• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Lets settle the b+ on cold tubes issue!

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I was concerned about the inrush on the filament as that's which seems to really shorten the life of lightbulbs. After reading about inrush current limiters being used on aircraft landing lights (which are very expensive) 10 yrs ago I tried it on a frequently used reg 100W bulb always turning it up and down with the slide pot switch. Lasted 7 yrs! Seemed I was always changing light bulbs before. I also noticed outside lights on photocells last for years as the ones I have slowly power the light up. I don't know about the off part or if down voltage speed matters. Reading the forms made me purchase a 10A variac last year and have used it power old tube amps and radios slowly that haven't been used for years.
This leads me to continue to use it to power up this equipment over a few minutes stopping at around 60v for the longest, then slowly up to 110-115 v as they were designed for 117V max. I try to stay at 111-112V for longer life? Is this good for filament? Maybe a good compromise?
Is this better than inrush limiters which I intend to install. Will this short period of below normal cathode voltage hurt? What about the life of the variac?
 
My own opinion is that the best medicine for long term reliability in tube equipment is a thermistor on the primary of the power transformer. Bendix recommended current limiting resistors on the plates of their high-reliability rectifiers and, get this, a 45-second warm-up time for their amplifying tubes but with plate and filament voltages applied simultaneously.

I would agree with your first point- but not because of fears of cathode stripping. It's useful for surge limiting in heaters, power supply caps, and rectifiers.

As for the Bendix data, very interesting!
 
I've been using and building tube amps for 40 years, not all of them for audio. The only time you need a delayed turn on for B+ is with high power transmitter tubes. Well over 1000 volts, up to 25000VDC and/or amps, not milliamps, of plate current.

It's just doesn't have any affect on tube life in audio amps. Now, the thing that will made a difference is a soft turn on. Thermisters can help or series resistors in the AC primary that are bypassed by relay contacts after some delay both can limit the current surge in the heater circuit and allow the B+ to charge the filter caps slowly saving your rectifier. That is the thing that will help extend the life of your tubes, a slow easy turn on that keeps them from being shocked by sudden full voltage.

BZ
 
I heard a theory as if 3 Great Pyramids on Giza Plateau are precise aligned to North Pole, and resemble projection of 3 stars in Orion belt in December 2012, in order to stop Earth's crust shift, caused by precession angle and planet's paradise.

Can somebody prove/disprove this "theory"?

The Earth's crust is more valuable than selected quad of EL156 tubes, so it is better to be cautious and believe in the theory, right? :D

Unproven potential after all. Better safe than sorry :rolleyes:

Seriously, if the B+ is applied with the cathode already hot, then it might be a good idea to make sure the g1 voltage is stable and that the g2 and plate voltages come up smoothly with g2 below plate. Also good to think about the case where power is briefly interrupted and restored before the cathode has a chance to cool off. Smooth B+ ramp-up will cover both cases and is IMO all that is needed.

Sure there are other reasons to sequence the B+ but IMO cathode stripping is not one of them.
 
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Unproven potential after all. Better safe than sorry :rolleyes:
I have several types of tubes in my box, and ONLY the 12AT7 from BRIMAR makes a big Flash in the filament when turned on, almost like a lightbulb,and after a 2 or 3 seconds goes normal, maybe they used different material in the filament?
I think this is not good for the tube .
Does someone has this experience with this Brimar tube?
Silvino
 
I was concerned about the inrush on the filament as that's which seems to really shorten the life of lightbulbs. After reading about inrush current limiters being used on aircraft landing lights (which are very expensive) 10 yrs ago I tried it on a frequently used reg 100W bulb always turning it up and down with the slide pot switch. Lasted 7 yrs! Seemed I was always changing light bulbs before. I also noticed outside lights on photocells last for years as the ones I have slowly power the light up. I don't know about the off part or if down voltage speed matters. Reading the forms made me purchase a 10A variac last year and have used it power old tube amps and radios slowly that haven't been used for years.
This leads me to continue to use it to power up this equipment over a few minutes stopping at around 60v for the longest, then slowly up to 110-115 v as they were designed for 117V max. I try to stay at 111-112V for longer life? Is this good for filament? Maybe a good compromise?
Is this better than inrush limiters which I intend to install. Will this short period of below normal cathode voltage hurt? What about the life of the variac?
A lightbulb ONLY brakes when it is turned on , i never saw a lightbulb blowing up when is on! why ? do you guys know why?
I would like to know!
Thanks
 
Jeez, what a lot of fluff.

If you have a tube amp and you expect your wife to turn it on, or off, you must have solid state rectification and no time buffer for the HT, along with one on off control and one volume control. If your wife is not involved, what difference does it make to a bunch of old farts, fooling around in their man cave, how many different switches must be thrown in the proper sequence?

Back in the day, no one expected their tubes to last forever. Electronics repair shops were legion and all of them had stocks of tubes, due to their limited life time. The tube manufacturers didn't have to be concerned about extended lifetimes, especially for tubes in televisions. Tubes were a commodity and it was a profitable business model to replace them often. We don't live in those times, so why would anyone advocate treating tubes as they were treated, once upon a time, in a far gone land?

AudioPrism got around the issue by adding a 50% extended primary winding as their first switch position step, with both heater and B+ coming on together, and this from the mind of a noted RF amplifier designer. I haven't noticed the tubes lasting any longer or shorter, than what I found with a Dynaco 70. They wear out, no matter what. So, just decide upon the matter by who has to use the amplifier and how complex an arrangement they are going to tolerate.

Or is this the new popcorn corner?

Bud
 
Jeez, what a lot of fluff.
Or is this the new popcorn corner?
I think it may be! Everyone seems quite happy to bicker over unsubstantiated theories from creditable people and crackpots alike. But so far I think I'm the only person to have quoted any real academic research. And among such genuine research, the mention of delayed HT is conspicuous by its absence. Since valves have been studied for a hundred years, it seems unlikely that there would be any magic new phenomena that haven't been investigated already.
 
A lightbulb ONLY brakes when it is turned on , i never saw a lightbulb blowing up when is on! why ? do you guys know why?
I would like to know!
Thanks

There is apparently a critical phase during warmup when current density is decreasing and resistance is increasing. Eventually the filament cracks somewhere due to the increased stress during this phase and creates a localized high current density region, which soon results in runaway local temperature increase and vaporization of the filament.

---------------------------------------------------------
Horacsek, O.
Research Institute for Technical Physics of the Hungarian Academy of Sciences, Budapest, Hungary

This paper appears in: Physical Science, Measurement and Instrumentation, Management and Education - Reviews, IEE Proceedings A
Issue Date: April 1980
Volume: 127 Issue: 3
On page(s): 134 - 141
ISSN: 0143-702X
Digital Object Identifier: 10.1049/ip-a-1:19800023
Date of Current Version: 11 November 2008
Abstract

The behaviour of doped tungsten filaments is discussed with particular reference to the life-controlling processes in incandescent lamps. Since lamp failure is a complex phenomenon depending on many conditions, it appears that there is no single mechanism of universal validity by which the finite life of the tungsten filaments can be explained satisfactorily in all cases. It is suggested that, in addition to the nonuniform evaporation of the filament owing to local defects, other mechanisms, like migration and growth of the potassium-filled bubbles within the wire, and grain-boundary sliding promoted by unfavourable grain shape, may also contribute to the failure of incandescent lamps. Presumably, on the basis of a better understanding of the doping effect and failure mechanisms, further improvements can be achieved in lamp quality by optimisation of bubble dispersion, grain structure and other life-influencing parameters.
 
... If your wife is not involved, what difference does it make to a bunch of old farts, fooling around in their man cave, how many different switches must be thrown in the proper sequence?

Back in the day...
Bud

Back in the day, 2 channel audio was a killer app for electronics

Today, it seems to be mostly about old and not-so-old farts fooling around in their man caves arguing over the internet as if 2 channel audio was still an important killer app... :(

Isn't the whole thing a popcorn corner?

Nice the weather finally warmed up, eh?
 
A lightbulb ONLY brakes when it is turned on , i never saw a lightbulb blowing up when is on! why ? do you guys know why?
I would like to know!

I saw.
Initially bulbs were very reliable, and they were mass-produced for new and new installations Until... There was their own bulb-crisis then...

After over-manufacturing of incandescent light bulbs they decided to save materials and shorten life of bulbs dramatically, so their mass production would be profitable again. Before that bulbs lasted decades, after that they had to be replaced in less than year. Switching on again and again creates irregularities of the filament, that may blow up any time: during switch on, when it is on, or even when it turned off! Turn off when damaged the filament almost in any case was spectacular: a drop of extremely hot metal made a tiny hole in the bulb, an air sucked in, and a white smoke inside appeared there.
 
Hello,
My front porch light has a photo cell feature that turns on the lamp with what is effectively a soft start feature. I do not recall the last time the lamp was replaced. It was a long time ago.
For a distraction take a look at the GE 5 Star 5670 Tube Data Sheet or MilSpec for same. This tube was specified and tested for long periods of being Hot and not suffering the ill effects of cathode poisoning. To design for surviving such an effect must mean the effect is real? If the tube is cheap and the aircraft will not fall out of the sky who cares?
DT
All Just for fun!
 
For a distraction take a look at the GE 5 Star 5670 Tube Data Sheet or MilSpec for same. This tube was specified and tested for long periods of being Hot and not suffering the ill effects of cathode poisoning. To design for surviving such an effect must mean the effect is real?

5670 datasheet:
http://tdsl.duncanamps.com/pdf/5670.pdf

Cathode poisoning happens when the cathode is hot but the tube is cut-off for long periods, and it certainly is a real effect. It can be almost completely eliminated by making the cathode tube out of very pure metal; some telephone repeater valves buried at the bottom of the sea used platinum for this reason.

But this is not the same effect as cathode stripping, which is what most of this thread is about.
 
Hello Merlinb,
From early in the thread there were several related effects mentioned as they relate to cold B+ and the need for “safety items” such as standby switches and avoiding thermal shock to the heater
One at a time, so far,
Thermal Shock (on and off cycling of the heater) is real and it seems that the consensus is that reducing TS extends the life of the tube. Disagree?
Cathode Poisoning is documented as real. This seems to be a good argument against a stand by switch alone. A bit complicated. The effect that was discounted is the justification for the device that we are saying is unnecessary? BTW cathode poisoning was listed in at least 6 posts before we began to kick it about.
Cathode stripping is next. Real? I say yes.
I get to go to work now.
DT
All just for fun.
 
Is "cutoff" the same as no B+ applied?

I thought cutoff was B+ applied and G1 far enough negative WRT cathode to reduce conduction below a specific level (50uA?).

I grew up with tube radios, tube TVs, etc. I remember a few incidences of needing to replace tubes in our TVs. According to my father the Telefunken stereo ate output tubes and he got rid of the amp in it and replaced it with a Qualasonic SS receiver in the early 70s (while I was in the military or I would have grabbed ithe Telefunken).

When I went to college (1977) after the military I got our family Setchell Carlson TV to take with me. I used it up till I graduated in 1981. Setchell Carlson went belly up in 1970 and I think we got the set in 1968, so it would have been 13 years old when I got rid of it. I remember I replaced a couple of tubes in it. It was one of the “Instant On” TVs that kept the heaters on when the set was turned off so CS would not have been an issue with it. Maybe Cathode Posioning was the cause.

Back to CS.....

To me it is not a question of whether or not cathode stripping occurs. I think we all can agree it has been demonstrated in Microwave tubes and other tubes requiring B+ in the 1000+V range and that the designs of such systems incorporate protective measures against it (turn on filaments first, etc).

Anecdotal evidence seems to support the assertion that CS does not occur at an appreciable level once B+ is below 1000V.

I expect cathode stripping to have a logarithmic relationship to applied voltage level, when applied with cold cathode and the cathode subsequently heated to normal operating temperature. There should be some voltage beyond which CS becomes significant.

The question in my mind thus becomes:

"At what applied voltage does cathode stripping become significant enough to warrant measures to protect against it?"
 
Hello TheGimp,
Is "cutoff" the same as no B+ applied?

I thought cutoff was B+ applied and G1 far enough negative WRT cathode to reduce conduction below a specific level (50uA?).

I believe that you are on target. Cutoff is yes there is B+ on the plate with the tube biased to the point that current is nil.
From the cathode’s point of cutoff and no B+ may be similar in that the cathode will emit some electrons. In the case of cutoff the negative grid blocks the flow of electrons to the positive voltage plate as a valve will cutoff water flow. In the case of no B+ the plate is not nearly so attractive to the electrons. In either case the cathode emits a limited number of electrons that go crashing about, however there is near 0 current flow through the tube.
What does this mean in terms of cathode stripping or cathode poisoning? Some people use the terms interchangeably.
DT
All just for fun!
 
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