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Is there a method to solving ratio and inductance with only impedance data?
Say its 20K:600 transformer gaped for about 10ma
What would one use in spice for inductance and/or ratio
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But if you know impedance AND -3 dB low frequency point. you can calculate inductance using the formula Z=2piFL. At -3dB frequency, Z is equal to transformer's primary impedance.
Determining the inductance accurately from the ratio is pretty much impossible. I simply use a value that's high enough not to adversely affect the circuit. For example, let's say I want to simulate a 5k:8 ohm output transformer. I use the rule of thumb that inductive reactance must be at least 5X the operating impedance of the circuit at the lowest frequency of interest. In this case, that means the 8 ohm secondary needs to exhibit 40 ohms inductive reactance at approximately 20 Hz. That's roughly 0.32H. The pri/sec impedance ratio is 625, so the primary will be 0.32 X 625, or 200H. This produces an ideal model with no losses. It's not the most accurate, but it gets SPICE working without the inductive reactance intruding on circuit operation.
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Buy Chinese. Bury freedom.
I guess for modeling purposes, just go high and trust the transformer winder will do a good job of winding something that works with your project?
Most winders only want the impedance and current, so I assume they work out the inductance and frequency response. My GM70 8K transformer is only 65H which replaced a Hammond 6.5K at 55H, in this case I think bigger is better (more inductance) since its about 25% larger and has about 20% more inductance.
I'm trying to wrap my head around this to figure out if an 01A tube would work with a Line transformer. If I need a impedance of 15K:600, that's a 25:1 ratio, a tube with a MU of 8 isn't going to work well unless another gain stage is added. A large plate choke with about 150H is required to get 20hz @ -1db. It sounds like it may be nearly impossible to get 150H on this setup and not have more than a 6:1 ratio.
It's all about compromise. Best thing is to simulate it with values that you think are achievable. Compare that to ideal values (XL of 5X or 10X the operating impedance) to see whether the degradation is acceptable.
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Buy Chinese. Bury freedom.
You get to the same number by starting with the reflected impedance at the primary.5 times 5k is 25k and the reactance of 200Hy at 20Hz is 25k ohms.
To me this brings up the question of how we can ever have a series feed SE OPT without the inductive reactance intruding on circuit operation?
I know of no 5K SE OPT with 200Hy of primary inductance.
My work around is to never use a SE amplifier full range and, through the use of a high pass filter, never let the output tube and OPT "see" low frequencies.
My SE amplifier uses a 200Hz high pass filter at it's input.
My 5k SE OPT with 30Hy of primary inductance has a inductance reactance at 200Hz of 37.6k. That is 7.5 times the reflected impedance.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 10/08/17
200H and 5K indeed is difficult/impossible to obtain via series feed.
If you allow for parallel feed, you can have 100H and a 2.5K transformer being driven by a 2A3. I don't see much available to get over 50H for a higher current plate choke intended for a 300B, but 50H and a 3K transformer works nicely.
Every component intrudes on circuit operation. Tubes, resistors, capacitors, inductors, etc. We just have to do the best we can with what we have. Considering the sonic excellence achievable with tubes, I'd say we're doing a pretty good job despite the real-world shortcomings of individual components. :)
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Buy Chinese. Bury freedom.
I agree but when the load line at low frequencies becomes a beach ball because of the lack of primary inductance it's not a pretty picture.
"It does not matter if your speaker does not go down to 31.25 Hz or not. If you are sending low frequency information to the amplifier, the amplifier will make the plate voltage move. If the plate voltage moves, the tube must spend bias current to charge and discharge the primary inductance as well as the actual primary load impedance. If the plate voltage or current "clips" because of low frequency plate excursions, the sound will suffer. "
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
What is the formula used to calculate .32H from Freq and impedance.
L = R / (2pi x F), where F is the lowest frequency of interest and R is the value of inductive reactance needed to avoid degrading response.
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Buy Chinese. Bury freedom.
At a high level, the turns ratio is sqrt of impedance ratio. A more complete model needs to include winding resistance, leakage inductance etc. If a sample is present some of this can be measured but the "etc." part may be difficult.
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