What is Cathode Poisoning?

[Axtman]

What is cathode poisoning and how can it be avoided?

[Helmholtz]

See here:

http://www.bustedgear.com/faq_Amp_standby_switch.html
https://en.wikipedia.org/wiki/Hot_cathode#Failure_modes
https://www.pearl-hifi.com/06_Lit_Ar...cuum_Tubes.pdf

It seems the most effective way to prevent early cathode poisoning is to avoid longer periods in standby mode.

[The Dude]

IMO, not something to worry about in normal situations.

[Axtman]

Well I was considering putting a switch on an amp to switch from tube to ss rectifier. From what I gather if the rectifier tubes sits too long without doing any work it will slowly degrade.

[Leo_Gnardo]

Well I was considering putting a switch on an amp to switch from tube to ss rectifier. From what I gather if the rectifier tubes sits too long without doing any work it will slowly degrade.

I don't know that the rectifier would suffer cathode poisoning from being lit up but not passing much in the way of current. It's more like the output tubes would eventually degrade with cathode at operating temp but no current being drawn from it.

If you select 5AR4/GZ34, that rectifier has a built in slow warmup feature. Not much of a problem powering up with no standby switch. However, no rectifier appreciates having current suddenly drawn, perhaps a bit more than it's rated for, when standby switch is thrown into operate position. In many amps, the inrush of current to the filter caps at this point stresses the rectifier, no matter which type. Some amps have the first stage of filters charged when standby isn't sending current into the rest of the circuitry. My best guess is this case is less stressful on the rectifier.

FWIW in Mesa amps with this dual rectifier setup my ears never noticed much of an audible difference between SS or tube. But that's me. Maybe others do. Sure, the amp delivers a pinch less power with the tube rectifier, the meters notice, you can see it on the scope, but my ears say "so what." To me, this is an example of a solution in search of a problem.

I'd say try it experimentally before committing to having such a switch in your amp. If you don't detect any difference, or the difference is miniscule, why bother.

[Enzo]

Cathode poisoning is a real thing, but it mainly applies to LARGE power tubes, like the kind in broadcast radio modulators or some such. WOuld it affect tube life in some Fender? Maybe, but I won't be too concerned that my 5AR4 lasts 10 minutes less than it would have.

[pdf64]

Cathode poisoning is a real thing, but it mainly applies to LARGE power tubes, like the kind in broadcast radio modulators or some such. WOuld it affect tube life in some Fender? Maybe, but I won't be too concerned that my 5AR4 lasts 10 minutes less than it would have.

May be thinking rather of cathode stripping? As I recall, that occurs when high plate voltage is applied to cold transmitter tubes, whereas cathode poisoning is the deterioration in emission over time when a tube has its heater powered but with no cathode current for prolonged periods.
See Tomer http://www.tubebooks.org/Books/Atwoo...um%20Tubes.pdf
I suspect that neither is an issue for a regular tube guitar amp, but the slow ramp up of HT provide by a GZ34 is surely a nice thing to have.

[Helmholtz]

That^^^^.

Already had the Tomer book in my list above (Pearl-hifi link). See pp 34,35,36.

[Enzo]

Hey, I am old, I get confused. Just in my experience all the various doomsday scenarios like cathode XXXXing of whatever kind tend not to show up in polite little situations like guitar amps.

[J M Fahey]

What is cathode poisoning and how can it be avoided?

Something that attacks people who read too many Forums on the Net or yellowing moth eaten books from the 40īs and donīt realize much of whatīs written there applies to 40īs level electronics, specially Radio equipment, running hairy deadly voltages most of the time.
Ham Radio was a hot item those days.
Not too applicable to humble Audio Electronics.