Now, take a look at this..

[nickb]

OK all you theory gurus. This little invention of Korg is bound to hurt your head:



Claimed voltage gain is 3.5 with 2k source and 100k load. Oh, don't ask about the current gain.

You can check out patent US7053710 for a description.

Enjoy!

[g1]

Interesting. Input to plate, output from grid. From the patent info, US Patent # 7,053,710. Vacuum tube amplifier having grid as output and plate as input - Patents.com the emphasis seems to be reduced power consumption (due to reduced heater voltage).
Scratching my head as to why Korg would be interested in this (from 2004). Perhaps having a patent on a "greener" tube circuit?
Coincidentally, just saw a Triton extreme for the first time the other day, the vacuum tube in it is very deliberately visible. Doubt it uses this circuit though.
Is there anything else Korg is using tubes in?

P.S. Had a laugh seeing some "smith" patents referenced in the article.

[J M Fahey]

Korg *is* so called "VOX", and they use this contraption extensively to claim working tubes inside their battery powered pedals.
Just *suppose* this works as claimed (really doubt emission from a cold 12AX7 cathode, receiving only 2.5V instead of 6.3V) and anyway, doubt the "Tubyness" of something used upside down, inverting grid and cathode.
Not forgetting that we do not have power gain, like on *any* active device, but power *loss*
Yes, voltage gain of 3.5 into 50X the load impedance, amounts to power *loss* of 14X.
Even a humble (obviously passive) transformer fares much better: easily capable of 10X voltage gain with only 10% power loss.
I repeat, even if working this is useless in practice, but of course fuels the Marketing Hype machine.

[Merlinb]

Notice the patent doesn't actually explain how it works!

Anyway, the grid is based positively, so it works like the screen grid in a pentode. Some cathode current goes to the anode, and some to the grid. When you vary the anode voltage, the ratio of current splitting is modulated, so you get a signal voltage developed at the grid.

[woodyc]

I'm surprised the grid current characteristics from one tube to the next are consistent enough to permit this.

But hey, if it sounds good and draws little current... what's not to like?

[Jazz P Bass]

It's all about the potential between the cathode & the plate.
Is the grid wire up to the task?

[exclamationmark]

Inverted triodes have been known about since at least the 1940's, though I don't know of any commercial uses for them. Since this particular circuit runs off only a 2.5V heater, they can probably fudge things and omit the cathode resistor. The main advantage of inverted triodes is their reduced output impedance in comparison to a normal gain stage, as well as lower input capacitance. IIRC Steve Bench did some articles on inverted triode operation.

[nickb]

Interesting info Mr "!". You pointed me in the right direction. A quick Google of "Inverted Triodes" brings up this from Radio Museum Inverted-Triode Circuit for measuring of small Currents

Now we know the true nature of the beast. I guess they got the patent based on the low power feature rather than the topology.