Another concern, apart from the risk to cathodes, is that of turn-on surge. If you turn on the HT quickly, by using SS diode rectifiers, the tubes are not yet heated and cannot conduct. There will be no current drain on the PS due to the tubes in the amp, so all the current through the rectifiers ia available to charging the smoothing and reservoir caps. This gives a great surge through the caps and some people think that's bad.
The remedy is to use a delay and a soft start, i.e. let the tubes get hot first, then allow the HT to build slowly. This can be achieved by using thermionic indirectly heated rectifiers, which can take up to half a minute before they start conducting, depending on type, and turn on quite slowly. This solution avoids the turn-on surge and also deals with any risk of cathode stripping, either real or imagined. It also results in a relatively
"clean" HT, free from SS diode "hash".
To minimize the impedance which is incurred with thermionic rectifiers, with the resulting voltage drop and "sag", thermionic TV damper diodes can be used. These have a long warm-up time and are quite rugged. They work well in either a "full-wave" circuit (center-tapped power transformer secondary) or in a Gratez hybid bridge, in which they make up the positive-going half of the bridge and SS diodes make up the negative-going half. TV damper diodes tolerate high PIV, high currents and usually high heater-to-cathode voltages.
my diy-amp has 4xpy500A for rectification.....
I make sure i always preheat the py500A
Nevertheless, within 2 years they have developed a 'mirror-blackish' on the top of the tubes...at the top-cap. (that is in this case the cathode connection)
i suspect this is.....'moved' getter and a residu of other materials
could this be cathode stripping also??
or is it just conatmination???
kathodyne: It could be removed cathode, but that could be down to your ripple current.
ray_moth: I thought TV damper (or efficiency as they're known in Europe) diodes would be slow warm-up too, but when I measured 12CL3, they took exactly the same time to start conducting as any other thermionic diode - about 11s.
gingertube: What makes you think helium? If your photomultipliers are as expensive as the ones we used in telecines (£1500 each in 1992), I can see why you'd want to protect them.
EC, I've chewed this one around and I still have some trouble with the mechanism you propose. Here's the problem: the concentration of gas in a tube is quite low. Now, when there's a significant electron flux, there could indeed be a significant cross section for collision and ion formation. Problem is that at start-up, the electron flux is miniscule. The probability of electron-ion collision will certainly be proportional to the electon flux and the gas molecule concentration- if both are low, that cross-section will be very, very close to zero.
Things are different when there's a very high electric field. But at normal operating voltages, the field between cathode and plate is not horribly big, in fact, no bigger than the field between an insulated wire carrying B+ and the grounded chassis a few mm away.
Frank has a good point I believe.
@ray_moth: how are your experiences withe 6d22s??????? they're not too expensive and could maybe very well substitute for my py500A's???
I just looked up some gas ionization potentials in the "Handbook of Physics"
Hydrogen: 13.598 eV
H2: 15.427 eV
OH: 13.18 eV
H2O: 12.60 eV
CO2: 13.79 eV
He: 24.587 eV
N: 14.534 eV
N2: 15.51 eV
O: 13.618 eV
O2: 14.01 eV
Ar: 15.759 eV
Kr: 13.99 eV
Xe: 12.127 eV
Rn: 10.745 eV
Neon bulbs and VR tubes strike over around 65V up to 100+V or so .
Maybe the important parameter is when the ions have sufficient energy to actually cause damage to the cathode coating. Which might be more like the voltages necessary to cause secondary emission or higher.
On page 1085 there is a secondary-electron emission table for the elements,
and these seem to be roughly 300 V to 600 V. No listing for any oxides though.
The interesting thing is how the IPs are determined. For nitrogen and oxygen, it's electron impact. For water vapor, it's photoionization (at least in the CRC book).
Here's an article addressing cathode stripping and other mythical failure modes:
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