I have seen schematics for putting a CCS on the first stage and on the output stage of amplifiers.
My question- for a single-ended design is the greater bang for the buck obtained by putting a CCS on the input stage or the output stage?
I'm thinking current sink, like an LM317 based CCS on a grounded cathode.
My question- for a single-ended design is the greater bang for the buck obtained by putting a CCS on the input stage or the output stage?
I'm thinking current sink, like an LM317 based CCS on a grounded cathode.
Plus anything you can do to make the input stage more linear pays double dividends at the output. I've been very pleased with CCS-loaded/LED-biased SE front ends.
Edit: Since the steaks are still on the grill, depends on circuit of course. LED biasing means no global feedback to the input cathode. Plate-to-plate or output in series with the OPT secondary still work. CCS also doesn't give as high a maximum possible driver plate swing, which may be important depending on output grid swing required (especially with cathode/secondary feedback) and the available B+. Nothing I've measured though has lower distrortion.
Edit: Since the steaks are still on the grill, depends on circuit of course. LED biasing means no global feedback to the input cathode. Plate-to-plate or output in series with the OPT secondary still work. CCS also doesn't give as high a maximum possible driver plate swing, which may be important depending on output grid swing required (especially with cathode/secondary feedback) and the available B+. Nothing I've measured though has lower distrortion.
I built an amp with a CCS plate load for the input tube (5842) AND the output tube (45). It sounds nice but is impractical and grossly ineficient. It needs 500 volts of B+ for 1.5 watts output.
I used that design as the driver for an 845, which is now my 845SE amp.
Since I have experimented with both, I would say "bang for buck" wise the input stage is a far better deal. Plenty of bang for little bucks. Output (plate load) stage CCS's are complicated and expensive.
Many people use CCS's in the cathode of the output tube. This only works right if the output tube remains firmly in class A operation. When you drive the amp hard enough to leave class A, the output tube tries to draw more current, which it can't do. This upsets the bias causing a distortion that may take several seconds to clear up.
I used that design as the driver for an 845, which is now my 845SE amp.
Since I have experimented with both, I would say "bang for buck" wise the input stage is a far better deal. Plenty of bang for little bucks. Output (plate load) stage CCS's are complicated and expensive.
Many people use CCS's in the cathode of the output tube. This only works right if the output tube remains firmly in class A operation. When you drive the amp hard enough to leave class A, the output tube tries to draw more current, which it can't do. This upsets the bias causing a distortion that may take several seconds to clear up.
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