Ian Iveson wrote:
Ian Bell wrote
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?
Ah, well, perhaps not. I have tried very many models and
all but one fail to accurately model the normal operating
region where the grid is negative wrt the cathode. The one
that does model the normal operating region accurately
(probably because it is based on The Audio Designers Tube
Register curves which are the measured curves of actual
tubes) does not include a grid model. Maybe I can make a
hybrid and nick the grid model from the Duncan Amps
models.
That would probably be OK but watch out for "LD" that links
the grid model to the anode model. You may need to rename
it, or add it's derivation into your anode model.
Thanks for the tip. I'll watch out for that.
I say probably because in reality grid current has a
relationship to anode current, probably, because I guess it
decreases perveance (less free electrons available so anode
current should fall such that total cathode current remains
the same, all other things being equal). But it should be a
negligible error where grid current is small.
Yes, and re-reading RDH4 on grid current there seems to be several
contributory factors and I doubt the spice model includes them all.
* 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.
Yes I did, I just gave a sample in the summary. I measured
several tubes in 1V increments from 1V to 10V rms and then
in 5V increments thereafter. As expected the distortion is
very close to being directly proportional to signal level
as expected until the onset of grid current when it rises
steeply. Only the lower level measurements are suspect as
they are approximately double the distortion of the
oscillator itself.
Right, I got that bit, and I can see why you expected what
you got. What I was wondering, though, was whether you had
tried a range of bias currents in between the two you
mention, to establish the relationship between standing
current and distortion for the same signal. I got the
impression that you were asserting that lower standing
current is related to lower distortion, but it's not very
clear...perhaps I'm missing something. The question is
slightly ambiguous, because of variation in mu and therefore
in gain for different standing currents, but that shouldn't
be so great as to prevent reasonable comparison. There is
also the problem of ensuring that the bottom anode remains
set at half the supply voltage. Two points aren't enough to
establish a trend. I ask partly because it seemed to be the
question that you set out with: why did MJ bias at such a
level?
What I did do was:
1. Try a lot of tubes at 8mA and -3.4V (bottom triode) and 8mA 1.3V (top
triode) and got consistent results between tube types and also got
distortion ruoghly proportional to output level.
2. Tried a few tubes at 8mA and 1.3V both top and bottom triodes and got
lower distortion at lower levels but grid current at higher levels. This
presumably means that a lower bias voltage means lower distortion at a
given current though it is by no means conclusive.
3,. Following Patrick's comment I did a quick test yesterday with both
triodes biassed at 5mA and -5V (actually turned out to be 4.55mA and
-4.55V). I got exactly the same distortion figures at 2V and 20V rms as
in the original experiment (1 above). Frankly I expected the distortion
to go up at the lower current but it did not - this is however one test
on one tube only.
I realise that standing current is also related to maximum
signal capability, but still wonder if I want to amplify a
small signal, where a low standing current would be
sufficient to stay far enough clear of grid current, will I
always get greater distortion by raising the standing
current? Or have I got it the wrong way round?
I am not sure but I would have expected distortion to decrease as
standing current increases simply because you move away from the region
where the curves bunch. And if you look at the spacing between curves
they seem to me to be less evenly spaced as the bias voltage gets more
negative which does explain why test #2 gave lower distortion
Whichever, I'm still wondering about sweet spots.
Me too. Test 3 seems to show that nearly halving the standing current
makes no difference to distortion and leads to a bias voltage a long way
from grid current problems.
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.
Not at the moment. 320V is as high as I can get with HT
and that's just high enough for the circuit used. I
suspect if I could raise the HT I could get even lower
figures. However I was regularly achieving 0.4% THD at
20Vrms which is at -48dB relative to the fundamental. For
most tubes, Morgan Jones only achieved -50dB second
harmonic with a pentode CCS plate load at 19.5V rms so I
do not expect there is much improvement to be gained by
raising the HT. Personally, I was surprised I got
distortion figures so close to Jones' with such a simple
circuit which speaks volumes for the inherent linearity of
the 6SN7 family.
At present both tube halves sit comfortably within the
SOAR. The bottom tube has a plate/cathode voltage around
140V which at 8mA is less than 1.2W dissipation and the
top one has about 100V across it which is 800mW both of
which are well within the SOAR of the 6CG7. I would be
more concerned about exceeding heater/cathode voltages
than SOAR. At present the heaters are raised to +75V (6CG7
has max +Ve dc of 100) adn the top cathode is at about
220V (6CG7 has max -ve dc of 200V which would allow
cathode to go to 275V) so there's no much margin on either
one.
How does simulated total distortion compare with your
real measurements?
As I mentioned above, one model reproduces the distortion
levels very accurately except for grid current. All the
others I have tried are way out.
This comes as a pretty big surprise. Do you mean that you
have measured simulated distortion and compared it to your
real measurements?
Yes, the spice model based on the The Audio Designers Tube Register
curves gives THD results that are extremely close to the measurements.
This makes a sweep of distortion level v
standing current even more interesting.
Yes, that might be easier to achieve in a simulation than on the bench.
Could you post a copy of your model, please? AFAIK, the
generic DM model underlying the 6SN7 I posted is the most
accurate and sophisticated available, with one possible
exception that you probably couldn't use on your simulator.
Considering the DM triode comes with a grid model that had
been omitted from previous, inferior triode models, and that
DM is the originator of nearly all available models and I
would guess *all* in common use, I surmise that *all* of the
improvement in the model you have used is due to the more
appropriate data used to generate it.
That would be my conclusion too.
It is therefore not unlikely that fitting the most recent DM
model to that data would result in a model more accurate
than the one you have, particularly in the BRH quarter of
the SOAR, IIRC.
Possibly. The model I have apparently used quite a sophisticated curve
fitting model so I am not sure why the DM model should be inherently better.
Which is why I would appreciate a copy of the model text. If
it is similar to the DM generic model, it may be possible to
transfer common parameters, and leave any extra ones as they
are. If not, then the data the model was fitted to would be
useful, although a complete refit is a tedious business.
No problem and I just realised I told a big fib. The model is apparently
taken from the GE data sheet not the The Audio Designers Tube Register
curves as I stated earlier - I wonder where I got that from?? - must
check that out. Anyway, here is the 6SN7 portion of the file:
*6SN7 LTSpice model from GE 6SN7 datasheet
..subckt 6sn7 P G K
Bp P K
I=(0.02003791851m)*uramp(V(P,K)*ln(1.0+(-0.07740549711)+exp((4.618036737)+(4.618036737)*((2 0.85288965)+(-110.4389272m)*V(G,K))*V(G,K)/sqrt((28.13407639)**2+(V(P,K)-(7.118597372))**2)))/(4.618036737))**(1.380047579)
Cgp G P 4.0pF
Cgk G K 2.6pF
Cpk P K 0.7pF
..ends 6sn7
Note the preamble to the set of models is as follows:
*Generated by Joel Tunnah using Curve Captor v0.9.1
*UPDATED 02/07/06
*Contents:
*6SN7
*12AX7
*12AU7
*12AT7
*ECC88
*6C4Pi
Cheers
Ian
Ian