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In Reply to: Power Transformer Model Inductance posted by deafbykhorns on August 11, 2015 at 17:19:39:
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".
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Follow Ups
- RE: Power Transformer Model Inductance - Tom Bavis 08/12/1508:43:08 08/12/15 (1)
- a good SE transformer might be 10 Henries at rated current - Caucasian Blackplate 12:27:22 08/12/15 (0)
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