6550 valves - longevity?

[anthonyTD]

Another possible reason given for short valve life is cathode stripping or more accurately cathode poisoning....opinions are divided on this and it's been furiously debated on many forums in the past but the theory goes along something like this:

If the HT is applied in advance of the heaters there is a possibility of ions bombarding the cathode reducing it's emissive capability and so shortening life...with the heaters properly warmed up the electron cloud surrounding the cathode repels the ions.

The old WAD amps with solid state rectification applied HT for approx 30s or more before the heaters had time to reach full temp....if you power cycle such and amp often enough the cathode is subjected to these ion attacks...as I said that's the theory...

The counter argument is that the potential difference between HT on the plate and earthed (effectively) cathode needs to be about 10Kv for this to take place and so at typical domestic valve amp HT of 300~500v it's hardly an issue...

One interesting bit I dug up was that of 'cathode sleepiness' where leaving a valve in an effectively cut off state by removing the heater voltage whilst still maintaining HT for extend periods of time turns the plate into a getter: The gas impurities present in the valve envelope (it's never a perfect vacuum) get attracted to the plate and stop it conducting...

yep, there is something in both your points, years ago when valve rectifiers were all there was, if you used in-directly heated rectifiers then there was virtualy no chance of cathode stripping at any voltage as the valve cathodes were already warmed suficiently before the HT voltage came up, obviously directly heated valve rectifiers come on almost as quickly as solid state types so like solid state little or no protection against potential cathode stripping, but as you quite rightly pointed out this is only realy considered a serious factor in short tube life in high voltage valve circuits.. cathode sleepiness or "lazy cathode" can also become apparent if as you say, the HT is up but little or no heater voltage is present, or when the valves heaters have been run on a DC supply. i have experience of valves that have become lazy from being in this paticular type of enviroment, and the good thing is, some valves suffering from this ailment are redeemable ie, with the right conditions aplied they can recover.
but alas both these senarios are not i fear the reason for the aparent short life of the valves in this paticular situation, its probably more due to the bias drifting, caused by coupling capacitors leaking DC onto the grids of the output valves, hence gradually reducing the negative grid bias, resulting in the output valves drawing more, and more current the longer they are left on.
hopefully i will know more when i have examined it.
anthony,TD...