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Old 8th July 2007, 02:25 AM   #1
Jeb-D. is offline Jeb-D.  United States
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Default Light bulbs as cathode resistors?

Obviously for a self biased power output stage. Think it would work or would there be stabilization problems?
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Old 8th July 2007, 02:52 AM   #2
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I don't see why it wouldn't work..
But I don't think light bulbs are terribly linear.
Could be interesting in a guitar amp though.
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Old 8th July 2007, 03:04 AM   #3
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Quote:
But I don't think light bulbs are terribly linear.
Yeah, there not. The resistance increases as power dissipation/heat increases. There almost a dead short when cold. Which is why I raise the question. Just thought it would be cool for bling factor, instead of just having some power resistor hiding in the chassis. It would take some experimenting for sure, to find the right bulb.
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Old 8th July 2007, 03:28 AM   #4
jlsem is offline jlsem  United States
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Quote:
Just thought it would be cool for bling factor
Mo' betta bling would be a mercury vapor rectifier in the cathode circuit.

John
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Old 8th July 2007, 03:28 AM   #5
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Use LEDs.
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Old 8th July 2007, 03:49 AM   #6
Jeb-D. is offline Jeb-D.  United States
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Quote:
Use LEDs.
I should have been more specific, For an output stage, 60Vcg @ 250mA.



Quote:
Mo' betta bling would be a mercury vapor rectifier in the cathode circuit.
Nice idea, if I can find any that meet the circumstance.


I was thinking lightbulb that's halfway lit so it glow's orange instead of bright. Kind of like in the ZEN amplifiers.
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Old 8th July 2007, 05:47 AM   #7
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Well, I have never used a light bulb in a cathode, but i have an amp running right now using the preamp tube's filaments as the cathode resistor for the output stage (class A el34 triode PP) ... which is pretty similar... so my answer would be: "go ahead and try it!"
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Old 8th July 2007, 07:26 AM   #8
OzMikeH is offline OzMikeH  Australia
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Not my amp. No idea if they are for the cathodes or not. You will notice they are part of the power supply section. They are used as slow regulators. more current = hotter filament = more resistance.

they end up about 3-5 times brighter than the tube filaments.

I suggest you experiment with a few different light bulbs, if you can't get close enough use a low value resistor to tweak the light bulb.
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Old 8th July 2007, 02:40 PM   #9
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I guess you'd also have to take into consideration the possibility of the bulb burning out, opening the cathode circuit.
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Old 8th July 2007, 03:45 PM   #10
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Theres a few guitar amp schematics where the preamp 12AX7 heater is wired to the output tube cathodes. Not sure if theres any advantage
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