[Ian Bell[_2_]]

I have just about completed my distortion tests of 6SN7, 7N7 and 6CG7 mu
followers using tubes kindly loaned or donated by group members (Thanks
Peter and Matthew). Here is a summary of the results.

Basic set up is similar to Morgan Jones with 8mA standing current, the
bottom triode biassed to about Vg=-3V and the top pentode CCS replaced
by a normal mu follower CF set to Vg=-1.3V, with 10K between the tubes
and a 320V supply and the output via a 0.1uF into 100K load.

I tested distortion at a variety of levels at 200Hz, 2KHz and 20KHz. As
the distortion was identical at all three frequencies the majority of
tests were done only at 2KHz.

I tested 4x6CG7 (Matsu****a), 2x7N7 (Sylvania) and 8x6SN7 (RCA) at
output voltages between 1 and 50V rms.

All the tubes produced remarkably similar results. The variation between
types was not greater than the variation within a type although the best
tube of all was a 6SN7. Typical THD readings we

2V rms 0.04%
10V rms 0.2%
20V rms 0.4%
50V rms 1.0%

At 50V rms we are pretty close to grid current but since the oscillator
used has a low output impedance no grid current distortion was observed.

A subsequent test feeding the oscillator via a 120K series resistor
showed no sign of grid current up to 25V rms output (as far as the
oscillator output would go).

I was curious why Jones chose to bias his test rig at Vg=-3.4Volts but
with tubes with mu about 20 and with 20Vrms output you need to bias
several volts -ve to be sure to be away from grid current. This does not
mean that this bias point produces the lowest distortion. Simulation
showed biassing the lower triode also at about 1.3V should give even
lower distortion but of course the simulation does not simulate grid
current. So I re-biassed the bottom tube to 1.3V and check the
distortion. With a 120K in series with the oscillator, grid current
distortion began at 6Vrms output. Below that the distortion was
exceptionally good. At 5V rms is was just 0.04%. Distortion at 2V rms
was hard to measure as it expect it approaches the limit of both my
distortion test set and the oscillator distortion. I would estimate it
to be no more than 0.02% at 2Vrms.

Cheers

Ian