Ian Bell wrote:
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.
Of course?
* Duncan Amplfication Generic Triode Model (Spice 3F4
Implementation)
* Copyright (C)1997-2002 Duncan Amplfication
* Unauthorised Commercial use prohibited
* Please refer to documentation at
http://www.duncanamps.com
..SUBCKT NH6SN7GTB A G K
* ANODE MODEL
BLIM LI 0 V=(URAMP(V(A)-V(K))^ 1 )* 0.0037
BGG GG 0 V=V(G)-V(K)- 0
BRP1 RP1 0 V=URAMP(-V(GG)* 0.02 )
BRP2 RP2 0 V=V(RP1)-URAMP(V(RP1)-0.999)
BRPF RP 0 V=(1-V(RP2)^ 2 )+URAMP(V(GG))* 0.002
BGR GR 0 V=URAMP(V(GG))-URAMP(-(V(GG)*(1+V(GG)*
0.006167 )))
BEM EM 0 V=URAMP(V(A)-V(K)+V(GR)* 19.2642 )
BEP EP 0 V=(V(EM)^ 1.4 )*V(RP)* 0.0000189
BEL1 EL1 0 V=URAMP(V(EP))
BEL EL 0 V=V(EL1)-URAMP(V(EL1)-V(LI))
BLD LD 0 V=URAMP(V(EP)-V(LI))
BAK A K I=V(EL)
* GRID MODEL
BGF GF 0 V=(URAMP(V(G)-V(K)- 0 )^1.5)* 0.000213
BG G K I=V(GF)+V(LD)
* CAPS
CAK A K 0.0000000000007
CGK G K 0.0000000000024
CGA G A 0.0000000000039
..ENDS
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.
Thanks, Ian. Nice to see actual measurements of real
circuits.
I wonder if you measured at any intermediate operating
points, in between the two you mention? Just wondering if
the change in distortion is a trend or a blip.
Are you able to vary the HT voltage, to explore other parts
of the safe operating area? Some idea of a trend would be
good there too.
How does simulated total distortion compare with your real
measurements?
cheers, and happy new year, Ian