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11-05-2018, 04:06 PM
Hello,
I was wondering about grid-leak biased input stages we sometimes see in older designs. What are the con's and pro's (if any) of such arrangement? I have this Supro Coronado schematic and channel one tube is biased like this: https://el34world.com/charts/Schematics/..._1690t.pdf
First of all it looks as if the channel was maybe designed for a microphone originally , but apparently the amp was not marketed for it. On the other hand a rather high level of gain is called for here and this way of biasing the tube saves the cost of one resistor and one electrolytic cathode bypass cap which otherwise would have had to be put in there.
I thought it was built this way to introduce some grid distortion as the whole amp seems intended for distorted sound and output signals from either channel easily drive the splitter grid to conductivity. I scoped the input stage expecting to see half wave clipping but there is none. I used a 1kHz 128mV (peak to peak) sine and got a clean output wave 50 times the input signal (6,4V peak to peak). With the 6,8M grid leak the bias voltage is -650mV with the plate sitting at 77V. I had to increase the signal voltage to almost 2 volts to clip the grid.
Looks like the triode could've been easily biased using a cathode resistor with a bypass cap and do exactly the same job.
Was there any particular reason why the first input stage was built this way that I'm unable to see. I'm aware this is most likely another case of bad amp design, I'm just interested in hearing some background about it.
Tomislaw
I was wondering about grid-leak biased input stages we sometimes see in older designs. What are the con's and pro's (if any) of such arrangement? I have this Supro Coronado schematic and channel one tube is biased like this: https://el34world.com/charts/Schematics/..._1690t.pdf
First of all it looks as if the channel was maybe designed for a microphone originally , but apparently the amp was not marketed for it. On the other hand a rather high level of gain is called for here and this way of biasing the tube saves the cost of one resistor and one electrolytic cathode bypass cap which otherwise would have had to be put in there.
I thought it was built this way to introduce some grid distortion as the whole amp seems intended for distorted sound and output signals from either channel easily drive the splitter grid to conductivity. I scoped the input stage expecting to see half wave clipping but there is none. I used a 1kHz 128mV (peak to peak) sine and got a clean output wave 50 times the input signal (6,4V peak to peak). With the 6,8M grid leak the bias voltage is -650mV with the plate sitting at 77V. I had to increase the signal voltage to almost 2 volts to clip the grid.
Looks like the triode could've been easily biased using a cathode resistor with a bypass cap and do exactly the same job.
Was there any particular reason why the first input stage was built this way that I'm unable to see. I'm aware this is most likely another case of bad amp design, I'm just interested in hearing some background about it.
Tomislaw
