Hey all, I’ve been toying around with different gain resistors on a couple of Safety Valve modules I built, and while poking around I realized the 27r resistor from Pin4 of the TL071 to pin 5 of the valve is insanely hot - enough that I can’t touch it without hurting myself. Has anyone else noticed this, and should I be worried about leaving them powered on for extended play sessions? Anyone got suggestions for circuit tweaks that might help?
Thanks,
-Wes
From a quick peek at the schematics, it’s in series with the heater filament which is speced as max 150 mA at 12.6 V so simplifying a bit total series resistance is 27 + 12.6/0.15 = 111 ohms which corresponds to 12/111 = 108 mA through the circuit from the −12 V rail, and the 27 ohm sees 27×(12/111)^2 = 0.32 W which is above the specs for a 1/4 W resistor. It’ll probably survive (the watt limits are set for a high ambient temperature) but it’ll be hot. You could replace it with a 1/2 W or 2-4 larger resistors in parallel to distribute the load.
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I have added a bypass switch to the circuit that connects the input of the circuit directly to the output in bypass mode AND disconnects the current to the heater filament. So when I’m not using the valve caster, it will pass its input signal 1:1 to the ouput without consuming power to heat the valve. Because I built 2 this unloads the power supply quite a bit when they are not used.
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maybe the little problem with that is when you trigger the effect the lamp is not hot right away so the distortion effect will gradually act 
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Fredrik, that’s an incredibly informative answer, thanks very much. I might order some half-watt resistors for a simple clip and kludge or something.
To anyone who wants to use Eurorack power, I came up with the following stripboard layout:
I also had issues with the bypass not working - maybe I had the 10k resistor in the wrong place or something. In any case, I’m using a DPDT switch here to provide a bypass that is working much better for me. Hope this helps someone:
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If you’re going for a kludge, you might as well solder a few 1/4 W resistors in parallel, on top of each other – e.g. 2×56 ohm, 3×82, or even 4×100 depending on what you have in your stash will all give you roughly 27 ohm but with more capacity. Since they’ll split the current, you’ll quickly get down below the 1/4 W limit:
EDIT: There’s also the option of removing it – the heater is designed for 12.6 V so can obviously handle 12, but that’ll result in higher power consumption and possibly also hum, see here.
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This has been an eye opener. It’s been about fifty years since I read Gilbert Davey’s books (by then out of date) about thermionic valve electronics. I was gifted a couple of valve radio sets that had been junked. The resistors in those cabinets were big and thick, mounted on heat-conducting metal chassis and well ventilated. If you’re putting a low value resistance into the valve heater circuit, show some respect for the device. Give the resistance element room to breathe. Good air flow design will help your circuit to stay alive for longer.
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