On Sun, 27 May 2012 04:10:04 +0000, TerryG
wrote:
flipper;957307 Wrote:
On Thu, 24 May 2012 01:19:25 +0000, TerryG
wrote:
-
[image:
http://tinyurl.com/bnnoyv5
This chart explains a few things. Not of g2 specifically, but of
distortion behaviors of Pentodes. An unbypasses resistor on g2 could
essentially be helping by lower reflected impedance to the plate, so a
little is good, but too much isn't.-
An unbypassed screen resistor might lower plate impedance, and lord
knows what else, but it isn't going to do a blessed thing to OPT
impedance.
-
This could be why small value resistors (relatively) make things sound
better and larger values do not.
Terry-
Why restate the obvious, can not anyone understand what I was inferring?
I get *now* what you were 'inferring', or what you're 'now' inferring,
but can only go by what you said and, in that post, you were talking
about affecting "reflected impedance to the plate," which it can't.
My point was that there is a relationship between plate impedance, OPT
impedance, and distortion type and amount. Therefore by effecting a
change in plate impedance with an unbypassed G2 resistor, we are
essentially controlling where this relationship lands on the chart I
provided for any tube.
Possibly but I think this perhaps explains what may be contradictory
results rather than being necessarily 'practical' because speaker load
is anything but constant.
What I'm suggesting is there may be more than one thing interacting in
the results. For example, let us imagine that, despite having nothing
to support the opinion, Patrick is correct and Rg2 feedback is
'non-linear' (g1 is also 'non-linear' or else we'd never have
distortion to begin with), which he then apparently presumes would
result in 'more' distortion rather than less. It's possible that at
some serendipitous point the reduction in distortion from a 'magic'
load line intersection might swamp that increase, but maybe only at
that one point under specific conditions. Now, if that were the case
you'd likely be better off to simply change the load, for the supposed
'lower distortion' intersect, without also adding the Rg2 distortion.
Except, of course, if it's only at 'one point', with the rest worse
off, it might not be of any 'real world' benefit since, as mentioned,
a speaker is anything but a constant load.
Just between you and me let's get one thing clear, since nothing is
ever clear with Patrick, I never said his opinion was 'wrong'. What I
said was he has little or nothing to back it up and that it's absurd
to open with "I always wondered" and then, in the next breath, start
making declarations of alleged 'fact' about what he "always wondered."
Therefore aside from the assumption that G2 produces feedback (linear or
not), it also can lower or increase distortions depending on the
relationship the tubes plate impedance has with the OPT load impedance
according to the chart for the 6Y6. I have a feeling every tube has a
similar plot like the one in the 6Y6 chart, I would really like to make
my own charts of current production pentode power tubes.
Given the parameters of the 'experiment' in which you readjust so
screen volts and idle current are returned to 'normal', lower plate
impedance would be the result of negative feedback so it isn't an
"aside" thing.
Every pentode will have a 'similar' set of curves because it's
characteristic of the 'pentode shape', so to speak, and, in
particular, the plate knee.
If you look at the vertical on the plate curves you see the plate
lines 'compress' the further down from 0 grid you go. That is 2nd
harmonic distortion. Now, if you look to the left at the knee you see
the upper end becomes 'compressed', relative to the vertical case. At
some 'magic point' knee compression will match the bottom end
compression and, so, we have '0' 2nd harmonic distortion, since the
peak excursions match, but that means increased higher order
distortion because the plate lines now go from 'compressed' to
(partially) expanded and back to 'compressed'. It 'balloons in the
middle', so to speak.
Go even further down the knee with your load line, past the 'magic
point', and we get 2nd harmonic distortion again but the phase is now
reversed because the 'top' is compressed more than the 'bottom',
rather than the other way around in the vertical case, and the higher
order harmonics keep increasing because we've retained the 'balloon
middle'.
You know this thread is typical of what happens often on the internet in
technical forums. There are egos and issolated facts presented in each
post. A lot of good information and I want to thank everyone that has
contributed so far, I have gotten some information from almost every
post. But there isn't much here that seems to be presented in the
spirit of genuine interest in helping explain what is happening with a
pentode with an unbypassed resistor on G2. Personally I feel several
things are happening all at the same time, stopper, affecting of plate
impedance, perhaps feedback, and a change in the total Gm and mu of the
circuit
We will see how this progresses further.
Terry
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