If I'm right do I get a prize?

:=))

smiles,

msl

the reflected load on the primary of an output transformer is the load (say an 8 ohm nominal speaker load) put across the secondary terminals multiplied by the turns ratio squared.

so if you have a turns ratio of 20:1 step down and you put an eight ohm load across the secondary you will reflect back to the primary an impedance of 3200 ohms.

generally when we "spec" or use "specs" for a power trans... we don't use the impedance language but instead talk the "voltage" and "current" language.

so that a 20 to 1 step down trans with say 120 volts across the primary will have 6 volts across the secondary. On power tranneys the moreso relevant and useful "numbers" are voltage and current.

cheers,

msl

in the example provided previously we had at 20 hertz an inductive reactance of 12566.8 ohms. At ten hertz an inductance of 100 henries will have a reactance of only 6283.4 ohms. At five hertz it will again be cut in half and an inductance of 100 henries would have an impedance of only 3141.7 ohms.

so as you go down in freq reactance or impedance produced by a given amont of inductance decreases... as you increase the freq that same amount of inductance will produce greater amounts of impedance or inductive reactance.

cheers,

msl

The general suggestion for avalve with a low grid circuit resistance spec has usually been, "use a choke". How big a choke, in terms of L and DCR has not been discussed or quantified to my satisfaction, for my application.

I am preparing to do some measurment on my custom grid choke capacititance. It is not that difficult, and I am quite suprised it is not a generally published spec. The number is probably embarassing....considering what the general percieved 'good' numbers are!

We'll just have to see! I have a few examples and I'll publish the origin of acquisition as well as manufacturer when I get my own C-core specials from Heyboer to compare to.

Look at the RCA data sheet for the 2A3. Or for that matter the 1619 or the 813. All specify a low value of grid circuit resistance. My theory is that the reason for having low grid circuit impedance is almost the same. Clearly the way in which the grid circuit deals with charge acumulation is important. The AC nature of the charge build up has led me to believe that a high impedance grid choke is going to have the same effect as a big resistor.

It isn't an absolute, but with a few big grid chokes to play with, I was able to collect some data which supported the theory.

So, the recomendation to design for just the inductance the previous stage will stand, along with the grid circuit in question. I am moving up the scale with bigger power valves. For the 813 we'll see how the experiments go. How much inductance they will stand, and which OPTx gets the not for E-Linear modification.

I am going to take better care of my lab notebook that is certain.
cheers,
Douglas

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

There`s a E182CC cascode driver + 300 B SE stage simulation. I substituted 220k grid resistor with grid choke model, consists of (constant) inductance in series with its winding resistance, and with stray shunt capacitance, Cw. The Cw=200pF value I "made up", like not too good example. This is a simplified model, but "good enough" for our simplified considerations :-).
Our driver has some good properties (low Miller capacitance, amplification ~35, good sound ), but unfortunately, has Rout~Ra, or 12kOhms in this example.
The simulated frequency response we can see in lower diagram - high frequency started to fall after 20kHz, and we have LF resonance (~8dB) on 10Hz. For later, if we want to avoid this and have a linear response down to 2Hz, we must use a much larger coupling cap Ci, about 4,7µF.
And for HF - use a grid choke with smaller Cw, or another driver - with lower Rout. If we use, say, common cathode 6C45Pi, our Rout would be much smaller, about 1/10 then cascode, and now HF "falling" problem is gone, but 10Hz "hump" would be even larger - need larger Ci...
Knowing Cw helps a lot in design process - especially with rel. high Rout drivers (cascode, pentode, some triodes).

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