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I added a 100uf cap across the lower triode section cathode resistor of 6SN7 beta follower to DC 2A3 finals.
Ran FFT spectrum w/1K sine wave input at 1W output.
There was no significant change in harmonic profile or THD%/THD+N% on the output of the amp with 8 ohm/50W resistor loading.
Ran 20-20KHz bandwidth sweep with FFT and nothing significant changed there either.
I guess a cathode bypass cap on the lower triode in not needed in this circuit.
The Morgan Jones book p.126 shows cheap red led's instead of a cathode resistor.
I'll try the LED's and see if anything changes.
Follow Ups:
If the lower triode in the beta follower is operated constant current (and from what I can tell, it is) then there would be no AC component of the current in the cathode circuit and it would not need to be bypassed.One easy way to prove this is a gain test.
If the gain of the circuit does not increase with a bypass cap in place then the bypass cap is not needed.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 04/06/14
The output level on the graph did not change for the given input signal level, so the gain did not change either.
Then I would say there's no need to bypass the cathode.The cathode bypass capacitor is intended to prevent local current feedback.
Local current feedback is degenerative. If the gain stays the same, bypass cap or no bypass cap, then that tells me that there is no feedback present which proves that the stage is running constant current.
If the stage is running constant current then a cathode resistor for bias is just as good as LED's or whatever else because all the current is direct current.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 04/06/14
There was a reference to using a depletion mode JFET as a CCS to replace the BJT and it related components in the book.
But the distortion performance of doing this was supposed to be worse.
The only thing left to try is another BJT with greater hfe.
I used a 2N2907A with a hfe = 180 instead of the BC558.
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