The theory behind starved tube plates?

[Pierre]

The more voltage = the more headroom. So if instead of 200V you run the grid of your typical 12AX7 at 9 or 12V... Do you get instant saturation, or simply not enough voltage to allow it to run at its best?

A lot of people mention tube pedals run on batteries or 9/12V supply are a lot marketing, little sense. Why is that? A lot of people seem to go for the starved plate system now.

[nebucanazza]

Re: The theory behind starved tube plates?

I was thinking of posting a similar query, but you got there first
What effect,if any, does a tube running at 12V have when placed in the signal path of a typical op-amp based distortion/OD pedal?
And I guess the tube's not totally useless, because changing the tube to 12AT7/12AU7 reportedly has a noticeable impact in such pedals.
Waiting for some enlightenment from the tube gurus....

[Chaos]

Re: The theory behind starved tube plates?

Ultra basic tube theory:

1.) A tube filament is heated to excite and liberate electrons from the filament’s surface.

2.) A negative voltage is applied to the tube’s cathode, which repels the negatively charged electrons. (Opposite charges attract – similar charges repel)

3.) A positive voltage is applied to the tube’s anode (plate), which attracts the negatively charged electrons. This repulsion / attraction creates current flow in the tube.

4.) A negatively charged grid is placed in close proximity to the filament. Since a negative charge repels electrons, and it’s since the grid is located closer to the filament than the anode, it can overcome the anodes positive attraction with a much smaller negative voltage.

Now by varying the negative voltage on the grid you can control the current flow through the tube like a valve. (Wonder why the Brits call tubes valves?)
If you reduce the controlling voltage on the grid enough, some control over the current flow through the tube will be lost causing over-saturation and loss of fidelity.

If the anode (plate) voltage isn’t high enough (starved plate) the liberated electrons will repel each other to a certain degree, acting much like a control grid themselves. The temperature of the filament will affect the number of electrons which in turn, affects control.

In low powered pedals, a tube is usually used as a simple clipping diode, replacing its silicon brethren. The slow response speed (compared to silicon) makes tube distortion sound different