I replaced the 27K resistor with 270K and got the 34K value for the plate resistor. The sound improved but still has no bass and the vocal is thin, treble improved, but the amplification still low.
DF96 : as you suggested, I connected a 1M resistor to DVM and measure the voltage, the different is only several mV. I think there is no oscillation.
The next step will reduce the source resistor of JFET to 22ohm to increase the current from 4mA to about 6mA. If still not success, then will add grid stopper resistor as suggested by Rod.
DF96 : as you suggested, I connected a 1M resistor to DVM and measure the voltage, the different is only several mV. I think there is no oscillation.
The next step will reduce the source resistor of JFET to 22ohm to increase the current from 4mA to about 6mA. If still not success, then will add grid stopper resistor as suggested by Rod.
Dear All
finally, I should have fixed my preamp. I replaced the source resistor of the JFET to 15ohm and increased the current from 4mA to about 6mA. It sounds much better now although treble is still little sharp and fast, not enough bass, I believe the components need run-in especially the Jensen Copper tube coupling capacitors.
Thanks again for all experts' help.
finally, I should have fixed my preamp. I replaced the source resistor of the JFET to 15ohm and increased the current from 4mA to about 6mA. It sounds much better now although treble is still little sharp and fast, not enough bass, I believe the components need run-in especially the Jensen Copper tube coupling capacitors.
Thanks again for all experts' help.
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