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In the Forum: Audio For Dummies ™
In the Thread: Get, or made up a tube tester, it’s necessary
Post Subject: something is movingPosted by hagtech on: 1/24/2007
I barely put 40W on this plate by testing. And that's a peak value! The curve sweeping action results in an average power dissipation much lower. The hysteresis is there with maybe 5W average power. And it is only on one side (76 heater) of this particular tube. I'm pretty sure it is not caused by the tester. So what is causing this loop? It could be grid current, as that would generate some small voltage across the 100 ohm series resistance I have at the output of grid step amplifier (total Ro might be about 200 ohms?). But what would cause it to shift in value? I like the thermal argument, which didn't occur to me at first because of the exceedingly fast transient response it must have. For this effect to occur, the thermal has a time constant of maybe 10ms. Fast. Maybe a grid wire has this sort of low thermal mass. And what does the thermal do? Is it a change in grid current or grid spacing? A mechanical displacement (like a vibrating grid wire) could be caused by heating/cooling. Maybe the efficiency of electron generation by the cathode can't keep up? I'm still at a loss to explain it.
The hysteresis didn't change over time. It is a short term (1ms to 100ms) effect, not really warmup related. You'll get a much better feel for this tube after watching the following video. It is the tube in question with one side heated. I then switch heating to the other side. You will see emission fall as one heater cools and the other warms up. I don't remember seeing loops like this on small preamp tubes under the same conditions. You can also see the powerful and direct relationship between emission and plate impedance. Also peak plate current.
http://www.hagtech.com/blog/media/100_1482.MOV
It's for quick-time player (6MB). Let me know if I should convert this to other format. Like WMV.
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