The experiment was about listening to how an active device overloads in real time from a live mic signal.
Signal path :
Grampian ribbon 35ohms mic - Pleiades 1:30 tranformer with 2.2mH primary - K117 Pleiades stage with 4.7K both drain and source resistor (VB=2.5V) - another ditto Pleiades stage - 1/2 12AU7 Pleiades stage, cathode to ground, Ra=33K, Rg=10M, Vb=12V!, 2/2 12AU7 Pleiades stage, cathode to ground, Ra=10K, Rg=10M, 50K, 436ohms divider - realistic disco mixer mic input - headphone atten pad - Sennheiser HD580
After noticeable distortion the 10MOhms resistor was removed from the final stage and a 3.3M was connected from grid to Anode!!!. Sound was grand! At such conditions anode voltage was 9.7V, Vg was -0.3V when no signal input and Va=10.7V and Vg=-0.7 when singing, inbetween values when speaking.
Output VU meter on disco mixer showed no increase from +3 even after singing quite loudly indicating limiting. Subjectively sound was getting louder and louder are harmonics were gradually being generated by the process.
Then the 12AU7 stage was replaced by the Pleiades triode connected EF183 stage powered by 6V and Rg=3,9M from anode to grid. Vg was measured to be -0.58V.
As this was 1 stage only it seemed to compress more than limit. It needs to be found if vary mu tube do compress more than limit.
A useful further experiment could be to supply the grid through a fairly high resistance and high voltage so as to make a higher release time (grid would discharge the charge it has accumulated when overloading more slowly.
Signal path :
Grampian ribbon 35ohms mic - Pleiades 1:30 tranformer with 2.2mH primary - K117 Pleiades stage with 4.7K both drain and source resistor (VB=2.5V) - another ditto Pleiades stage - 1/2 12AU7 Pleiades stage, cathode to ground, Ra=33K, Rg=10M, Vb=12V!, 2/2 12AU7 Pleiades stage, cathode to ground, Ra=10K, Rg=10M, 50K, 436ohms divider - realistic disco mixer mic input - headphone atten pad - Sennheiser HD580
After noticeable distortion the 10MOhms resistor was removed from the final stage and a 3.3M was connected from grid to Anode!!!. Sound was grand! At such conditions anode voltage was 9.7V, Vg was -0.3V when no signal input and Va=10.7V and Vg=-0.7 when singing, inbetween values when speaking.
Output VU meter on disco mixer showed no increase from +3 even after singing quite loudly indicating limiting. Subjectively sound was getting louder and louder are harmonics were gradually being generated by the process.
Then the 12AU7 stage was replaced by the Pleiades triode connected EF183 stage powered by 6V and Rg=3,9M from anode to grid. Vg was measured to be -0.58V.
As this was 1 stage only it seemed to compress more than limit. It needs to be found if vary mu tube do compress more than limit.
A useful further experiment could be to supply the grid through a fairly high resistance and high voltage so as to make a higher release time (grid would discharge the charge it has accumulated when overloading more slowly.
No comments:
Post a Comment