hornlogo small

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.

6SN7GTB 1) 6SN7GTB 1) 6SN7GTB 2) 6SN7GTB 2) ECC82 ECC82 E80CC E80CC
Cath. Decoupl. n y n y n y n y
Max out Vrms 86 115 86 115 90 - - 115
100V - 9,1 - 8 - 8,6 3,6 5,4
80V 3,4 6,2 2,65 5 4 6,7 1,7 3
50V 2,3 3,7 1,8 3 4 5,4 1,1 1,8
20V 0,88 1,45 0,68 1,1 1,4 1,9 0,4 0,66
10V 0,44 0,73 0,34 0,54 0,65 0,92 0,2 0,32
8V 0,35 0,58 0,27 0,44 0,55 0,74 0,16 0,27
5V 0,22 0,36 0,17 0,28 0,46 0,34 0,10 0,16
2V 0,1 0,15 0,086 0,11 0,15 0,18 0,062 0,07
1V 0,12 N 0,1 0,12 N 0,086 N 0,15 N 0,1 0,1 N 0,065 N
0,5V - 0,14 N - 0,13 N - 0,12 N - 0,1 N
Vb 323V eq eq eq 321 eq - -
Ra 100k eq eq eq eq eq eq eq
Rk 3,9k eq eq eq 2,2k eq eq eq
Gain 9 15,5 9 15,5 10 13 15 25

1) Stock RCA 6SN7   2) Small base RCA

ECC81 6201 3) 5965 5965 ECC85 ECC85 2C51 2C51
Cath. Decoupl. n n n y n y n y
Max out Vrms 110 110 - - - 120 - 115
100V 11 10 6,7 12,5 13 23 7,4 13
80V 5,8 6,7 4,6 8,4 7,7 14,5 4,8 8,8
50V 3,3 4 2,7 4,6 4 8,2 2,7 5
20V 1,25 1,5 1,0 1,8 1,5 3 1,0 1,8
10V 0,62 0,72 0,5 0,88 0,74 1,5 0,5 0,96
8V 0,50 0,60 0,4 0,70 0,6 1,2 0,4 0,78
5V 0,31 0,37 0,25 0,44 0,38 0,8 0,25 0,48
2V 0,14 0,16 0,13 0,18 ,015 0,3 0,11 0,2
1V 0,18 N 0,15 N 0,081N 0,08N 0,1 0,15 0,12 N 0,1 
0,5V - 0,14 N - 0,13 N - 0,1 N - 0,05
Vb 313V eq eq eq 321 eq ~ ~
Ra 100k eq eq eq eq eq eq eq
Rk 1,8k eq 2,12k eq 3,6k eq 4k eq
Gain 22,5 20 18 31 18 36 12 22

3) 6201=E81CC    

6AM4 6AM4 ECC40 ECC40
Cath. Decoupl. n y n y
Max out Vrms - - - -
100V 13 18 5 6,6
80V 7,6 - 2,4 3,5
50V 4 - 1,4 2,1
20V 1,7 2,4 0,53 0,78
10V 0,86 1,2 0,25 0,38
8V 0,68 0,98 0,21 0,30
5V 0,42 0,6 0,13 0,18
2V 0,17 0,24 0,056 0,076
1V 0,09 0,12 0,04 N 0,04
0,5V 0,06 N 0,074 - 0,028
Vb ~320V eq eq eq
Ra 100k eq eq eq
Rk 570 eq 2,2k eq
Gain 14 20 16 25

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.

ECC40 ECC40 E80CC E80CC E88CC E88CC 6SN7GTB 2) 6SNYGTB 2)
Cath. Decoupl. n y n y n y n y
Max out Vrms - - - - - - - -
100V - - - - - 21 - -
80V 15,5 20 18 24 5 8,4 - 13
50V 2,4 4,3 2,35 4,3 2,2 4,4 2,9 4,7
20V 0,77 1,45 0,86 1,6 0,78 1,6 1,1 1,8
10V 0,36 0,7 0,42 0,8 0,38 0,8 0,54 0,9
5V 0,18 0,34 0,23 0,4 0,18 0,4 0,27 0,43
2V 0,074 0,14 0,086 0,16 0,077 0,15 0,11 0,17
1V 0,048 0,072 0,054 0,083 0,056 0,084 0,06 0,084
0,5V 0,064 N 0,065 0,065 N 0,068 0,074 N 0,08 N 0,06 N 0,05 N
Vb ~320V eq eq eq eq eq eq eq
Ra 22k eq eq eq eq eq eq eq
Rk 1k eq eq eq eq eq eq eq
Gain 10 20 10 18 11 23 8 13

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.

Vo rms Tetrode triode triode
50 4,3 4,4 7,2 19 4,2 6,2 2,8 2,15 4,7 6 - 6 5,8
20 1,8 1,2 8,2 1,8 1,6 2,2 1,05 0,61 0,74 2,2 2,8 1,35 1,05
10 0,73 0,52 7,0 0,92 0,78 1,05 0,94 0,29 0,25 1,2 1,4 0,96 0,32
5 0,4 0,27 4,4 0,45 0,38 0,5 0,54 0,15 0,12 0,68 0,74 0,18 0,19
2 0,5 N 0,48 N 2,4 0,18 0,16 0,22 0,25 0,15 N 0,17 N 0,34 0,52 N 0,25 N 0,22 N
Ra 220k eq eq eq 15k eq eq eq 47k eq 100k eq eq
Rk 1k eq eq eq 890 600 500 180 1,5k 1,5k 750 3k 3k
Rg' - 470k - - - - - - - - - - 470k
Rg2 1,2M eq eq - - 68k 51k 100k 100k 220k 1,2M 220k eq
Vb 313 eq eq eq eq eq 330 331 300 307 305 320 320
Cath dec y y y y y y y y y y y y y
Gain 330 260 225 - 14 52 61 68 78 97 180 145 125

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.