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In Reply to: RE: But it will matter.. posted by dave slagle on November 13, 2014 at 13:28:41
I thought DC was anything (moving, periodic or whatever) that doesn't reach both sides of the zero voltage line?
The above is all DC. The grid current, even though it's periodic, is DC, no?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
...the waveform you posted is "pure" DC with an AC waveform superimposed on it. IOW, there is a DC component and an AC component.
Yes, and I think Dave's point is the AC component will see the reactance of the inductance and the bias of the output tube will be thrown off and not recover quickly.
So one of the two things that most people think of as being a real plus of having a grid choke just went out the window. :-(
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I guess my test would be how the inductance of a choke would respond to that waveform. If it is pure DC then only the DCR of the choke matters, if it is pure AC then it is the DCR + the inductance that matters.
In the case of a grid choke, I want an example of a situation where the transient signal causes a waveform that only responds to the DCR.
dave
I guess if it is moving and/or periodic then it's going to "see" the reactance of the choke.If it's caused by musical peaks driving the tube to grid current, from say a song at 120BPM with the kick drum and bass guitar pounding on the down beat, then the period of the occurrence of the grid current might be 30Hz.
A grid choke with 7000Hy has a reactance, at 30Hz, of 1.3meg ohms and the bias of the tube will be thrown way off and for a long time.
WRT bias stability a person would be better off with a grid resistor.
BTW Dave, this is a setup just for you. Now run with it. :-)
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 11/14/14
Been trying to discuss it for 5 years... even tried on DIY audio and nobody seemed to care.
dave
and I would like to understand.
It's been said and I've always understood that a grid choke has two advantages over a grid resistor.
1. Higher impedance presented to the driver stage helping to keep the load line for that stage horizontal, lowering the harmonic distortion and increasing the gain.
2. Presenting a low DC resistance to the output tube's grid, lowering the bias offset voltage when the tube is driven to grid current and reducing the time it takes for the bias to re-stabilize.
If the grid current is periodic and thus will "see" the reactance of the grid choke and not it's DCR, that changes everything and I would like to understand the ramifications.
I would think a lot of people here would like to understand the ramifications.
Thanks Dave.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
It is your point 2 that I would like to discuss.... maybe it required its own thread but nobody seems to want to discuss it.
this sums up my stance for the past 5 years or so....
.
Have Fun and Enjoy the Music
"Still Working the Problem"
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