Inductance of various output transformers

Inductance is very important as it dictates together with driving impedance the low frequency cutoff.


It is not that the transformer cannot pass the low frequency.


It is not that the electron tube cannot reproduce the low frequency. Electron tubes like transistors go down to 0Hz. Descrete active components have tremendous bandwidth. It is surrounding components including parasitic components such as stray for example enclosure capacitance that create restriction of bandwidth. [Storey].


The low frequency voltage never arrives at the transformer primary because it is dropped at the output impedance of the electron tube. R and L form a potential divider. We know that the magnitude of the impedance of an inductance L decreases with frequency. The lower the frequency the lower the impedance the more of a short circuit the electron tube sees, the more the voltage drop at the terminals of the electron tube or any other source due to its internal impedance (conservation of energy).


For reproduction we can aim for such inductance that frequency response starts from say 20Hz or 10Hz. The higher the output impedance of source the higher the inductance must be.


For production we may aim for an inductance, winding resistance (R,L) such that for example when connecting a microphone to such input transformer flat frequency response results from producer's brain to listener's brain. Otherwise severe bass heaviness can result due to many reasons including psychoacoustic ones [Lowe, Morgan]. See also Pleiades (R,L) filters.


While preparing Pleiades lab for making a single ended WE417 low power amplifier in class A operation (electrons flowing all the time) some transformers had been measured with the Escort digital LCR meter.


Inductance of some output transformers:


Sowter SK05S, 13.4H to 22mH


Hammond 1626, 14H : 11mH


Saratoga Collins 9.8H : 6.2mH (1H for 600Ω winding) see other posts


Secondary of output transformer of Bouyer ST3 amplifier:
6mH, 12mH, 22mH for 4Ω, 8Ω, 15Ω respectively.


2 interstage transformers:


Sowter 9040s (no air gap by sample mistake?) 200H : 10H


Sowter 8423S, 95H : 18H








References:


Electronics - Neil Storey


Sound Picture Recording and Reproducing Characteristcs - D. P. Lowe, K. F. Morgan - Journal of the Society of Motion Picture Engineers


See also:
Magnetic Circuits and Transformers - Staff of MIT (Massachusetts Institute of Technology)