I get a decent overdrive out of a ValveCaster, just like this one : http://www.beavisaudio.com/projects/ValveCaster/
but with a NoS russian tube. Usually, the signal straight from a guitar (without booster) cannot overdrive a single tube stage unless it's badly biased. But that first stage can overdrive a second stage. The ValveCaster uses both triodes of a preamp tube for that reason, and a 12V power supply.
That 12V is convenient for the 12A?7 tubes that can have series heaters. Mine doesn't, the heaters are always in parallel, so i've added a 7806 regulator and its heatsink to provide 6V, 300mA to them.
One fundamental difference between transistors and tube is the voltage they need to work well. One can also say that tubes are high-impedance devices, whereas transistors are low-(output)-impedance devices. Let's have a look at the basic properties of each device :
- Bipolar Junction Transistor : the current at the base commands the current going from collector to emitter.
- Junction Field Effect Transistor : the voltage at the gate commands the current going from drain to source. That voltage sort of "pinches" the channel where the current flows.
- Metal-Oxyde-Semiconductor Field Effect Transistor : the voltage at the gate commands the current going from drain to source. The gate has a big input impedance (causing sensibility to electrostatic discharge), but some capacitance also. The same "pinching" effect occurs, but within a capacitor.
- Triode: the voltage at the grid commands the current going from plate to cathode. The grid has a huge input impedance, because it's basically alone in vacuum. The electrons that jump from the heated cathode are attracted by the high-positive-voltage plate. The grid can reject the electrons (when negative) or attract them (when positive, but that's usually not recommanded).
So what happens when a tube is used with a low anode voltage, aka "starved plate" ? The electron cloud around the cathode is not much attracted by the anode, because it's far away (a couple of millimeters) and the electric field decreases with the square of the distance. The tube will have a very low gain, if any. It may need also to be driven into positive grid, to accelerate the electrons towards the plate : this requires providing substancial current into the grid, which passive guitar pickups cannot do.
The trend in "cloning" tube circuits with JFETs is cool, but it's a bit of over-simplification. It doesn't take the Miller capacitance of the tube into accound, the drain trimpot is not the best way to bias, etc. Just look at these ROG article for more details (and acknoledgement that pin-for-pin replacement is not faithful enough) : http://www.runoffgroove.com/fetzervalve.htmlhttp://www.runoffgroove.com/sd.html
But the most important thing is often overlooked : a ~300V plate voltage is translated into a ~9V drain voltage. When doing the opposite, shouldn't you also scale to a high voltage ?
A starved plate design, how convenient and safe it is, does not react the same way.
On the other hand, there were "space charge
tubes" designed especially for that mode of operation. I suppose that's the case of most of the submini, "hearing aid" tubes used in Frequency Central designs.
trad3mark wrote:Do i just use the same circuit but put 18V into the fillament and 9v into the same places it was (via a resistor into the plate i think it is?)
That's the best way to fry your tube ! It's the other way around : you'll want the heaters to get 6.3 V (in parallel) or 12.6V (in series), and the plate to get the highest voltage you can get (up to 350V).
trad3mark wrote:I'm basically looking for an easy conversion from using a FET in a SHO to using a triode (half a 12AX7).
Basically, you won't get any boost from a single starved plate triode alone. You'll need either a higher voltage, a second triode gain stage, or some transistors/opamps to do the actual amplification.
I recommand reading this : http://www.freewebs.com/valvewizard1/gainstage.html