Hi,
I'm new to tubes and bilding a small amplifier with a 12AU7 triode.
The plate voltage should be 90 V.
In some datasheet the heating time is given with about 30sec. Does this mean that the plate voltage must be enabled after 30sec or it is ok too when the plate voltage is applied at the beginning of switching on the amplifier.
THX!
I'm new to tubes and bilding a small amplifier with a 12AU7 triode.
The plate voltage should be 90 V.
In some datasheet the heating time is given with about 30sec. Does this mean that the plate voltage must be enabled after 30sec or it is ok too when the plate voltage is applied at the beginning of switching on the amplifier.
THX!
Some purists like to warm the heaters up slowly to avoid thermal shock to the filament, then add the B+ when everything is warm to avoid cathode stripping.
However, look at most commercial valve designs from the valve years and you will see that most of this was not practised in the hey days of valves.
However, look at most commercial valve designs from the valve years and you will see that most of this was not practised in the hey days of valves.
Cathode stripping is a persistent myth from the era of directly heated tubes. It has no scientific basis whatsoever for indirectly heated tubes like the 12AU7. There's no need with these small current triodes to preheat them.
The time stated in the datasheet is usually to indicate how long it takes to get the tube to it's normal operating point.
Controlled heater warmup can be done, but again not necessary with the 12XX7 series. Power tubes is another story. These 1.6A (KT88) heaters draw a significant amount of current when cold, stressing the power transformer. Inrush current limiting is advised.
The time stated in the datasheet is usually to indicate how long it takes to get the tube to it's normal operating point.
Controlled heater warmup can be done, but again not necessary with the 12XX7 series. Power tubes is another story. These 1.6A (KT88) heaters draw a significant amount of current when cold, stressing the power transformer. Inrush current limiting is advised.
Heaters are more robust than 'purists' think. Significant heater overvoltage is routinely applied when activating the cathode at the factory.
Obviously this 30 secs are the minimum time suggested by the manufacturer, but you must consider tube manufacturers want sell more tubes to you soon, and 30 secs is not suffice to heat and stabilize the cathodes and less yet the plates.
I would like 10 minutes for cathode heating and more 10 or 20 minutes for plate heating.
The plate B+ in amps must be manually operated as in ham radios for a slow power delivery, but hifi amps are commaded by (low) cost rules.
FullRangeMan wrote "Obviously this 30 secs are the minimum time suggested by the manufacturer, but you must consider tube manufacturers want sell more tubes to you soon, and 30 secs is not suffice to heat and stabilize the cathodes and less yet the plates.
I would like 10 minutes for cathode heating and more 10 or 20 minutes for plate heating.
The plate B+ in amps must be manually operated as in ham radios for a slow power delivery, but hifi amps are commaded by (low) cost rules."
The data sheet on an indirectly heated ECC81/2/3 or 12A*7 is quite clear and states nothing about any damage caused whatsoever to the cathode if the Anode has voltage applied before 30seconds of pre-heating. What is this rubbish about 5-10minutes?
If the "Plates" are heated, they will not be as efficient as electron flow is to a cold Anode, not a hot one unless it is made of carbon, like the 813 that will keep working until the glass implodes!
I would like 10 minutes for cathode heating and more 10 or 20 minutes for plate heating.
The plate B+ in amps must be manually operated as in ham radios for a slow power delivery, but hifi amps are commaded by (low) cost rules."
The data sheet on an indirectly heated ECC81/2/3 or 12A*7 is quite clear and states nothing about any damage caused whatsoever to the cathode if the Anode has voltage applied before 30seconds of pre-heating. What is this rubbish about 5-10minutes?
If the "Plates" are heated, they will not be as efficient as electron flow is to a cold Anode, not a hot one unless it is made of carbon, like the 813 that will keep working until the glass implodes!
I think you mean say IF THE PLATES ARE OVER HEATED beyond the regular temperature.
A two pages tube datasheet can not contain all the tubes data or recommendations for long life, in fact long life procedures are hard to find in tubes datasheets.
Honestly speaking, the intention is to keep the tube user buyer of valves not aware of these facts and data.
Yes, there's nothing like a conspiracy theory to try to convince someone of something with no scientific bases at all 🙄. Tube aging is mainly caused by cathode heating. Pre-heating will make it age just as well. Cathode poisoning might occur is the valve is heated without HT for hours on end, so please don't consider adding e.g. a standby switch.
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You might like 10 minutes. The valves are happy with 15-20 seconds. Please don't spread such silly misinformation. Applying heater and HT supplies together is perfectly fine for most valves. Under these conditions it is quite usual for NOS quality small signal valves to last for 20-30 years. Shorter life is usually caused by overheating due to lack of ventilation (e.g. in a 1960s TV set).FullRangeMan said:
The situation with high power valves, such as some of those used in RF power amps, is quite different as they sometimes have anode-based gettering. Don't confuse these with normal small-signal valves or most audio power valves using normal construction techniques.
The original poster asked about preheat of 12AU7 tubes. Perfectly serviceable 12AU7 tubes are $10 at the local Guitar Center. 10 minutes for this, 10-20 minutes for that is silly. You will spend more on electricity by far than any possible extension of tube life.
Sounds like a Monty Python skit. "Every tube is sacred". I don't think so.
Not an engineer by any shot, but I have a basement full of old, ahem, perhaps mature tube musical instrument amps full of 12??7 series tubes, most with no provision to preheat. Most of the 12??7 series tubes are original and are still fine. Many of those amps gigged hard for over 10 years.
Relax, don't worry. Turn the gear on and enjoy. If in 20 or 30 years one finds the need to replace that poor little 12AU7, know that I have spent more on a pack of smokes than on that tube.
Sounds like a Monty Python skit. "Every tube is sacred". I don't think so.
Not an engineer by any shot, but I have a basement full of old, ahem, perhaps mature tube musical instrument amps full of 12??7 series tubes, most with no provision to preheat. Most of the 12??7 series tubes are original and are still fine. Many of those amps gigged hard for over 10 years.
Relax, don't worry. Turn the gear on and enjoy. If in 20 or 30 years one finds the need to replace that poor little 12AU7, know that I have spent more on a pack of smokes than on that tube.
No, the 30 second heating time quoted in the data sheet refers to how long it takes the valve to heat up and become fully operational from a cold start, presumably due to a special heater design. It is nothing to do with waiting 30 seconds before applying anode voltage.
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Well, how a person read/understand a datasheet has a important role in this issue.
To me all datasheets are abscure in this regard to intentionally cause misuse of valves, except the Michael Boele 6C33 2005 datasheet, which inform the heating time for this tube is 10 minutes.
As the 6C33 has a huge metal plate the 10 minutes is suited, so seems you are saying small tubes need less time due it use a smaller plate.
I you like the word of the factory about this important subject, but they are hiden so far.
10 min to reach thermal equilibrium sounds about right for a large tube. That does not mean you have to wait 10 min+ before swithing on the HT or even using them.
I've been around KW level RF triode amplifiers most of my life, and they certainly heat up in less than a minute. Even if there may be an HV delay, it is 1 min or less, for the ones I've seen. Mostly the HV comes on immediately.
I've been around KW level RF triode amplifiers most of my life, and they certainly heat up in less than a minute. Even if there may be an HV delay, it is 1 min or less, for the ones I've seen. Mostly the HV comes on immediately.
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Unfortunately many if not all manufacturers of tube amps do not like to inform the public that his amp requires a long heating time.
They prefer to say that its amp has a new innovative magic circuit which exempting heating time.
They prefer to say that its amp has a new innovative magic circuit which exempting heating time.
Stop spewing nonsense. There's absolutely no harm in applying HT immediately. If anything, it provides a nice soft-start for the PSU.
The 6C33 datasheet you're referring to states "Warm-up time to steady state > 600s". That has got nothing to do with possible damage or reduced life, just the optimum operating conditions.
Why are you so eager to believe some kind of conspiracy?
The 6C33 datasheet you're referring to states "Warm-up time to steady state > 600s". That has got nothing to do with possible damage or reduced life, just the optimum operating conditions.
Why are you so eager to believe some kind of conspiracy?
I dont consider the electronic bussiness practice a conspiracy, just a way to do more sales.
There is no way to break the laws of physics even inside a semi-vaccum tube.
These 600 seconds to steady state are informed to military use which are supposed to 1K hours service life, for hi-fi use the heating time must be much more longer, if not similar time life will result(2-3K hours).
Also Russian army has zillions of these 6C33 tubes and money is no object.
If some one can show a tube factory official or technical doc dispensing or informing small heating time I would like to read it to my own knowledge, if not... you guys are on my side e.g. we are just guys with a opinion.
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No, you have a fantasy, from who knows where? We have an understanding, based on facts. For many series heater valves (often with an A suffix) the heating time is stated as 11 seconds (to 90% emission IIRC).FullRangeMan said:
You are talking complete nonsense. Don't assume that most valves require the heating time of a few special valves.
Is cathode stripping the same as cathode poisoning where the filament is supposed to have a life reducing getter action when run while under heated? I imagine the higher the dissipation the tube is called to do during the power on and power off stages when the cathode is under heated would exhibit the most of this behavior? Seems pretty insignificant for small signal tubes if so and more so for power tubes and indirectly heated rectifiers. Still seems pretty insignificant since the cathode passes through this state for only a moment at each power on and power off. And while cold the anode and cathode are just distant materials in a vacuum.
Or is cathode stripping when B+ is turned on instantly when the filaments are warm?
Everything seems happier when the tubes are heated first and then the DC is ramped up gradually as Jon Snell mentioned. This is what I do because the turn on action just seems a bit violent to me, but this is a completely non consumer oriented practice.
I can see maybe more of problem of cathodes being turned on and off over and over in combination with mechanical jarring and the doping loosening from the cathode.
Personally, I don't mind since I buy guitar tubes.
I doubt the tube companies are trying to cheat anyone. I doubt there is even the capability for that kind of 'sophistication'. It seems at this date, they are just trying to make something work that is close enough to the original specs to designate the tube as a substitute for the original. Thanks. -Fred
Or is cathode stripping when B+ is turned on instantly when the filaments are warm?
Everything seems happier when the tubes are heated first and then the DC is ramped up gradually as Jon Snell mentioned. This is what I do because the turn on action just seems a bit violent to me, but this is a completely non consumer oriented practice.
I can see maybe more of problem of cathodes being turned on and off over and over in combination with mechanical jarring and the doping loosening from the cathode.
Personally, I don't mind since I buy guitar tubes.
I doubt the tube companies are trying to cheat anyone. I doubt there is even the capability for that kind of 'sophistication'. It seems at this date, they are just trying to make something work that is close enough to the original specs to designate the tube as a substitute for the original. Thanks. -Fred
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