
Originally Posted by Half a Mullard View Post
Just out of curiosity would it be possible to achieve the same result by running the valves with heaters only - i.e. without HT applied, maybe by say removing the rectifier in a Quad II?
I'm afraid not. The heater in a KT66 (6.3V, 1.27A) dissipates just 8W. That will get the valve's glass envelope, where the getter is, warm but not hot. In a Quad II the anode-cathode voltage for the KT66s is 340V or so and the quiescent cathode current is about 72mA, so the power dissipated by the electrons is now 24.5W which has to be added to the heater power to get the total power - 32.5W. This is four times the heater-only power and it gets the glass a good deal hotter. Getters work better as the temperature goes up, so for maximum getter pumping you'd run the valve at its absolute maximum limit which is 34.5W electronically plus 8W for the heater. At that power the outgassing rate will start to rise too though.

If you have a pair of Quad IIs then I'd recommend switching them on for a few hours once or twice a year. But if they've been out of service for several years I'd use a voltmeter to keep an eye on the voltage across the output transformer's cathode windings, V-U and V-W. If either of them rises above 1.6-1.7V then that's a sign of gas and if they get above 1.8-1.9V then it's time to switch off, let the valves cool for 10-15 minutes or so and see if things are better when you switch on again. I've sometimes managed to de-gas valves by repeating this process a few times.

Cheers,

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Old 17th Oct 2018, 10:55 pm   #11
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Default Re: Wanted: Clear glass GEC KT66s

Excellent advice GJ, thank you very much
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Old 18th Oct 2018, 10:18 am   #12
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Default Re: Wanted: Clear glass GEC KT66s

Thank you GJ, don't think it is the first time you have mentioned outgassing in power valves, I remember it with EL34's. Caused me to consider what to do with valves I have had for decades, so far done nothing, need a simple warming up jig.

As a matter of interest, what happened to the date codes after 1968? All my KT66's have the usual yyww date code, last one in 1983. All marked with a Z so Hammersmith?
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Old 18th Oct 2018, 12:48 pm   #13
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Default Re: KT66 storage precautions

Quote:
Originally Posted by GrimJosef View Post
My advice to anyone who owns any of these valves would be to run them for 2-3 hours, perhaps once a year, in an autobias circuit which gets them properly hot and which holds their grids very firmly to zero volts.
Cheers,
GJ
Have you got a schematic that you could suggest using for this purpose please?

Thanks

John
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Old 18th Oct 2018, 2:55 pm   #14
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Default Re: Wanted: Clear glass GEC KT66s

Quote:
Originally Posted by woodchips View Post
what happened to the date codes after 1968?
From 1969 to about 1983, the date codes are four digits with the first two the year and the second two the month. So a valve with the code: 8204= April 1982. Although you may be right if the second pair of digits are week then 8204 would be January 1982!
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Old 18th Oct 2018, 4:10 pm   #15
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Default Re: KT66 storage precautions

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Originally Posted by John M0GLN View Post
Have you got a schematic that you could suggest using for this purpose please?

Something like this will do the trick.

The HT voltage is not critical (within sensible limits); the resistor dissipates about 1 watt and so a 2 watt component will be suitable.

Expect to see about 15 volts developed across the resistor.
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Old 18th Oct 2018, 4:53 pm   #16
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Default Re: KT66 storage precautions

KT61 used in the Leak Point One TL/10 amplifier also suffers from this effect. Here the problem is aggravated by the very high value grid leak resistors at 1MΩ.

Thanks, a timely reminder to recheck this situation as the amplifier has not been used for a while!

Cheers
Rich
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Old 18th Oct 2018, 5:44 pm   #17
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Default Re: KT66 storage precautions

I'm late to this discussion so if I talk complete rubbish - please excuse.

If the getter still has some useful coating, or some vapourised material remains on the glass and it just needs to get hot again to de-gas the vacuum, what is needed to fire it..?

Do I need to put my KT66 in the microwave? I guess not since the getter was a single turn of <10mm dia.
It just piqued my interest...

Originally it was fired by close induction heating we suppose.
Cathode poisoning is of course quite different and not to be confused with out gassing.
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Old 18th Oct 2018, 10:46 pm   #18
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Default Re: KT66 storage precautions

Getters will work at the 'normal' temperature of the hot valve's glass - they're not just a one-off device, although they might well pump quite a lot of stray gas molecules when they're first laid down.

I have heard it suggested that if the valve is irredeemably gassy then it might be worth seeing if just a second or so in a microwave can liven up the getter at all. I only tried this once, with a very gassy KT66s, and a lot of sparks were generated. When I tried the valve again the experiment had made its performance even worse.

I did also wonder whether the KT66's getter, which is at the bottom of the valve where the glass meets the base glue, might be running a bit cooler than optimum because of conductive cooling of the glass and slightly cooler air at the bottom. I considered either trying to insulate (thermally) the bottom of the valve or playing hot air from a heat gun over that part of the glass to see if this helped at all, but I've yet to try this properly.

As far as the schematic goes Colin's is very much the sort of thing I've used in the past. I might put a 100R between the anode and the screen grid, just to reduce the likelihood of any possible oscillation. I might also put a 47k from the control grid to ground with a push-to-open switch across it. That could be used as a grid current monitor. A valve in good condition will have much less than 1 microamp of grid current, so less than 47mV across the monitor resistor. Finally I might raise the HT voltage to 350V or so and adjust the cathode resistor for 80-85mA cathode current. If no gas appears at that dissipation then the valve is probably in good shape.

Cheers,

GJ
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Old 19th Oct 2018, 6:33 am   #19
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Default Re: KT66 storage precautions

Thanks Colin for the schematic and GJ for the comments, I'll make one up in the next few weeks and see how an old KT66 that I have reacts, I have a variable power supply up to 400V that I can use with it.

Thanks

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Old 19th Oct 2018, 8:42 am   #20
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Default Re: KT66 storage precautions

I'll be interested to hear how you get on. The only other bit of advice I'd give is just to remember that thermal changes can be slow, so a good deal of patience can be necessary. In terms of gas, practically none may show up for the first 5-10 minutes after switch-on. This leads to the tragic syndrome where we turn something on for the first time, nervous that it will go bang. The rectifier warms up, there's no bang, all measures well, music even comes out. Phew ! We decide to celebrate by going to make a cup of tea. When we get back there's smoke and red hot metalwork .

Cheers,

GJ
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