Distortion in small signal tubes

This is a collection of measurements made on some of the tube types I have, done around 2001, when I had a HP distortion analyzer available. I wanted to find the most linear tubes to use in the amplifiers I make. These are THD measurements, but I viewed the output from the distortion analyzer on a scope, and usually the second harmonic dominated. Measurements are made over a wide range of levels, from 1Vrms (or 0.5Vrms) to 100Vrms across the anode resistor. At the lowest levels, measurements are strongly influenced by noise, so don't take those figures too seriously. Everything above 5V should be reliable.

(Note: these measurements may not be all that useful without a listing of the harmonic structure. However, they give you an indication of what tubes are the most linear, even if the fine tuning has to be done with other equipment (or your ears)).

I have used the standard RCA 6SN7GTB as reference, it will show up at the beginning of each table.

The following measurements are made with 100k anode load, so the tube works like a straight voltage amplifier. y(es) or n(o) on the Cath. Decoupl. means exactly that. Voltages are in Vrms, the numbers are THD in %. N means that noise makes the value of THD partially meaningless. Usually, there is only noise left when the fundamental is removed. 'eq' means that the value is the same as in the box to the left. Gain is measured with 1Vrms input.

1) Stock RCA 6SN7   2) Small base RCA

3) 6201=E81CC    

The best tubes seem to be E80CC, E88CC and ECC40. They all have mu in order of 20-40, so it seems that low mu triodes are more linear than high mu ones. But there are exceptions, like ECC82.

The following measurements are made with 22k anode load with the tubes that seemed to be most linear as voltage amplifiers.

In descending order: ECC40, E88CC, E80CC, 6SN7GTB

C3m measurements

C3m is a special quality tube made by Siemens and others (I have a Telefunken, and Philips have them in their data book.) Using pentodes opens very many more possibilities than using triodes.

Some general notes:

This tube seems so far to work best as pure pentode. It can degenerate into a very distorting tetrode (which had mostly 2nd harmonic) or a low mu triode. I haven't explored the full possibility of triode connection, but I think there are many other good triodes that could be used instead.

As pentode, it has distortion figures comparable with E80CC, and much higher gain. (at 5Vrms out, E80CC has, with bypassed cathode, a gain of 25 and 0.16% THD. C3m has a gain of 78 and 0.12% THD).

To get good results, the screen voltages should be about 200V (+/-20% should be OK). The tube also seems to have lower distortion with a load to work into. This hasn't been fully explored, but it is an interesting feature. It also makes me worry less about loading a C3m stage.