I've been thinking about the topic being discussed and the interest in this phenomenon... well... here are several thoughts on the subject...

first thought... we should separate out anode (plate) chokes from grid chokes. As they perform different functions and are used and spec'd differently. the plate choke must ordinarily be designed to accomadate unbal dc plate currents and thus generally requires an air gap. A "grid choke" typically is used with a dc blocking condensor and is designed as an "ac only" device. The magnitude of L that you want from each may also be quite different.

second thought....

the example you provided... quoted below...

:::300pF in parallel with 80pF input capacitance of 300B is 380pF. And with high impedance driver (cascode, pentode, high rp tubes common cathode...), say Rout~15kOhms, we have f-3= 1/(2*3,141*15000*380*10^-12) = 27,9 kHz , limited frequency response of the driver.:::

this is with a very high impedance tube... and generally when you want to drive a 300B grid your going to use a tube with a much, much lower internal plate resistance. Say anything btwn a low of 800 ohms to 5000 ohms or even 7000 ohms would be much more typical (I think)...

with a low impedance driver and a high shunt capcitance the -3db cutoff frequency would then still be much greater than 27.9khz.

as with all devices and circuits... the designer must optimize his/her circuit and use a topology that makes sense and produces good results.

For example... it is much tougher to build a plate choke for a tube with an r sub p of 15K... and generally these tubes are operated at much less plate current.... so they are not even generally spec'd or used to drive the grids of triodes which do have higher internal capacitances... so with triodes most designers aim at using a tube with much lower r sub p and perhaps greater bias currents.

if your going to use a tube with an r sub p of 15K I would *most* (not absolutely though) likely not recommend that you use a plate choke and instead might recommend a plate resistive load instead.

third thought.... the whole issue of shunt capacitance also applies to any transformer that carries an audio signal through it...

push pull output transformers will have shunt capactances across the primary...

single ended output transforemrs will have these same capacities...

so... why the focus on shunt capacitance (in seeming isolation) of a
plate choke or a grid choke?

In most real world apps... shunt capacitance is only one of dozens of parameters and quantities that must be taken into account in achieving a good design...

whenever the focus of any discussion on the merits or demerits of a design boils down to ONE parameter... it's bound to lead to mischief...

the trick in design is not to aim at or focus on optimizing any single parameter... whether it be minimizing shunt capacitance, maximizing inductance, minimizing DCR, or minimizing flux density level, or etc...

good designs are designs that pay attention to a whole range of factors or considerations that go into a design and optimizing for the "whole" of the device and not just any single paramter...

as an addendum to this post I will put up a link to a post written by voltsec which I think you may enjoy reading.

cheers,

msl

I've been thinking about the topic being discussed and the interest in this phenomenon... well... here are several thoughts on the subject...

first thought... we should separate out anode (plate) chokes from grid chokes. As they perform different functions and are used and spec'd differently. the plate choke must ordinarily be designed to accomadate unbal dc plate currents and thus generally requires an air gap. A "grid choke" typically is used with a dc blocking condensor and is designed as an "ac only" device. The magnitude of L that you want from each may also be quite different.

second thought....

the example you provided... quoted below...

:::300pF in parallel with 80pF input capacitance of 300B is 380pF. And with high impedance driver (cascode, pentode, high rp tubes common cathode...), say Rout~15kOhms, we have f-3= 1/(2*3,141*15000*380*10^-12) = 27,9 kHz , limited frequency response of the driver.:::

this is with a very high impedance tube... and generally when you want to drive a 300B grid your going to use a tube with a much, much lower internal plate resistance. Say anything btwn a low of 800 ohms to 5000 ohms or even 7000 ohms would be much more typical (I think)...

with a low impedance driver and a high shunt capcitance the -3db cutoff frequency would then still be much greater than 27.9khz.

as with all devices and circuits... the designer must optimize his/her circuit and use a topology that makes sense and produces good results.

For example... it is much tougher to build a plate choke for a tube with an r sub p of 15K... and generally these tubes are operated at much less plate current.... so they are not even generally spec'd or used to drive the grids of triodes which do have higher internal capacitances... so with triodes most designers aim at using a tube with much lower r sub p and perhaps greater bias currents.

if your going to use a tube with an r sub p of 15K I would *most* (not absolutely though) likely not recommend that you use a plate choke and instead might recommend a plate resistive load instead.

third thought.... the whole issue of shunt capacitance also applies to any transformer that carries an audio signal through it...

push pull output transformers will have shunt capactances across the primary...

single ended output transforemrs will have these same capacities...

so... why the focus on shunt capacitance (in seeming isolation) of a
plate choke or a grid choke?

In most real world apps... shunt capacitance is only one of dozens of parameters and quantities that must be taken into account in achieving a good design...

whenever the focus of any discussion on the merits or demerits of a design boils down to ONE parameter... it's bound to lead to mischief...

the trick in design is not to aim at or focus on optimizing any single parameter... whether it be minimizing shunt capacitance, maximizing inductance, minimizing DCR, or minimizing flux density level, or etc...

good designs are designs that pay attention to a whole range of factors or considerations that go into a design and optimizing for the "whole" of the device and not just any single paramter...

as an addendum to this post I will put up a link to a post written by voltsec which I think you may enjoy reading.

cheers,

msl

I've been thinking about the topic being discussed and the interest in this phenomenon... well... here are several thoughts on the subject...

first thought... we should separate out anode (plate) chokes from grid chokes. As they perform different functions and are used and spec'd differently. the plate choke must ordinarily be designed to accomadate unbal dc plate currents and thus generally requires an air gap. A "grid choke" typically is used with a dc blocking condensor and is designed as an "ac only" device. The magnitude of L that you want from each may also be quite different.

second thought....

the example you provided... quoted below...

:::300pF in parallel with 80pF input capacitance of 300B is 380pF. And with high impedance driver (cascode, pentode, high rp tubes common cathode...), say Rout~15kOhms, we have f-3= 1/(2*3,141*15000*380*10^-12) = 27,9 kHz , limited frequency response of the driver.:::

this is with a very high impedance tube... and generally when you want to drive a 300B grid your going to use a tube with a much, much lower internal plate resistance. Say anything btwn a low of 800 ohms to 5000 ohms or even 7000 ohms would be much more typical (I think)...

with a low impedance driver and a high shunt capcitance the -3db cutoff frequency would then still be much greater than 27.9khz.

as with all devices and circuits... the designer must optimize his/her circuit and use a topology that makes sense and produces good results.

For example... it is much tougher to build a plate choke for a tube with an r sub p of 15K... and generally these tubes are operated at much less plate current.... so they are not even generally spec'd or used to drive the grids of triodes which do have higher internal capacitances... so with triodes most designers aim at using a tube with much lower r sub p and perhaps greater bias currents.

if your going to use a tube with an r sub p of 15K I would *most* (not absolutely though) likely not recommend that you use a plate choke and instead might recommend a plate resistive load instead.

third thought.... the whole issue of shunt capacitance also applies to any transformer that carries an audio signal through it...

push pull output transformers will have shunt capactances across the primary...

single ended output transforemrs will have these same capacities...

so... why the focus on shunt capacitance (in seeming isolation) of a
plate choke or a grid choke?

In most real world apps... shunt capacitance is only one of dozens of parameters and quantities that must be taken into account in achieving a good design...

whenever the focus of any discussion on the merits or demerits of a design boils down to ONE parameter... it's bound to lead to mischief...

the trick in design is not to aim at or focus on optimizing any single parameter... whether it be minimizing shunt capacitance, maximizing inductance, minimizing DCR, or minimizing flux density level, or etc...

good designs are designs that pay attention to a whole range of factors or considerations that go into a design and optimizing for the "whole" of the device and not just any single paramter...

as an addendum to this post I will put up a link to a post written by voltsec which I think you may enjoy reading.

cheers,

msl

gapping the core will tame the increasing L among other things. makes for more turns>>more Cw( same general geometry ) to get the same L at some small signal.

More importantly, what does gapped L sound like compared to not-gapped L?

The measurements from the non-gapped device showed a dramatic increase in L with increasing signal. It makes sense to me that this would not be a good thing.

I'm sure the 'White Paper' writers and supporters will have something silly to say about gapping inductors with no DC in their use profile...:) I got the C-core ones made so I can experiment and those measurements I'll share, including how they sound. These I got wound so as to maximize leakage L, and minimize end-to-end capacitance. I will include some comarisons to other available grid chokes for Cw. To hell with the idea of not publishing that info...:)
cheers,
Douglas

If you have the grid of any stage running to positive values, the sonics are going to suffer for some non-zero recovery period. This period will be longer than the one oscillation, and is one mechanism by which a 22 MHz oscillation can make itself known.

I tried a high gm cascode and found it singing along with 20V peak-peak of 22 MHz oscillation. Resistive dampers cured it, and the circuit souded better. No easy way to be sure you've got things under control save to measure carefully.
cheers,
Douglas

and speaking of high gm, how 'bout a 100,000 mA/V device?