This is in particular reference to the Pleiades BD139 one transistor amplifier with no negative feedback.
A small core transformer completely failed. See previous post.
Fortunately just after graduating and then graduating the army service, the first Pleiades transformer had been build for an MJ2955 one transistor amplifier.
It had been sitting for 20? years in a cupboard. It come out rusty. Dusted off...
Fortunately it had many taps for various impedance values.
The 4Ω (Tesla drive unit) linoleum Pleiades Auratone 5C type cube monitor speaker was connected to the first tap.
The Pleiadss BD139 was firstly connected to this tap too. Sound was normal. Good news. There was no core saturation from DC quiescent current.
Then amp out was connected to 2nd tap. Loudness increased.
Then 3rd tap, loudness increased.
4th tap loudness increased.
5th and last tap, loudness further increased. Is it a max? Not known yet.
Amp was disconnected and the Escort LCR impedance meter was connected. The 4Ω speaker voice coil impedance is being transformed to 25Ω.
Next logical step was to connect the Zeis Ikon DEW 12Ω speaker at the first tap and repeat the listening experiment.
Exactly the same happened at all taps indicating that the output impedance of the amolifier is even higher. Maximum loudness was again at last tap. Impedance was measured 58.5Ω.
This time there was just noticeable difference between tap 4 and 5 indicating a close maximum in power transfer. So is the BD139 output impedance about 60Ω at such current, voltage conditions?
Sound quality is big at almost a laughing point from such a small transistor operating in class A (electrons flowing all the time) at quiescent current of just 70mA and Vb just 12V.
See also previous posts.
See also next post on connecting a transformer rather than autotrasformer.
https://euroelectron.blogspot.com/2019/06/pleiades-bd139-one-transistor-amplifier.html
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