6C33C-B, Reliability--A theory........ - diyAudio
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Old 14th December 2005, 01:47 PM   #1
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Default 6C33C-B, Reliability--A theory........

Maybe Im just barking, but I have a theory concerning the 6C33 triode and its reliability. I suppose similar type triodes, with a very small spacing between the plate and cathode could also be affected.

I think its fair to say, that as the cathode is very close to the plate on these tubes, --Probably less than 1mm that there will be a 'radiated heat-sink' effect from the cathode to the anode.

If the heater at its correct voltage is sufficient to achieve correct temperature at 0 anode dissipation, then surely when the amode is dissipating say 40-50W the heat-sink effect is much less, and the cathode will get hotter, thus reducing its life?

It appears a well known fact that an increase in cathode temperature will shorten the emission life, as well as a reduction in temperature (within certain limits) will extend life.......)

I have actually seen an increase in the colour temperature of the cathodes in a 6C33C, when running at different dissipations from say 20W p Diss, and at higher levels like 45W plate diss--The effect takes some time to become noticable as the plate takes time to thermally stabilise in these tubes as its just so massive.

I think the real way to achieve longevity is as is popularly thought, Keep the anode dissipation below 40W, and maybe even reduce the heater voltage to the lowest recommended by the makers, which If I recall correctly is 5.7V per heater half and use good quality sockets with adequite cooling by using thick hook-up cables to act as heat-sinking and periodical socket to pin mantainence/cleaning, as well as running the heaters in series per valve from a 12V source..........

Ive been doing some experiments along the lines of a Circlotron OTL, where I have a very low plate diss, of less than 25W, The tubes stay relitively cool, (For a 6C33c-b!) with no apparent increase in colour temp of the cathodes, in comparison to a tube with just heaters powered. (Doing a similar test, running a tube at 40w and above, a definate increase in temperature is observed in comparison with the heater only tube.....)Even though I have a very low plate diss, the sound is very good as there is 100% feedback to a pair of diff-amp driver-stages, which take care of the output-impedances, which effectively drive the output pair harder to compensate .....


Or maybe Im just barking..............
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Old 14th December 2005, 04:08 PM   #2
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Hi,

I don't doubt that the cathode will be slightly warmer if there is high anode dissipation but the I think the effect will be very slight and attributed to the fact that something that is heated with a fixed amount of power will reach a higher temperature if the ambient temperature is higher. So the radiation from the anode should make the cathode area a bit hotter, however I am quite sure that this is a very minor effect as there can not be much radiation received by the relatively small cathode.

What make me curious is why you believe that this would have a major effect on the reliability of the tube? Do you assume that heater failure is the main reason for tube failure?

I have only minor experience of failure in 6C33C, actually the only failure I have encountered of more than 30 used tubes is one who shorted or was already shorted beteen cathode and grid, all others are still working perfectly.

In my experience of tubes I have never experienced a tube fail due to heater failure but I have experience of tubes fail due to decreased emission, (after very long use) internal shorts and due to gas leakage because of faulty or bad sealing.

For power tubes like the 6C33C I think it is much more likely that it will fail to some of the reasons mentioned above rather than due to heater failure.

BTW I don't agree with the idea to run 6C33C heater at low voltage, there will be much higher risk of cathode stripping and subsequently reduced emission if the heater is not as warm as it should be, I think that heater under voltage could be a major source of failure due to reduced emissions.

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Old 14th December 2005, 05:03 PM   #3
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Hi Hans!,
My thinking was not about the heater, which is probably the most reliable part of a large tube like this.

My reasoning was that the cathode itself would de-generate due to the higher temperatures attained--Perhaps if you have some time you could rig up a similar experiment that I did, running a tube with only the heater, and another running say 45W anode dissipation--After an hour or so, there is a definate difference in the colour temperature of the cathode.

I dont advise running the heater at below the minimum specified rating, but at or just above the lowest recommended value specified bu the makers.

I doubt that cathode-stripping would be an issue, (although maybe Im wrong.....)as most of these oxide-coatings have an emissive current in the tens of Amperes per square cm, so the 6C33 with say, 12cm of cathode could easily handle the 2-3 A of current experienced in an OTL

At the moment I only have 8 6C33c's all of which were new cheapies off fleabay, and all are good. I have hammered a few in a SE project a while back, and now a pair in my OTL--Im really surprised at the power that can be passed through one of these things, but in all Ive read about the 6C33 has advised to keep the standing bias dissipation to a minimum, to avoid socket problems (heater supply) and a run-down in emission.

My ideas about running the heaters series per valve, was to do with the effects of a tiny resistance may cause--Just ask anyone whose owned a 6V battery vehicle, like a VW Combi/Bug!
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Old 14th December 2005, 05:24 PM   #4
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Hi,

No I iunderstand what you mean but I still think the effect is very minimal. I have not seen any of this effect when I have done my reliability tests running continous full power at an average cathode current of about 0.8A for a full 24 hours, everything gets very hot but I haven't seen the effect you describe.

Any cathode has a limited life and cathode stripping or degeneration of the cathode surface is a part of that. If it is so that bad heater connection is a primary source of premature failure of the 6C33C then the failure is probably due to some sort of cathode degeneration due to the too low cathod temperature. The heater voltage range is not a fixed range where you can expect the same performance, for high reliability it is as important to not go below nominal voltage as exceeding max voltage esopecially if the tubes is wiorking with very high current.

The easiest solution for the reported bad heater connection problem is to find good sockets, I use real russian made ones and I have not encountered any connection problems neither short term or with tubes working for several years.

As I have learned the main reason to run the tube at low dissipation like 2/3 of max is to reduce the risk of gas emission inside the tube which will majke the performance drift and finally shorten life, the so "bad heater connection problem" I have seen as a separete problem with its own solution.

Regards Hans
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Old 14th December 2005, 07:25 PM   #5
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Hi,

Something to add to my last post, a cathode has an emissivity that is dependant on both the heater power and the surface area. For oxide coated cathodes a typical value is 10 - 200mA per sqcm per watt, (source F.E. Terman Radio engineering handbook). the lower figure is valid for larger cathodes, the emissivity figure is dependant on the heter power per sqcm where higher power density will give higher emissivity.

This means that a cathode of the size in a 6C33C that has a surface are of ~10sqcm and a heater power of about 40W should be able to emitt up to 10 x 10 x 40 = 4000mA minimum which could be about right, I have never exceeded a cathode current of about 2.5A but I know that this is not the absolute maximum.

What is important for this discussion is that if a cathode is run at lower voltage it will have less than maximum emission, this will obviously give a shorter life as there is less margin to begin with.

Regards Hans
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