Cathode poisoning and stereo poweramps

[Meson Boogie]

I've read a bit on cathode poisoning recently and as I understand it happens when the amp is left in standby.
Anyway, I have a mesa 395 with separate standby switches for each channel, and since I only use one side the other is kept in standby pretty much all the time it's on. Occasionally I switch sides, but I found the more used side sounds better (it was obvious which side the previous owner used).
Could this be because of cathode poisoning on the unused channel?

Any feedback is appreciated.

[edit: Regardless, common sense tells me the best way to preserve tubes is to go through the hassle of removing them when I wont be using them.]

 

[stokes]

Keeping the amp in standby will not wear out the tube more than having full power on.When in standby the only thing functioning in the tube is the heaters.There is no current passing thru the tube and therefore will not be any electrons being emitted from the cathode.In standby the heaters are heating the cathode and there may be some wear,but not nearly as much as there is when the circuit is taken out of standby and allowing current to flow thru the tube.When you take the amp out of standby,even with no signal going thru the tube there is still current flowing,idle current to be exact.This will wear out the tube faster than leaving it in standby.Leaving it in standby will wear out the heater to some degree,but wont "poison your cathode",whatever that means.

 

[Meson Boogie]

I'm not familiar with the works of tube audio, only reason I thought this was due to a few recent threads on the matter like this one
http://forum.grailtone.com/viewtopic.php?t=27574

Perhaps I'm missing the point there.

 

[stokes]

I saw that post too,but in almost 30 years working with tubes I have never heard the term "cathode poisoning" before that post.Emissions cant be measured without some current flowing thru the tube,so it makes sense that if you have no current flowing,you have no emissions.A tube in standby has only the heater operating so you will wear out the filament to some degree,but nothing else.And a heater is either good or not,it doesnt get weak and cause the tubes tone or performance to suffer.Leaving your amp in standby as you describe will not cause the tubes any more stress,in fact it will cause less stress, than hitting them with full power for no reason.

"The activated electrodes can be destroyed by contact with oxygen or other chemicals (eg. aluminium, or silicates), either present as residual gases, entering the tube via leaks, or released by outgassing or migration from the construction elements. This results in diminished emissivity. This process is known as cathode poisoning."
This is a quote from that post,he obviously quoted this from some source.If you read it,nowhere does it describe in any way leaving the tube in standby,rather it describes a vacuum leak or an otherwise gasseous tube,which wont be caused by the fillament left on.

 

[stokes]

that's one thing i really like about my dual monobloc, completely independent operation (meaning 2 standby and 2 power switchtes)

maybe i'm wrong, but it seems to me that leaving a tube in standby is like leaving the lights on in a light bulb (filament concept there as well, no?)

Yeah,it is similar,a filament in a tube is like a filament in a bulb,except the one in a tube is much tougher than a bulb.I have tubes that have been used for forty years and the filament is still good.That is why I wouldnt be too concerned about leaving a tube in standby.

 

[dmt]

Standby mode is only meant to be used for a short period of time to warm up the tubes so they are not shocked by high voltage and overly stressed as well as keeping them warm in between sets so that you are not continually cycling the power on the tubes and further stressing them. Standby is not meant to be an 'always on' solution.

How can you say that you can leave a tube on standby, and therefore partially functional, and have no damage or changes made to the tube?

My advice would be to either use the amp in stereo as it is mean to be, or find a separate mono amp. That would be ideal, but not necessarily financially feasible.

I prefer to leave my amp on even for a short break; this is less wear on the tube, based on what anecdotal evidence and research i have done on tube duty cycles. Leaving an amp on in Standby for long periods of time is going to affect the tube, and while i do not mean to question your long years of experience, i am simply stating to the best of my knowledge what i understand to be true about the operation of tubes. I am by no means an expert.

I will however consult with a good friend of mine who IS an expert and knows more about tube design, function, and electronics than anyone i have met or spoken to. I will get back to you on this.

 

[stokes]

"How can you say that you can leave a tube on standby, and therefore partially functional, and have no damage or changes made to the tube?"
Because the heater has no bearing on the tone or functionality of the tube,as long as it is working,and a heater either works or it dont.As you say"the heater is meant to be used....to warm up the tube so they are not shocked by high voltage.... So leaving the amp on in full power is going to stress the tube more than in standby simply because of the fact that you have current running thru it the whole time,no?So leaving it in standby will only stress the heater or filament,which in my experience is really no big deal.In a good tube they will last a very long time,unlike a light bulb.

 

[boola]

I remember reading somewhere that poisoning is caused by gassing from the metals in the tube which form a 'crust' on the cathode and make it less effective.

I don't know the details but I seem to remember that it can happen when the tube is not on standby but has no signal going through it, on standby it wont happen.

 

[stokes]

http://www.tube-tester.com/sites/nixie/different/cathode poisoning/cathode-poisoning.htm

what does the revered RCA tube manual say about this topic?

Slow degradation of the emissive layer and sudden burning and interruption of the filament are two main failure modes of vacuum tubes.

http://en.wikipedia.org/wiki/Hot_cathode

Okay,but where is the proof that leaving the amp in standby causes slow degradation of the emissive layer or sudden burning and or interuption of the filament?In standby there is no current flowing,if there is no current flowing there is no or at least no measureable emission taking place,even if you want to take the argument that with just the heater warming the cathode,there is some emission taking place,it is not measurable,so to me that is negligable at best.To me,if you cant measure it,it aint there.As for "poisonig",if it is caused by gasses,and I can buy that,leaving the heater activated doesnt cause gas.I'll give you that leaving the heater on will cause some wear to the filament to some degree,but given its lifespan,which in an otherwise healthy tube,will outlive the plate and grid,you arent going to do any considerable damage leaving the tube in standby.

 

[Meson Boogie]

A nice resource:
http://books.google.co.uk/books?id=...&hl=en&sa=X&oi=book_result&resnum=5&ct=result
I don't know what kind of cathode is used in a standard 6l6gc, but it's likely tungsten with an oxide layer.

Anyway, note "Poisoning occurs quickly if the filament voltage is too low" but I think this refers to when the amp is full on and properly set up ie. at high cathode temperatures.
It is likely that warming up the cathode in standby would cause it to react with any stray oxygen in the vacuum, just as the getters react with it during the production process, but that is assuming the getters haven't already done that which is a pretty big but.

In conclusion, I don't think I'll be avoiding standby...

 

[stokes]

A nice resource:
http://books.google.co.uk/books?id=...&hl=en&sa=X&oi=book_result&resnum=5&ct=result
I don't know what kind of cathode is used in a standard 6l6gc, but it's likely tungsten with an oxide layer.

Anyway, note "Poisoning occurs quickly if the filament voltage is too low" but I think this refers to when the amp is full on and properly set up ie. at high cathode temperatures.
It is likely that warming up the cathode in standby would cause it to react with any stray oxygen in the vacuum, just as the getters react with it during the production process, but that is assuming the getters haven't already done that which is a pretty big but.

In conclusion, I don't think I'll be avoiding standby...

If the cathode is reacting to stray oxygen in the tube,the tube aint long for this world anyway,since there should be no oxygen or gasses of any kind in a tube,hence the name "vacuum" tube.So regardless of standby or full power,the lack of vacuum is going to kill the tube,not cathode poisoning.

 

[Meson Boogie]

I'm agreeing with you on this subject, but a few stray atoms of gas inside wont harm the tube and there's no way the vacuum is even close to perfect.

 

[stokes]

The vacuum seal has to be perfect,or the tube will leak and die.I'm no engineer,but my understanding is that the tube is put under vacuum then the flash/getter is ignited to eliminate the last few atoms of any gas remaining.If the getter doesnt do its job and any gas remains,the tubes life will be shortened.Do they routinely leave a few atoms of gas?I dont know,but again I'll go back to"if you cant measure it,it aint there".Who knows how many atoms of gas it would take to adversly affect the tubes performance or life?So if a tube lasts a good long life,I would go under the assumption that there is/was no gas present at the begining of its service.Of course gas can develop as the tubes components wear out,so maybe we should try to figure out a way to get some bi-carb into a tube,and be the first to develop a tube that wont die.Sorry,I couldnt resist that one.