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In Reply to: RE: The short answer as to way I like them... posted by dave slagle on November 9, 2014 at 07:02:27:
I'm going to try and interpret this (which you have done a dozen times already I guess).I'm just thinking out loud here...
Looking at your former graph, I conclude the blue line represents the result with the undamped grid choke not?
> the output is simply the input signal subtracted from the signal reaching the grid of the 801
So, the green trace still represents the input signal, the blue trace not. My conclusion would be that in the grid choke case 'more' of the signal reaches the grid, since no 'residual' signal is left if input and output are subtracted.
This is on par with the knowledge that when one uses a grid choke the gain is also higher (maximal).
A 100K resistor gives a fixed reactance on all frequencies, while the grid choke gives a much higher inductance (except for the lower frequencies, depending on inductance).
My guess is that if you simulate this using a higher value grid to ground resistor, the green trace signal will be smaller. When I find the time, i will do some simulations too.
When discussing grid-chokes and their low resistance to ground, I guess we are only worried about negative grid current. This should actually not be much of in issue in low mu tubes, and more of an issue in fixed bias cases than for auto bias.
The negative grid current is (mainly) caused by Gas Ionization, Ions hitting the grid will cause NEGATIVE grid current and the grid will take on a negative voltage from this current developing a potential across the grid to ground resistor. It seems clear then that a lower resistance from grid to ground, will cause a smaller voltage (error) to develop.
So, when bypassing a grid choke with a high value bypass resistor, I guess you can have your cake and it too. You maintain a low resistance path to ground, while create a higher reactance at low frequences. This at the loss of some gain, since the reactance of the grid choke will always be higher then the resistor at higher frequencies.
(Actually not so high. Reactance will exceed 350K at only 8Hz using a 7000H grid choke, so basically the resistance as seen by the grid is there as a result of the resistor, and not because of the grid choke. So basically the grid choke does nothing when bypassed, except providing a low DC path to ground).
The high inductance at low frequency (by using a high inductance grid choke), enables one to use a smaller coupling-cap, generally resulting in better results.
Of course the proof of the pudding is in the eating. I will implement the CL coupling of the VT25 as below, and listen to to results.
Edits: 11/10/14
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