With that class of tubes (6336, 6528, 6337, 6C33...), long warmup is needed to activate the zirconium getter on the plates. The tube will in fact be damaged if B+ is brought up before the specified heating time is finished. This consideration is irrelevant for common small signal and power tubes.
There's actually quite a bit of info on this in Tomer and in the various RCA internal technical texts posted on Pete Millett's site. The idea that the datasheets were read by consumers of tubes is novel. Back in the day, most people bought tubes from the bottom part of drugstore tube testers.
Thanks for that. I read a writeup concerning AC vs DC directly heated cathode heating. Something about DC heating the cathode less evenly and AC more evenly. It might have been a reference to an Eimac "proper care of your giant tube" sheet. -Fred
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Be careful, that article contains some nonsense and misunderstandings about cathode poisoning/stripping and standby switching. Standby switching is not useful for receiving valves. Peavy made guitar amps, and appears to have got his info from popular books and hearsay. He wasn't a rigorous academic.
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As far as I know, cathode stripping is a non-issue with the relatively small electrical fields used in tubes like e.g. the 12XX7 series. Cathode poisoning can occur, but only if the heater is kept on for hours on end without HT, which would negate the need for a standby/preheat function. So I have to disagree with this article.
The datasheet disagrees with you and specifically calls for warmup before applying plate voltage.
So you are saying that as an example the 6s19 with zirconium getter which has a 6.3W heater (6.3V/1A) reaches a temperture of 550K (kelvin) only with this little heater?
I don't believe that the minium nescecarry for activation of the getter will be reached.
Do you have (any) source?
Non-Evaporable Getter - Wikipedia, the free encyclopedia
Indeed, some tubes need preheat !
But in a "common" audio amp there are a lot of bypass and link caps which ALL act as a short when HV is applied suddently.
Imagine the "surge" anode current of a cathode biased 6550 having its cathode shorted to ground and its grid to B+ tru the anode load of the driver.
So, forget and switch on, caps will load slowly while tubes heat.
An NTC in the PSU transfo's primary helps to save fuses 😀
Yves.
But in a "common" audio amp there are a lot of bypass and link caps which ALL act as a short when HV is applied suddently.
Imagine the "surge" anode current of a cathode biased 6550 having its cathode shorted to ground and its grid to B+ tru the anode load of the driver.
So, forget and switch on, caps will load slowly while tubes heat.
An NTC in the PSU transfo's primary helps to save fuses 😀
Yves.
Pre heated means the time till the tube reaches at least 90% emmision. In some cases it is 10s, for an power tube 30-50 s and the 6c33 needs 120s. I found no datasheets that this should be done without hv!
The real power tubes with direct heated thoriated thungsten needs much less time.
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At the sort of voltages used in most audio amps cathode stripping won't happen.
Two dangers can happen when people foolishly try to avoid cathode stripping by using standby switches or naive sequencing:
1. cathode interface - also known as sleeping sickness. Happens when the heater is full on, but little or no cathode current. An insulating layer forms between the nickel and the oxide coating. Some valves (e.g. for computers, or mobile use) have special cathodes which avoid this. Most ordinary valves do not.
2. cathode bombardment - also known as cathode poisoning. Happens when the heater is on but at low power, and cathode current is drawn so there is little or no space charge. Any positive ions produced from gas in the valve can bombard the cathode. Under normal hot cathode conditions the space charge prevents this.
The rule to follow is simple: if the datasheet specifically calls for power sequencing then do it, if not don't.
Two dangers can happen when people foolishly try to avoid cathode stripping by using standby switches or naive sequencing:
1. cathode interface - also known as sleeping sickness. Happens when the heater is full on, but little or no cathode current. An insulating layer forms between the nickel and the oxide coating. Some valves (e.g. for computers, or mobile use) have special cathodes which avoid this. Most ordinary valves do not.
2. cathode bombardment - also known as cathode poisoning. Happens when the heater is on but at low power, and cathode current is drawn so there is little or no space charge. Any positive ions produced from gas in the valve can bombard the cathode. Under normal hot cathode conditions the space charge prevents this.
The rule to follow is simple: if the datasheet specifically calls for power sequencing then do it, if not don't.
Yes, the datasheets. If you have data which contradicts the written recommendations of the people who manufactured the tube, let's see it.
DF96's common-sense recommendation is... common-sense.
Since Philips didn't make any of those tubes, this is hardly surprising.
So, I take it that you don't have any data to contradict the manufacturer recommendation.
If the datasheet says "activate the heater for N minutes before applying HT/B+" then that is what you need to do. If it doesn't say that, then you don't have to do it - and in many cases it would be harmful to do it when not required.
Datasheets don't spend pages on telling you all the things you don't have to do, although they may mention a few. They tell you what to do, and what you should not do.
Datasheets don't spend pages on telling you all the things you don't have to do, although they may mention a few. They tell you what to do, and what you should not do.
If you mean for tubes who has zirkonium getters, maybe philips didn't make them.
But anyway this is of no importance, a tube has a vacuum and this vacuum must be very good otherwise a tube will not work at all ( or very very short).
Zirkonium getters just give you the garanty that the vacuum is good over a longer period of time. Barium getters don't do this.
Btw, the data sheet of the Tung Sol 6336a dos say a 30s pre heating time but not because the reason you tell. Read yourself......
http://frank.pocnet.net/sheets/127/6/6336A.pdf
Oh, and i really like a link where i can find the data you are talking about.
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Well, you found one of the references I mentioned which contradicts your recommendations. I have great confidence that you can find the others as well.
Seems this is time to listen a tube manufacturer talk on this subject for some useful info.
I wonder if Vynil Savor member would contact EL ROG TUBES on this issue and inform us??
Even for ElRog tubes only it could be very useful info and a reliable source.
Thanks in advance.
I wonder if Vynil Savor member would contact EL ROG TUBES on this issue and inform us??
Even for ElRog tubes only it could be very useful info and a reliable source.
Thanks in advance.
If a spec sheet gives figures for heater warm up, it refers to "controlled heater warm up". This is important if you're "daisy chaining" heaters for applications where you aren't using a PTX, just deriving the DC directly from the AC mains. In these cases, you want an even split of heater voltages across each heater from cold to hot. There were a lot of radios and TVs made back in "the day" that didn't include PTXs. That's why you find types with a wide variety of heater voltages.
Otherwise, don't worry about it. Except for some specialized types, there is no need to preheat before applying the HV. The only time I split heater and DC power is when using SS power supplies. There, you want the cathodes hot before switching on the DC as the DC comes up very fast with silicon diodes, much faster than heaters can warm. This avoids over voltage with DC coupled stages.
We are listening - we read datasheets; tube manufacturers have already spoken.FullRangeMan said:
What they are unlikely to do is to pre-empt all possible misunderstandings by spelling out in detail all the things which are untrue. Their silence then leaves scope for people like you to claim that as they have not prohibited something (e.g. 10 minute warmup) they must require it or are secretly hoping we will shorten valve life by omitting it.
Take the instructions for almost any piece of technology. It will not tell you that the first time you use it you must be standing with your feet in a bowl of water. Failing to do this means that the groundside electrons are not properly discharged so the device life will be shortened. They don't tell you this because they want you to buy a new one very soon. Now I have exposed their duplicity.
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