At your risk. All safety life saving precautions should be strictly followed. Any voltage or current can be dangerous.
Continued from previous posts.
Instead of the military 5KΩ : 4Ω transformer a Sowter SK05 2.5KΩ to 8Ω was used.
The sound was louder and better.
With the Zeis Ikon typ:31-06 12Ω speaker instead of the Pleiades linoleum Auratone 5C type speaker with Tesla 4Ω speaker, the sound was even louder and possibly better.
A 10MΩ rssitor is connected from grid to cathode on these tests. Signal fed to grid though a coupling capacitor. Operating conditions were from 20mA to 35mA, anode potential of the order of 140V.
Very nice sound in Carlos Jobim CD but not as good on Ray of Light - Madonna. Bass sounded as dining out of steam. It may be due to the grid rectification compressing effect.
The standard practice of using a cathode resistor for bias might be tried too.
The one transistor Pleiadss BD149 power amplifier with output transformer from 60Ω to low voice coil impedance sounds much louder and of great sound quality. Much simpler or of much less complexity. No input transformer used as in the case of the electron tube. Operating point 70mA, 12V. Less power consumption. Later today's addition. Listening back to the BD139 amplifier, the sound is louder but it is more harsh. Electron tube sounds more smooth. Why? Is one reason that the transistor amplifier is currently using a power transformer core for the output trasformer? On very low component count amplifiers the slightest change make a difference.
Both of these one amplifying device amplifiers are operating in class A (electrons flowing all the time).
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