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Q3: Cathode Poisoning

Q&A (3): Cathode Poisoning

3. Some tube amplifiers have a “stand-by” intermediate position between “on” and “off” positions. It is recomended to heat up the tubes, and only then go to the “on” position. But when I am switching the amplifier “off” … should I be switching the amplifier from “on” to “off” instantaneously, or should I allow it a few seconds to rest in the intermediate “stand-by” position prior to switching it fully off ?

cathode poisoning

cathode poisoning

Let’s face it, that some ready made tube amplifiers, especially those originating from the ‘far east’, do not have a very good design, when it comes to these aspects. So you simply have an “on” and “off” switch, and that is it. When switching to “off” – you basically just disconnect the power supply, the tube heating filaments start to cool down, but you still have the full impact of the anode voltage on the anodes. Why ? Because you have a significant amount of capacitors in your high voltage filter within the HV DC power supply. So, what do you apply ? You apply “hope”.

And you apply it as follows: you “hope” that the anode voltage drops in value, which means that the anode voltage conditioning filter unloads the charge of it’s (huge) capacitors, before the cathode turns cold.
However, this hope may or may not have merit. If your HV DC power supply filter is a passive one, and has a significant amount of capacitance installed (such as a good passive filter would), then your chances of quickly getting rid of all that stored energy are rather slim.

But then again, it is reasonable to assume that the ‘far east’ manufacturers do NOT put in excessive capacitance into thos HV DC filters, in order to cut costs. So it well may be that there is ‘merit’ to your hope, and that there will not be to much damage done.

Therefore, for the sake simplicity, just switch it off, or pull the power plug, and be done with it.

{ … hey, did he just say something about ‘damage’ ? WHAT damage ?? … }

Well yes. Not big damage. Very small damage. Damage that probably occurs each and every time you switch the unit off. Damage considered as a slow process, that over time (switch-off count) gradually destroys your tubes.

Why ? Because tubes generally “do not like” to have a very high voltage on the anode, at a time when their Cathode is cold, or not at the right temperature. That is the reason, why when you switch your amp “ON”, it actually waits a bit for the cathode heaters to fully heat up, prior to applying the anode voltage. Well, OK, switching “ON” is the easy part. You can have timer circuits that postpone the switch-on of the anode voltage, so as to give the heaters some time to heat up properly. That is probably why your “music” appears only after a few tens of seconds in your speakers.

But switching “off” is just as important – maybe even more important, and constitutes the other side of the same “coin”.

Why the “damage” ? Because ther is a physical phenomenon that occurs, when you have a cold cathode and a high voltage on the anode. In such a scenario, an acceleration of heavy, positively charged ions of the particles of the residual gas takes place. There is no such thing as “perfect vacuum”. When produced, the vacuum tube gets nearly all of the gas pumped out from it’s inside, in order to achieve a high level of vacuum. But, there are traces of the gas particles left within the tube, as no pump is perfect. These traces of gas, when exposed to a strong electric field, tend to separate from their electrons (going “north” to the anode). Gas atoms convert into positively loaded, heavy Ions. Now, when we have a cold cathode, there is an absence of a heavy stream of electrons flowing from cathode to anode. Such a stream would normally swamp the Ions, that are interested in going southbound, to the cathode. But, alas, there is no such heavy stream of electrons, because the cathode is cold an not yet emitting. In the absence of “flow” of electrons rushing “northbound”, these heavy ions of gas, positively charged, begin to accelerated “southbound” in direction of the negatively charged cathod. They bombard the cathode. They have a big mass, so they are heavy (in comparison to an electron at least). So, they have significant kinetic energy. The higher the anode voltage, the higher the energy of the impact upon the cathode.

Now, it just so happens that your preamp and driver tubes are indirectly heated ones (most probably). They are coated with a thin layer of delicate substances (oxides, etc), of fine oxides that enhance the emission properties of the cathode. Suffice it to say that this bombardment of the delicate oxides layer can sometimes damage or rather gradually degrade the oxide layer applied to the cathode, and hence gradually degrade the emission properties of your tubes.

This phenomenon is called “cathode poisoning”. It takes place over a long period of time (switch-off cycles!) and can lead to a premature loss of emission by the tube.

But there is more. If you bombard the cathode and these delicate oxide substances, they tend to dislodge from the cathode and are sprayed “all around”. And guess what, who is the next neighbor of the cathode ? It is the grid.

It might so happen, that these oxides spray and accumulate on the grid, which is close by, adjacent to the cathode.

If these oxides land and settle down on the grid … hmmm, …. guess what happens. Your grid is converted into an “unwanted cathode”. Your grid begins to possess emission properties (which should NOT posess!) – the more so, that is close to the cathode, and catches a bit of that high “temperature” that the cathode is heated up to.

This in turn may lead to a phenomenon called “positive grid current”. Well, ok, you say. Positive grid current occurs when the grid is not biased with a negative voltage. Well, true, but not quite true.

It might turn out, that your “cathodized” grid, can can actually start displaying symptoms of positive grid current much earlier, than would be expected when analyzing the technical parameters and graphs describing the tube.

It may well be that you THINK that you are far away from positive grid current territory, when in fact you are already suffering from positive grid current. Especially annoying, if it takes place within circuits / topologies, that are not prepared to cope with it.

So, just to be on the “safe” side – consider an additional relay (with a few additional elements, probably). A relay added to the anode HV DC filter circuit. A relay that is powered off the heater circuits, and normally, when powered up – establishes a connection between the HV DC filter output and your anodes. BUT, when power from the heater circuits dissapears ( you switched the amplifier off ), the anchor of your relay releases, instantaneously disconnects the anodes from the high voltage, and falls into its resting position, where it actually now connects those fully charged filter capacitors to an appropriately dimensioned power resistor, and hence helps to discharge the lethal charge of those capacitors, via shorting them, via the power resistor, to ground.

So, please kindly consider – this one little relay may actually significantly increase the life span of your tubes.

Powering OFF, or the ‘other side of the coin’ – is a much neglected issue in tube amplifiers.

And Yes, if you have a three position “OFF” – “Standby” – “ON” power switch, it does make sense to switch off in two steps: a). from “ON” to the “Standby” position. Here – you wait for the music to totally stop playing. b). Once the music stops playing, switch from “Standby” to “OFF”.

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