Hey everyone,
I'm at the beginning of a long journey to create my first amplifier. I tend to learn best by doing, so after a lot of reading I'm building a simple low-voltage test circuit using a 12AT7.
The problem is, I'm already stuck on the heater elements. The datasheet specifies either 6.3V @ 0.3A in parallel or 12.6V @ 0.15A in series. So to start, I set a bench-top DC power supply to 6.3V @ 0.3A and wired pins 4 and 5 to (+) and pin 9 to (-). After a moment to warm up, only a single filament glows and just barely, only visible in near complete darkness. A multimeter across either pin 4 or 5 and pin 9 reads 1.3V.
I have also tried in series, wiring pin 4 to (+) and pin 5 to (-), with the power supply at 12.6V/0.15A. In this scenario, only 1 filament appears to glow. A multimeter across pins 4 and 5 reads 4.92V.
I've read that supplying DC to the filaments is okay. Though, clearly I'm misunderstanding some part of the hookup or power supply. Through searching this forum I've seen issues with high current inrush problems so I have disabled over current protection on the power supply. Any thoughts?
I'm at the beginning of a long journey to create my first amplifier. I tend to learn best by doing, so after a lot of reading I'm building a simple low-voltage test circuit using a 12AT7.
The problem is, I'm already stuck on the heater elements. The datasheet specifies either 6.3V @ 0.3A in parallel or 12.6V @ 0.15A in series. So to start, I set a bench-top DC power supply to 6.3V @ 0.3A and wired pins 4 and 5 to (+) and pin 9 to (-). After a moment to warm up, only a single filament glows and just barely, only visible in near complete darkness. A multimeter across either pin 4 or 5 and pin 9 reads 1.3V.
I have also tried in series, wiring pin 4 to (+) and pin 5 to (-), with the power supply at 12.6V/0.15A. In this scenario, only 1 filament appears to glow. A multimeter across pins 4 and 5 reads 4.92V.
I've read that supplying DC to the filaments is okay. Though, clearly I'm misunderstanding some part of the hookup or power supply. Through searching this forum I've seen issues with high current inrush problems so I have disabled over current protection on the power supply. Any thoughts?
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Are you setting a current limit function on your power supply? The tube doesn;t need that. If you are doing that, try turning the current limit up to an amp or something so it won;t trigger. The tube is only going to draw what it needs, you can't force current into it. The fact you measure only 1 volt tells me your supply is dropping, most likely from a limiter.
It is perfectly OK to run tube heaters on DC, it is done all the time. I suspect the problem is in your power supply setup.
Also, while the tube draws 0.3A steady for the heaters, when they are cold at first, they momentarily draw more current than that. Just for a second or two. But if you have a current limit set, it will trip it, and then the tube never gets a chance to warm up and operate normally. The supply stays in limit.
At least that is how I see it from here. I could be wrong.
It is perfectly OK to run tube heaters on DC, it is done all the time. I suspect the problem is in your power supply setup.
Also, while the tube draws 0.3A steady for the heaters, when they are cold at first, they momentarily draw more current than that. Just for a second or two. But if you have a current limit set, it will trip it, and then the tube never gets a chance to warm up and operate normally. The supply stays in limit.
At least that is how I see it from here. I could be wrong.
Well as it tends to go, you have to ask the question before you can figure it out on your own. After a lot of troubleshooting I finally checked the tube for continuity between each pin. It turns out both filaments and the cathode were shorted. I must have torched something along the way. Oh well, part of the learning process.
What do you mean by "continuity"? You should expect to see only a few ohms on the heater pins, as you are measuring cold resistance - not the 21 ohms when hot. If you are not careful this could look like a short circuit.
What is the cathode shorted to?
What is the cathode shorted to?
...and which cathode? (The 12AT7 is a double triode; two cathodes.)
I measure the cold heater resistance at about 20°C room temperature at about 11 ohm between pins 4 and 5 (I.e. across 12,6V application) - subtract say 0,5 ohm for meter leads.
(Good luck with your endeavours; keep courage! Although I am a professional engineer long since, I would hate to tell how I struggled all those decades ago with simple electricity. But all that serves to entrench the basics; most essential for later success.)
I measure the cold heater resistance at about 20°C room temperature at about 11 ohm between pins 4 and 5 (I.e. across 12,6V application) - subtract say 0,5 ohm for meter leads.
(Good luck with your endeavours; keep courage! Although I am a professional engineer long since, I would hate to tell how I struggled all those decades ago with simple electricity. But all that serves to entrench the basics; most essential for later success.)
I should clarify; I measure 0Ω from the cathode at pin 3 (section 2?) and both filaments. The short also exists between both filaments and from either filament to the heater center-tap at pin 9.
Unlikely that everything in the valve is shorted, but I suppose it is possible. Buy another valve. Check that one. If that is OK then the first one is duff. If the second one seems faulty then there is something wrong with your meter or your technique.
Just an update, received a replacement 12AT7 today and wired it up. Looks like it was indeed a fried tube. The new one works great as expected.
Thanks everyone.
Thanks everyone.
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