This is from email and a few interesting topics were starting to come up so I brought it here.
All you guys and your Low current High Rp tubes are making me think about a few things differently.I'm looking for some real decent plate chokes to load type 112A, as an input tube. (5k Rp, 8mA Ip)
Given that it is an input tube swinging very low voltages it seemed logical to me to shoot for 80% nickel as a core and I pointed him to these plots for a starting point.
The .016 gap will indeed net you a nice linear inductance. Assuming your 5k source simple math suggests a -1dB point of around 15hz with that particular gap (125hy) you can also interpolate the graphs a bit. For example I would expect a gap of .012 to fall between and give you around 160hy and show the onset of saturation a bit earlier. The one thing often overlooked is the actual AC flux in use plays a large role in what part of the curve you can operate in. For this choke your max of 2V @ 20hz represents an AC flux of only 65 gauss! this around 1% of the Bsat of the material and remember that as voltage goes down and frequency goes up flux decreases rapidly we really are talking about tiny signals here (which is why I suggested 80% nickel)There seems to be a range of current for which the inductance is at least fairly constant, with some of the air gaps. 0.016" with the 80% nickel for example, 8 to 13mA or so
Look at it this way, with a .012 gap increasing the standing DC current 1 ma form 7ma to 8ma will change the DC flux 400 gauss which is almost 7X the largest AC signal the coil could ever see. Some people will claim this to be headroom, others will suggest that the gap could be made much smaller and due to the small AC involved the device will still be linear. Going to the .008 gap would net you 200hy's and even though the line appears to be on the edge of saturation the small ac signal will not even register unless you zoomed in 100 times.
We can theorize about this all day, but when it comes down to it, it's your amp and not mine. So rather than picking an arbitrary value and slapping the device in a can, I encourage you to listen ot all three gaps and push it even more to see what actually works best for you. With this feedback combined with the details of the rest of your system I suspect we will start to see some patterns emerge.
What a fresh diversion from the "grunt grunt, I need a billion henries" world audio has becomeInductance is not so important
Do tell... And post a schematic if you can.I would prefer to go with DCR as a higher priority. I can detail the reasons for this when the time is right, at the forum; LF poles and overall response needed from the amp etc, but I am in no hurry at all, so take your time.
I can give you a few theories and a couple of hunches but I honestly do not know.What are the consequences of an uneccessarily high DCR
The relationship of DCR to the source impedance plays a role on how your LF rolloff occurs. Spice can tell you what it will look like, but who knows how that relates to sound. Interestingly enough DCR is also a barometer of other design choices soapart from voltage drop (non issue).
This is where it gets screwy. A CCS will have a horizontal load line, does that make it lower distortion than a choke? By linearity I mean a device that behaves as expected over the widest range of voltages and frequencies. While I agree that the simple models suggest a higher inductance will make a circuit more linear, they miss the fact that what you do to increase that single parameter makes other parameters suffer. I tend to step back and consider the big picture.By linearity, are you meaning the higher (inductance)
impedance load having a more horizontal load line?
When talking about choke loads, the truth is you start out with an inductive load and slowly transform to capacitive load as frequency goes up. At the point where the inductive and capacitive loads are equal (resonance) the load actually appears as resistive! What a mess. The problem is this "mess" always seems to sound better than anything else I have tried.
Well I see this getting long winded and better end it now... Attached below is the schematic.