Understanding plate loads

[Carlp]

I'm working on a preamp design that would allow me to switch between two different topologies and I need to switch between plate supplies that provide different voltages (and potentially currents). My supply thus far provides a voltage that is close to the necessary plate supply for the design at the intended current. What I want to know is what kind of plate "load" is needed for a vacuum tube.

Basically, what I want to figure out is what's the purpose of a plate load, and how important is it? If the supply voltage (in my case, after a VR tube or combination of tubes) is close to (or even spot on) the required plate voltage, can I get away with little or no plate load? Does the load perform some function other than helping to regulate the voltage and current (which in my case could, perhaps, be set via the regulating resistor for the VR tubes)? In other words, could I go straight from a 150V supply regulated by an OD3 tube to the plate for the B+?

I've tried to do the research, but I don't have Morgan Jones and haven't found an obvious answer in my web research so far.

Thanks,
Carl

 

[SY]

While you're waiting for your copy of Jones to arrive, do a search for "load lines."

 

[Yvesm]

Mmmh ?

And where will you pick up the signal ?

Yves.

 

[ray_moth]

A plate load allows the varying current through the tube to develop a varying voltage (across the load) for input to the next stage. Without a plate load, the output signal would be shorted to ground (via the power supply smoothing / decoupling capacitors).

Assuming you're thinking about a triode, the plate load needs to be at least 3 x the internal plate resistance. The higher the better, in fact, to maximize gain and to get good linearity. Using a resistor, though, there is a limit because the plate needs a voltage and too high a plate load will drop too much voltage.

There are ways of simulating a very high value of plate load without dropping too many volts - using an 'active' plate load, for example, or a choke.

For the basics, look at the technical articles at this site.

 

[Carlp]

Ray,

Thanks, that makes a TON of sense, now that you explain it. I hadn't considered how the signal varies the DC voltage, especially if there's a VR tube sitting there holding it steady. I'm still new enough to valves that none of this is intuitive to me, and though I've built a few amps with good success (from others' designs), I'm only starting to play around with small mods to designs. I guess it's off to find Jones.

This forum is VERY helpful. Thanks to all of you.

Carl