The heating filament of the tube has a positive temperature coefficient, so its resistance will go up when it heats up. I used a 27 ohm resistor in series with the heating filament to keep the inrush current a bit at bay. It is a big (chunky) resistor so that it can dissipate the heat. I took that value from other schematics involving this tube. I hope this will prolong the filament’s life.
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CONFIRMED! Well, this schematic works!
I’m finally posting this with a “CONFIRMED” tag on it.
Thank you to everyone for your input and advice on this one. I hope this will help others in the future!!!
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You may have explained this already, but why did you swap the filament connections with respect to what the LMNC diagram shows?
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In the process of testing why it wasn’t working, I used the strip board diagram version from the website as a guide. In that version, pins 4 and 5 were swapped.
I guess it actually doesn’t matter which pin goes to ground or not since there’s only the filament between 4 and 5?
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You’re right, the polarity of the cathode heater doesn’t matter.
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Congrats!
This bodes well for my strip-board layout.
That said, strip-boards are fickle beasts.
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It’s alive!
I wouldn’t trust it much further than I can throw it, but the output is frazzlier than the input and nothing has exploded so I’m taking this as a win.
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Nice!! What Switches did you add?
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I’ve got 2 2-pole switches and 1 1-pole switch.
The 1st 2-pole is a bypass that disconnects the output of the opamp and sends the input jack to the output jack.
The 2nd 2-pole is the power switch, it disconnects the power from the valve and turns off the power LED.
The 1-pole is the squelch switch.
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Just noticed that I have a short from power to ground on my build, which is quite concerning as I’ve been using the module on and off for a couple of weeks now… 
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I dont suppose you happen to have a stripboard layout of this. 
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If it worked despite shorted power rails, it cannot have been a very good short can it 
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Dav’s strip board layout works, you can use that.
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That is what’s confusing me.
Where’s the smoke, popping and tears?
Just curious, did you ever share those PCBs/Gerbers? I’ve got a big JLPCB order coming, would love to throw a PCB of this on there…
I didn’t share them, but I can now! Let me know if these work for you. I haven’t generated a Gerber file from EasyEDA before, but it seemed pretty straight forward!
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Hell yeah! Thanks homie!
Beautiful layout too!
Oh good! I’m glad it worked for you.
The layout isn’t exactly “best practice” (components close to each other like in the schematic), but with an area this small, I’d rather go for making it look good and being easy to assemble!
Hello,
my first attempt is not working, the 27R resistor gets hot and can’t find the reason.
I want to move to PCB to make some prototypes, i have doubled the valve for stereo out and added some leds to light the bulbs. I was using the schematic on the first post and after reading all this topic i understand that is not correct.
I search for a Kicad file to modify the schematic of a working one. Anyone have already done that?
Thank you in advance, i love this group.
I used 2 bigger 56Ohms in parallel.
See first comment from 'Fredrik here: Safety Valve - 27R is smokin' hot
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