It is generally accepted that pentodes are on average a good bit less linear as voltage amplifiers than triodes. Today I'm going to show how to take a pentode and make it more linear than any triode, or at least any triode that I have any experience with (if there is one that beats this performance, I wouldn't mind knowing about it).
I've previously written on this blog about highly-linear power stages using KT88s. The approach here will be similar, using voltage feedback that is parallel applied and driven with a p-channel FET, so the overall input impedance of the circuit will be very high and easy to drive. The p-channel FET makes things work out with a minimum of components. There are other variations of this circuit that would be possible with n-channel devices or tube followers/active loads should one desire to keep it all tubes.
Conceptually, this is kind of what I'm attempting to do as far as feedback scheme goes:
And here is a more fleshed-out conceptual drawing of the actual circuit:
What I've done is make a very high gain amplifier by putting an active load on a pentode, then I am using local feedback to reduce my distortion down to minuscule levels. I drive the feedback network from the source of the mosfet in the active load (a low impedance drive point) to keep the feedback network from loading the high-impedance point at the pentode plate. The circuit works very well and is very simple.Feedback-phobia is unwarranted as will be shown in the distortion spectra, which reveals very little distortion and what is there being low-order.
However, before I get too far perhaps I should take a slight detour and explain why I did these experiments with big power pentodes like the EL84 and EL34. I started this experiment trying to find a replacement for the driver stage in my Unity-Coupled amp. It requires 160Vrms to drive the output stage to clipping so it requires a tube capable of idling at 350 or more volts. I also wanted to develop a driver for an SE amp with a follower output stage, which would require a tube that can idle at 500V or more on the plate. EL34 was the cheapest tube I could find that fit the bill. It is overkill but it was the cheapest one. I also had an EL84 laying around so I ran some tests with it. I also had several beam tubes around and tested some of them but the results were not as good as the true pentodes. This makes a bit of sense, since the characteristics of beam tubes get kinky at low plate voltages and low currents. The performance of the beam tubes were still quite good, just not as good as the pentodes.
These concepts could be applied to small signal pentodes as well. I haven't tried it yet, but if/when I get the chance I will.
Here is a simplified version of the practical circuit (I have left out gate protection diodes and gate stoppers):
With these feedback resistors I get a gain of a little over 13. I spent a lot of time optimizing plate voltage and screen voltage and found that the higher the plate idle voltage, the lower the distortion. The opposite was true of screen voltage, the lower the voltage the lower the distortion. Of course, with the screen you can only go so low until you clip at the Vg = 0 line so you have to find that point and back off a little. For the EL34, this was 35V at a 10mA operating point.
I also had a bit of peaking in the frequency response that showed in square wave testing. I solved this with adding 12pF in parallel with the feedback resistor. This also limits bandwidth to 200kHz, which is really plenty. Earlier tests showed the response to be 3dB down at 350kHz. With a 10M90S as the upper device in the active load, there was no peaking in the response but with a 1700V IXYS part, there was peaking and ringing in the square wave before adding the capacitor.
Here are the distortion results I got with the plate set at 600V and 35V on the screen:
10kHz Square Wave
Here are some earlier tests on an EL84 with the plate idle voltage set to 225V and 100V on the screen:
I think that I could have gotten the EL84 distortion down more. I performed the EL84 testing before I discovered that lowering screen voltage reduces distortion. I will revisit that testing when I get a chance.