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In Reply to: RE: Really? posted by Triode_Kingdom on July 08, 2015 at 11:29:38
With 10k ohms reflected you will need 80Hy to have a inductance reactance of 10k ohms at 20Hz giving you a -3db point of 20Hz and a load line for that frequency that's not optimum.
You would need 160Hy to get 20Hz at -1db.
Even then the load line would not be totally optimum.
Your damping factor, about 3k ohms plate resistance into 10k reflected//the reactance of the inductance at the low frequencies, might not be enough for every speaker.
In the presents of frequencies that cause the load line to become an ellipse, do to the low reactance of the "less than really needed" inductance, all the frequencies follow the elliptical load line and that adds distortion to everything.
How much primary inductance does the FS-100 have?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
In order to calculate the -3dB point you need to take into account the source impedance in parallel with the reflected load so in the case of a 3K source and a 10K load with 80hy of inductance you get -3dB just under 5hz.
This doesn't take into account the ability of the source to deliver current to inductive leg and that needs to be addressed on a case by case situation.
dave
I see your point. My calculation is wrong.
It would only be right if the source impedance was equal to the reflected impedance.
I should have just stayed with my concern with the distortion and the shape of the load line.
When the inductive reactance is equal to the reflected impedance the load line gets bad and the distortion goes up.
If there was an infinite amount of inductance (and no shunt/winding capacitance) the load line would remain that of the reflected impedance and the distortion would stay low and be predictable.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
When the inductive reactance is equal to the reflected impedance the load line gets bad and the distortion goes up.
This premise falls apart when you realize that a plate choke can be considered as a 1:1 transformer with a very high reflected load. Since a plate choke has finite inductance, by your premise we should be introducing a finite load to match the inductance to reduce distortion.
dave
You're misunderstanding me.We want the reactance of the inductance to stay out of the way. We want that reactance to be high when compared to the reflected impedance. Otherwise it messes up the load line.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 07/09/15
I understand what you are saying. So how does a plate choke work with an extremely large reflected impedance? I'll stick with my belief that the inductance needed should be based on the source impedance (tube Rp) and not the reflected load. Sure in a perfect world we would want infinite inductance but in the real world, a 10K transformer designed for a 2A3 needs less inductance than a 10K designed for a 211 and designing a 10K transformer without concern of what will be driving it is just the beginning of that slippery slope of compromise.
dave
"So how does a plate choke work with an extremely large reflected impedance?"
The intended load for the output tube is the reflected impedance of the parafeed output transformer not the reflected impedance of the plate choke, so (since the two are in parallel) the higher the reflected impedance of the plate choke, the better as that will lead to more of the total power getting to the speaker instead of being lost in the plate choke.
We would be better off with a CCS in the plate circuit instead of a choke but as long as the inductance of the plate choke is large enough so that it's reactance at low frequencies stays out of the way of the parafeed output transformer's reflected impedance it will work and not mess up the load line for the output tube too much.
In other words, a plate choke with too little inductance (in a parafeed circuit) leads to the same problem as a series feed output transformer (in a series feed circuit) with too little inductance.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
forget about the parafeed circuit and lets keep it to the simple model.
Assume you need to use a plate choke to drive a 10K load and a 100K load from a 1K source. Does the 100K load require more inductance?
dave
Yes if you want the 1K ohm source to see the same percentage of the midband load at the lower frequencies.
With a 10k load in parallel with the reactance of a plate choke with 80Hy (10k ohms at 20Hz) the tube sees a load of 5k ohms at 20Hz.
The reactance of 80Hy at 1kHz is 500,000ohms. 10k //500k = 9800 ohms
5k ohms is 51% of 9800 ohms.
If we use that same choke for a 100k ohm load the 1kHz load for the tube is 83.333k ohms but the 20Hz load for the tube is 9090 ohms, only 10.9% of 83.333K ohms.
When we have a 100k ohm load, for the 20Hz load impedance presented to the tube to be the same percentage of the midband load impedance, 51%, then we would need there to be 800Hy of inductance in the plate choke.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I think that is a backwards way to approach a design. I prefer to consider the worst case first and then move onward. in this case the load at 20 hz is 5k. if we deem that as acceptable, and we are driving a tube grid, do we then add additional loading in an attempt to keep the load unchanged?
i much prefer the results of the lighter loadat higher frequencies
dave
"i much prefer the results of the lighter load at higher frequencies"
Me too but you've missed my point.
If 20Hz is present in the signal the load for the "higher frequencies" will, in effect, be 5k not 10k (9800).
With the low frequencies filtered out the load for the higher frequencies will be much closer to the value (and shape) of the reflected impedance.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
If 20Hz is present in the signal the load for the "higher frequencies" will, in effect, be 5k not 10k (9800).
I don't see it that way. The higher frequencies will still see the value of the inductance as a load. here are two loadlines of a small signal 1000Hz signal plus a large signal 20Hz signal for a fixed amount of inductance. I do not see the width of the loadline as anywhere near the problem as the overall range of current.
Can you share the asc file pease?
Naz
It is just a twist on a file Stephie Bench did and is attached below.
I am still trying to get my head around what is happening particularly in a dynamic sense.
dave
Naz
Tre', you are correct regarding the shunt inductance of the transformer. The required inductance is a function of the *operating impedance* of the circuit, and that in turn is determined by the impedance reflected back onto the primary from the load. This operating impedance can be represented by △E/△I, and that term is controlled by load Z and turns ratio. It is relatively unaffected by the source Z, which generally has more effect on output power and efficiency than anything else.
--------------------------
Buy Chinese. Bury freedom.
I don't know the FS-100 inductance, but the FS-030 lists at 35 henries for a 3K primary. That's -3dB at 14Hz, and the best bass extension I know of in an airgapped SE transformer.
In the deep bass, parallel feed can maintain bass flatness and keep the phase near zero to a significantly lower frequency than the plate choke alone might suggest. Plus, the parafeed capacitor causes the load impedance seen by the tube to increase below cut-off, instead of decreasing as in series feed. Both effects offer deeper bass at lower distortion, for the same transformer or plate choke inductance.
Nevertheless, I agree with Triode Kingdom that the perceived lack of bass in SETS is often lack of power rather than lack of extension per se.
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