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In Reply to: RE: Thats what I don't want... posted by Nickel Core on November 08, 2014 at 11:51:41
Bypassing the choke with a resistor, would remove the peak while maintaining the benefits of having a low DC resistance path to ground.However, I guess AC resistance would suffer, but would still be high (100K @ 4Hz, 200K @ 20Hz and then moving to the value of the bypass resistor when frequency rises). Output impedance of the preceding stage is 25K.
It would never be as high as when not bypassing the choke of course, so in this sense the bypass resistor would defeat the high reactance of the grid choke at higher frequencies.
Hmmm.... not sure what to do. But having the smaller, higher quality coupling cap in there is preferable I think.NC
Edits: 11/08/14Follow Ups:
What is the source of DC that requires the low DC resistance path to ground?
dave
You are the supplier of the custom made interstages and TVC's for my amp many moons ago, remember?
Thank you soo much for that. The end result has beaten all my expectations...
NC
Sure I remember... and I also do grid chokes which have made lots of people happy... yet I still want to see a discussion as to why people like them.
dave
PM me to discuss a proper grid choke for my design. I'm now using the silk unit, but I think it would be better to wind custom one, with the correct amount of inductance.
NC
Is that your amp sounds way better using a grid choke then a grid to ground resistor ;)
However, this is what I learned from Andy Grove on this subject:
The DC resistance of the grid circuit must also be kept low to control the effect of another rather annoying bugbear, grid current. Unfortunately, the vacuum in many modern valves is far from perfect, so there are quite a few gas ions floating around inside the bottle. Some of these ions will collide with the grid and draw electrons from the grid circuit. If the grid resistance is high, the grid bias will be modulated in tune with the signal, a real no-no in my book.
Also the grid may occasionally be driven positive on signal peaks, causing the grid cathode diode to conduct, rectifying the input voltage in the manner of a shunt diode supply with the decoupling cap as the reservoir. This action makes the bias voltage more negative, reducing the quiescent current through the tube, sometimes to the point where it will only conduct on peaks (Class C). In fact, a severe peak can cause the amp to cut off altogether, resulting in a total loss of output.
Worse still, the grid resistor/coupling cap combination as an RC time constant, so the effect lasts for some time after the overload has passed in sort of a time-delay distortion mechanism.
Reducing the grid resistor to combat these effects is no solution. We want a DC grid resistance similar in magnitude to the impedance of the driver valve, i.e. a few hundred ohms, not a few hundred kilohms.
Making your grid resistor 600 ohms will likely kill the driver stage and, anyway, would require a coupling capacitor so big that the RC time constant would put us right back where we started.
To cure the voltage swing problem requires a circuit element which has low DC drop but a high AC impedance. Plus, we need a low DC resistance in the grid of the output valve.
I have no problem with the concept that they sound better. I do have issues with the idea that they help grid current recovery since grid current is an AC phenomena which means that the inductance must also be considered.
dave
Did you every try to *add* a grid choke on top of an existing grid to ground resistor?
If it still sounds better, then grid current recovery is not the issue. It must be the higher reactance then (besides other phenomena I don't know about).
But the higher reactance won't make your frequency response more 'flat' if the response is already 'flat'.
NC
Never tried the series resistance but I do not like the parallel resistance since it sucks some of the life out.
this is an interesting comparison.... one trace is a undamped grid choke and the other is a 100K resistor. Source is a sine that transitions to some music. the output is simply the input signal subtracted from the signal reaching the grid of the 801.
Hi,
I tested the using the parallel resistor, only to find out that it *not* removed the low frequency oscillation :(
I changed back to a 43K series resistor, and again the oscillation was still there :( :(
The oscillation is on one channel only however, so maybe something else its wrong.
I listened quickly between the two options however. Using the series resistor, the presentation was more 'powerful'. There was more 'energy' in the presentation, however there was also a metallic 'glare' which was not present using the parallel resistor.
It was a quick listen only, but the presentation using the parallel resistor it was more natural and fluid, but lest dynamic and powerful.
Before I continue, i first need to get rid of the oscillation. No clue why that is there now...
NC
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
The bias is supplied by a bias supply with a precision adjustment pot with one leg to ground.
So the DC resistance from grid to ground is determined ultimately by the setting of this (50K) pot. At the setting used, I will have a DC resistance of 15K or so.
The supply does not need the low DC resistance, but the grid likes not to be offset by high DC resistances to ground.
Love the 'sound' of grid chokes.
NC
I'll give you that grid chokes can sound good but I want to see a compelling argument "why"
consider what happens to this input waveform....
when coupled via grid choke to a -30V biased 801.
dave
Is this behavior present always with grid chokes I wonder?
when the LC is well tuned well, this amount of ringing seems avoidable.
As to the why of grid chokes. I guess its a combination of:
1 very high impedance at high frequencies
2. high impedance at low frequencies (when enough inductance is available)
3. very low DC resistance which enables the grid to have maximum control, especially important with low mu triodes.
NC
I've always thought grid chokes were a very difficult way to go, an absolute last resort. The engineering challenges relating to both construction and application are enormous.
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