Plate and cathode resistor question
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audiosalvage
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Plate and cathode resistor question
In the usual high voltage B+ situation, 100k/820 is the norm. Now say I was running a 24v plate voltage, would these still apply? Or would one go for a much lower resistance pair? I'm attempting to marry a tweed type preamp with a parallel 12k5 power section at 12-24v B+. Thanks
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Re: Plate and cathode resistor question
Try it, but I recall low voltage operation being a completely different thing, positve grid, 12AU7 being preferable.
Expect much lower stage gain levels.
Edit - found it, for some reason Merlin doesn't have a link to his document on this http://valvewizard.co.uk/Triodes_at_low ... encowe.pdf
Expect much lower stage gain levels.
Edit - found it, for some reason Merlin doesn't have a link to his document on this http://valvewizard.co.uk/Triodes_at_low ... encowe.pdf
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Re: Plate and cathode resistor question
What you're proposing is generally called, "starved plate", meaning the plate voltage is much lower than the tube would normally operate at. This mode is often found in effects pedals, where high voltage is not available.
If you google up, "starved plate triode" you'll find a lot discussion.
The article at Valve Wizzard is correct, the usual plate curve graphs and SPICE models are useless at this point of operation. The common twin-triode tubes we use in guitar amps were not meant to operate at these very low plate voltages. This is well outside their normal operating condition.
I've not found a starved plate configuration that I liked much, other than perhaps as a distortion stage, and honestly, there are easier ways to get there, if one foregoes the novelty gimmick of having a magical tube in a pedal.
But, there ARE tubes that were meant to operate at low voltages. The RCA Nuvistor, which is a vacuum tube that's about the size of a germanium transistor, and any number of small glass vacuum tubes with soldered leads do operate at much lower voltages, well under 100v plate supply. This is kind of an unexplored area, but such things existed. You'll have to do your own research and learn how to draw load lines on the plate curve graphs of any potential devices you can get your hands on.
There's also the Korg Nu-Tube, which is derived from the vacuum fluorescent display tube technology. Again, a novelty, but they do run at relatively lower voltages.
The other path is solid simulations of tubes. Menatone offers a number of pedals that use JFETs in classic amplifier simulations. I have "Top Boost In A Can" pedal from 20+ years ago, that's actually pretty good.
One could also look into the darlington pairs of BJT's that mid-1990's Peavey Transtube employed. Sondemeyer was really clever engineer. I kind of recently stumbled onto this application and have found it actually works really well. There's a good patent that explains it too. If you wanted to build a "Princeton in a Pedal" kind of thing, I'd probably go down that route.
If you google up, "starved plate triode" you'll find a lot discussion.
The article at Valve Wizzard is correct, the usual plate curve graphs and SPICE models are useless at this point of operation. The common twin-triode tubes we use in guitar amps were not meant to operate at these very low plate voltages. This is well outside their normal operating condition.
I've not found a starved plate configuration that I liked much, other than perhaps as a distortion stage, and honestly, there are easier ways to get there, if one foregoes the novelty gimmick of having a magical tube in a pedal.
But, there ARE tubes that were meant to operate at low voltages. The RCA Nuvistor, which is a vacuum tube that's about the size of a germanium transistor, and any number of small glass vacuum tubes with soldered leads do operate at much lower voltages, well under 100v plate supply. This is kind of an unexplored area, but such things existed. You'll have to do your own research and learn how to draw load lines on the plate curve graphs of any potential devices you can get your hands on.
There's also the Korg Nu-Tube, which is derived from the vacuum fluorescent display tube technology. Again, a novelty, but they do run at relatively lower voltages.
The other path is solid simulations of tubes. Menatone offers a number of pedals that use JFETs in classic amplifier simulations. I have "Top Boost In A Can" pedal from 20+ years ago, that's actually pretty good.
One could also look into the darlington pairs of BJT's that mid-1990's Peavey Transtube employed. Sondemeyer was really clever engineer. I kind of recently stumbled onto this application and have found it actually works really well. There's a good patent that explains it too. If you wanted to build a "Princeton in a Pedal" kind of thing, I'd probably go down that route.