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This is a spawn from the "Don't trust schematics" thread. First schematic is Dave's example.
Dave Slagle clarified how to model a power transformer in LT Spice.
"The inductance is simply the square of the voltage ratio so a 120:240+240 transformer would be a 1:2+2 voltage ratio or a 1:4+4 inductance ratio."
That being said, I modeled a power supply with a little higher inductance using his ratio above which gives me the same results (I believe it has a higher series resistance and poorer regulation?). If I zoom in on ripple voltage and show .7v ripple peak to peak (as an example). That would be less than .5v rms, correct? Can I apply that ratio to the OPT to calculate hum with given ripple (considering the gain also)? I hope I worded that correctly....
BTW, I had to play with the data label expression to correctly show the output voltage as text. Don't understand whats up with that, here's the formula to get the data- "round($*10)/-1". Weird, I normally don't have to do this with this in tube gain stages. Not sure how to express AC volts at the first label that shows 0V.Also, does this inductance ratio apply to OPT's? If so, how would I calculate inductance on a 5000K:8ohm OPT? Do I assume secondary is .1H and calculate from there to model OPT's? Or am I over thinking this and simply call the OPT manufacturer for spec? I know some might like a certain (custom) transformer for a given design.
I think this method of power supply simulation has many advantages over the traditional PSUD program DIY'ers have been using. Although, the PSUD warns you when exceeding the limits of the device. I don't have an engineering background and have learned how to use the program quite well in a few months (about couple hours a week). I would recommend DIY'ers to give it a try, it's also cheaper to blow something up in a SIM ;-)
Edits: 08/11/15 08/11/15 08/11/15Follow Ups:
Hum at the speaker is reduced by the ratio of the output transformer - but what's across the primary? Very little with pentode drive, since the high plate resistance of the pentode is in series. For a triode, the plate resistance is lower than the transformer - ignore it for a rough estimate.
Inductance ratio is same as impedance ratio or the square of voltage ratio. A minor point - winding resistance will make the effective impedance a little higher than the voltage ratio would give - maybe 10%.
Measure primary inductance if you can, pull a number out of the air if you can't... it varies with frequency and signal level anyway. Which means that if you measure primary and secondary, the ratio may be wrong, unless you measure both at the same frequency and corresponding signal levels (say 10V on the primary, 0.5 on the secondary for a 20:1 ratio). Voltage ratio is easy to measure accurately.
Good push-pull transformers might measure more than 300 Henries at 10V, 20 Hz and a good SE transformer might be 10 Henries at rated current, a bit more if measured "dry".
A good SE guitar amp transformer might be 7-10H (assuming a 3-5K primary impedance). A good SE output transformer will achieve 8-10H per 1000 Ohms of primary impedance.
Magnequest, Sowter, and even Hammond have some units that achieve this specification. As others have mentioned, inter-winding capacitance and leakage inductance can get out of control in the process of prioritizing other parameters.
"does this inductance ratio apply to OPT's? If so, how would I calculate inductance on a 5000K:8ohm OPT?"There are two general aspects to this. First, you can derive values that will allow your SPICE simulator to work as though the transformer is perfect. That's how I usually handle it. I don't consider the degree of coupling or exact inductance of the transformer useful for most sims. I simply accept the fact that actual performance of an output amplifier in terms of response and efficiency (output power) will be somewhat less than shown in SPICE. That's OK for my purposes. If SPICE says 20W output, but I only see 17W from the prototype, I accept it for what it is.
The second way to derive these values is from actual transformer data provided by the manufacturer. This is much more complicated, but still not likely to yield real-world results in the simulator. For one thing, as with many characteristics of iron core devices, winding inductance isn't a constant. It varies with frequency and power. In addition, it's unlikely a manufacturer will provide data such as leakage inductance, inter- and intra-winding capacitance, distortion vs power and frequency, etc., etc. Even if you had access to all the manufacturer's data for a given transformer, it's my opinion such things are best measured in terms of the resulting performance in a live circuit, and that SPICE is best used as a guideline, not as an end in itself. Also, the level of precision possible by entering all the data relative to every component still won't produce sufficient accuracy to substitute for live testing and measurement. The process will, however, consume massive amounts of time that could be used instead to build and listen to the amplifier. :) Incidentally, the concept of a near-perfect simulation is much more difficult with tubes than with SS. I do this sort of thing almost daily for circuits using opamps and the like, but it just isn't useful in the same way for tubes.
Getting back to your questions, inductance varies as the square of the turns. So does impedance. Therefore, when you tell SPICE that a particular transformer exhibits 1H:4H, it knows the impedance ratio is also 1:4. It also understands that the impedance ratio is the square of the turns ratio, so that in this example, the transformer's turns ratio (voltage ratio) is 1:2.
As for actual values, you're trying to analyze and optimize your circuit, not the transformer itself. So, I simply pick an inductance value that won't "interfere" with the circuit's operation. The general rule of thumb for this is that the winding must have XL not less than 5 times the operating impedance of the circuit it's across (and 10 times is better, nearly perfect from both theoretical and practical standpoints). Of course, XL varies with frequency, and more XL is required as frequency decreases. So, I would typically assign a value for XL equal to 10 times the operating impedance at 20 Hz. For an output transformer driving an 8 ohm speaker, this means a secondary XL of 80 ohms is required, or 0.637H. If the transformer is 5000:8, the required inductance of the primary will be (5,000 / 8) X 0.637, or 398H. These values will allow you to install a transformer in the sim that will produce approximately the same voltage stepdown as the real one. The simulated transformer will also "stay out of the way" while you're attempting to analyze and adjust other components in the amplifier. Easy peasy. :)
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Edits: 08/11/15 08/11/15 08/11/15 08/11/15 08/11/15 08/11/15
So basically I would always start out with .637h at the secondary as a general rule of thumb (realizing this is rough figure for Sim purposes).
A 6.5K transformer would be 6500:8, (6500/8) X .637=518H. As you said, Spice is a tool used as a good starting point, for me it could help avoid some costly mistakes.
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