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Re: we 300b life

Posted by Janos (A) on January 5, 2007 at 17:20:27

In Reply to: Re: we 300b life posted by Nickel Core on January 2, 2007 at 13:21:44:

NC,

My gut feeling is that when the amp is switched on/off, the tube ages because the cathode undergoes rapid cooling / heating, damaging the cathode. The cathode is a complex structure, and its ability to release electrons can be influenced a lot by annealing it. And that's what occurs during on/off cycling.

There was one study on tube life and aging, involving hundreds of vacuum tubes, when they built Eniac that showed that slow turn-on of B+ does not influence tube life. The only evils they found was heat, that was not dissipated from the anode.

Something must be ocurring during turn-on.
When I hook a stereo70 up to a variac, and turn it on, there is an initial current draw of over an amp. Then the current draw drops to quarter amp or so, then a couple seconds later starts going up as the rectifier starts to conduct.

What is that initial huge current draw? As the rectifier is not operating yet, it's not HV. The filaments are drawing that excess current, and their current draw drops to the nominal as the tubes heat up. My hunch is that this initial huge current has a hand in ageing the tubes...

My idea is to turn on the amp with a variac slowly, to prevent this rush of current. (I tried it out, and turning the st70 on with a variac prevents the initial huge current surge). I'm curious if on the long run it can save tube life..... there was recently a post saying that RCA had theater amps on which they starved the cathode during the breaks, increasing tube life. I also heard from my mentor that when not using the amp, run 30% B+ voltage instead of switching the amp off, and that will extend tube life tremendously.

Looking at the physics of cathode heating: the cathode needs to be 900-1100C hot in order to emit electrons. Warming up a thin piece of metal from room temp (20C or so) to 1000C in a matter of few seconds is an immense stress to the material. After this immense stress, comes up the high voltage. As I see, the high voltage is not that stressful on the cathode (unless it is of too high current), it stresses the plate: as the electrons bombard the anode, their motion energy transforms to heat. However, the plate is a much bigger structure than the cathode: the current density on the plate is on the order of a few percents compared to the cathode. This translates to lower stress on the anode, than on the cathode, for a given current.

The heat generated by the flow of electrons has to be dissipated. If it is not dissipated by the tube envelope, it starts building up inside the anode structure, and a significant portion of it is reflected back to the cathode, increasing its temperature. When the cathode is heated above 1100C, its deterioration speeds up, tube life declines sharply as it increases.

If the B+ is decreased to 30%, then, with the same bias, the cathode current is basically shut off, plate dissipation and reflected heat (& stress) on the cathode is minimal. I would think this is better than starving the filaments on full B+, where there is a full current demand on the cathode, but the cathode is not hot enough to satisfy it. I beleive a straightforward way to kill a tube is put HV on it without filament voltage....
However, it also depends how much the filaments are starved, and what is the operating point of the tubes. If they operate in deep A, starving the filaments would probably wreck the tubes, as they have high current demand on a lukewarm cathode. If they operate in class B, there is but a trickle of quiescent current, that would not hurt the starved cathode.

It's worth a try to toy around........

Janos

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