I've noticed a lot of audiophile tube schematics with no grid stop resistor in the circuit.
Do they know what its really used for?
Why do they omit this important piece?
Do they know what its really used for?
Why do they omit this important piece?
The little graph says it all. They would prefer that graph to remain as flat as possible, as long as possible. If Rg<<Zsource it won’t move much, but the fact that it moves in at all is more than some people can stomach.
But do you really use anything over a 100Khz in the signal? That is where that low pass is. I think it has to do with people not knowing what kind of resistor it supposed to be. Because most resistors are not suitable.
In most cases, they simply are not necessary.
Very important in our full of RF noise world, for both tube and solid state circuits.
You may get by without it for a while, but things change. And it should have low inductance.
All amplifiers are inherently low pass, it's just a question of where the bandwidth limit is.
Because they fear a resistor will "screw" the sound. They get obsession and paranoia from additional components, that could be omitted. It is called audiophile "minimalism".
Simple as that. 😎
Simple as that. 😎
The picture in the first post surely does not show what a grid stopper is for...
Yes, in general they are $#1+, but even metal film is supposedly “too inductive” for a proper RF grid stopper. I’ve never had trouble with metal film g1 stoppers, but I suppose one of these days I’ll get burned. Like the person who gets burned not using one at all. CC’s just aren’t common anymore and I don’t keep very many on hand. And I use IF strip tubes in voltage gain stages, even with upwards of 30 dB of GNFB. I’ve already run into situations where without g1 stoppers it oscillates, but a cheap resistor fixes it.
It’s in a position where it won’t inject “much” noise at audio. Or even doubling in value being “much” of a problem. Hell, I use MOX films for g2 stoppers which aren’t all that stable either, but they serve a different purpose there.
Many metal film resistors are designed with non-inductive construction, like some wire wound resistors are.
For example the Vishay PR02-FS Non-Inductive Metal Film Resistor.
https://www.vishay.com/docs/48179/_pr02-fs_vmn_pt0514_1702.pdf
Some have used ferrite beads on the grid instead of resistors, since they appear resistive at RF.
For example the Vishay PR02-FS Non-Inductive Metal Film Resistor.
https://www.vishay.com/docs/48179/_pr02-fs_vmn_pt0514_1702.pdf
Some have used ferrite beads on the grid instead of resistors, since they appear resistive at RF.
I use Mills MRA5 or MRA12. They are non inductive wire wound made with silver alloy wire and does not have the typical insertion loss metal film have.
Just an option, as I haven't tried them.
But it's best to use a physically small resistor for this purpose, due to lower inductance.
In the neighborhood of 1/4" to 1/2" long, so probably more like 1/4W to 1/2W rating.
Of course if there's gonna be much grid current, the power rating may have to go up.
You want to keep that sensitive node where the grid terminal and the resistor connect together
as compact as possible, and that includes the resistor body.
Over sizing results in lower thermal noise.
Another reason why we build it that way in point to point like in old radios is the mechanical lead strength as well.
These are non inductive and the leads are non-magnetic. A lot of metal film resistors are not suitable for tube applications because their leads are magnetic under voltage. They also stick to the magnet. Non inductive wire wounds don't.
Also, once you use one as the cathode resistor, you probably wouldn't go back to metal films once you hear the night and day difference. Really elevates the tube circuit to the next level.
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I would take a few different views. CC resistors are bulk devices and well suited to grid stopper use. They pass no current, so have no excess noise contribution, and size does not effect thermal noise. Hoping that Marcel will comment on the significant part of the circuit inductance, inside the valve.
All good fortune,
Chris
All good fortune,
Chris
Here's the horse's mouth, Miller himself, in 1919 discussing instability in the grid circuit.
The added external grid impedance should make the total input impedance positive.
The added external grid impedance should make the total input impedance positive.