At your risk. All lifesaving safety precautions must be taken.
Continued from WE417A grid at space potential post.
This time it is time to connect grid through a high megohm resistor to either cathode or anode. Latter case is more strong pull up bias to be only used for low voltage operation. In all cases grid should still be negative with respect to cathode since cathode is already positive by missing electrons to electron cloud.
When connecting 10MΩ from grid to cathode
Anode current is about 10mA at 80V anode potential.
When connecting 10MΩ from anode to grid (Pleiades bias)
Anode current 10mA at 50V anode potential
Anode current 20mA at 70V anode potential
10mA at just 50V is not bad. 500mW plate dissipation. 250mW maximum output in class A operation. The BD139 transistor see Pleiades BD139 one transistor power amplifier sounds huge at such output to load by matching speaker impedance.
Listening tests:
Pull up bias to anode sounded more distorted than pull up bias to cathode. Contrary to low noise Pleiades microphone booster pre amplifiers which operate at about 3.7V anode potential. The sound is amazing with a nice gain. See Pleiades V6. It can't seem to operate if grid return resistor is connected to cathode at such low plate potential.
So 10MΩ from grid to cathode worked with very nice sound.
Cathode is connected to ground.
Signal path, setup:
Carlos Jobim CD Verve - Sony Walkman line out bridged to mono - Pleiadss WE417A - Pleiades linoleum Auratone 5C type 4Ω speaker with Tesla drive unit
There is a 1:3 microphone transformer at (electron tube input) capacitor coupled to grid
A 5KΩ to 4Ω speaker military output transformer, see previous post.
There is a series wire wound 22KΩ wire wound potentiometer plus a 10KΩ power resistor to adjust anode potential.
At about 10mA 100V sound was nice and clean. More mA at more Volts did not make any difference in sensitivity. Even with volume control at max there was no distortion. Octal base Sotwer 1:3 transformer will be replaced by an octal base Sowter 1:7 in order to hear maximum output power from electron tube.
2 surprises today.
1 when volume control is at max anode voltage rises from 80V to 100V. This indicates overload compression due to grid rectification effect of input signal. Bias self changes anode current decreases so anode voltage increases. Yet no audible distortion can be heard as this automatic gain adjustment like electron tube compressors at music recording production. Anode current stays surprisingly the same. This must be due to the high series power resistor before the supply decoupling capacitor. This power supply is a constant current source doe tomthenhigh series resistor. Very handy. No matter how the volume control is increased there is no distortion in sound. Only change in objective dynamics. Wow purely accidental. There is a nice release time constant of a few seconds. Like FM transmitter processing before the exiter.
Is this trick taking place also on legendary Neumann U47 microphone internal VF14 electron tube circuit?
2nd surprise is that if the grid reistor is disconnected from cathode ie open grid or grid at space potential operation, sound quality, gain, sensitivity, anode voltage or current are unchanged. Very handy too. Just an electron tube, 2 signal transformers and a coupling capacitor to grid.
Later addition: The grid to cathode 10MΩ resistor discharges slowly the grid to undo gain reduction when loud music passage is over. See next day's post:
https://euroelectron.blogspot.com/2019/06/comparing-we417a-open-grid-to-pull-up.html
Continued from WE417A grid at space potential post.
This time it is time to connect grid through a high megohm resistor to either cathode or anode. Latter case is more strong pull up bias to be only used for low voltage operation. In all cases grid should still be negative with respect to cathode since cathode is already positive by missing electrons to electron cloud.
When connecting 10MΩ from grid to cathode
Anode current is about 10mA at 80V anode potential.
When connecting 10MΩ from anode to grid (Pleiades bias)
Anode current 10mA at 50V anode potential
Anode current 20mA at 70V anode potential
10mA at just 50V is not bad. 500mW plate dissipation. 250mW maximum output in class A operation. The BD139 transistor see Pleiades BD139 one transistor power amplifier sounds huge at such output to load by matching speaker impedance.
Listening tests:
Pull up bias to anode sounded more distorted than pull up bias to cathode. Contrary to low noise Pleiades microphone booster pre amplifiers which operate at about 3.7V anode potential. The sound is amazing with a nice gain. See Pleiades V6. It can't seem to operate if grid return resistor is connected to cathode at such low plate potential.
So 10MΩ from grid to cathode worked with very nice sound.
Cathode is connected to ground.
Signal path, setup:
Carlos Jobim CD Verve - Sony Walkman line out bridged to mono - Pleiadss WE417A - Pleiades linoleum Auratone 5C type 4Ω speaker with Tesla drive unit
There is a 1:3 microphone transformer at (electron tube input) capacitor coupled to grid
A 5KΩ to 4Ω speaker military output transformer, see previous post.
There is a series wire wound 22KΩ wire wound potentiometer plus a 10KΩ power resistor to adjust anode potential.
At about 10mA 100V sound was nice and clean. More mA at more Volts did not make any difference in sensitivity. Even with volume control at max there was no distortion. Octal base Sotwer 1:3 transformer will be replaced by an octal base Sowter 1:7 in order to hear maximum output power from electron tube.
2 surprises today.
1 when volume control is at max anode voltage rises from 80V to 100V. This indicates overload compression due to grid rectification effect of input signal. Bias self changes anode current decreases so anode voltage increases. Yet no audible distortion can be heard as this automatic gain adjustment like electron tube compressors at music recording production. Anode current stays surprisingly the same. This must be due to the high series power resistor before the supply decoupling capacitor. This power supply is a constant current source doe tomthenhigh series resistor. Very handy. No matter how the volume control is increased there is no distortion in sound. Only change in objective dynamics. Wow purely accidental. There is a nice release time constant of a few seconds. Like FM transmitter processing before the exiter.
Is this trick taking place also on legendary Neumann U47 microphone internal VF14 electron tube circuit?
2nd surprise is that if the grid reistor is disconnected from cathode ie open grid or grid at space potential operation, sound quality, gain, sensitivity, anode voltage or current are unchanged. Very handy too. Just an electron tube, 2 signal transformers and a coupling capacitor to grid.
Later addition: The grid to cathode 10MΩ resistor discharges slowly the grid to undo gain reduction when loud music passage is over. See next day's post:
https://euroelectron.blogspot.com/2019/06/comparing-we417a-open-grid-to-pull-up.html
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