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Would I better off with 2 series HLMP-6000 or just single one to replace the 431? any sonic degradation using series led vs single?...
As a reminder, from my earlier post (edited now to fix a typo):
1) replace the 431 with a resistor of 500 to 690 ohms (metal film preferred). This may reduce the high treble a dB or two unless it is bypassed - damaging the sonics again. I would not drop below 499 ohms. The 45 will not have this problem; its Miller capacitance is half that of the 2A3. This is similar to the SEX amp circuit.
2) replace the 431 with an HLMP-6000 LED. I'm not entirely comfortable with such a low bias voltage, due to the risk of
risk grid current on transient peaks - though we did use it in the Paramour II with satisfactory results.
I'll add 3) replace the 431 with two HLMP-6000s in series. Two LEDs risks the opposite problem, running out of compliance at high voltages as the 12AT7 ages. They will be 3dB noisier that a single LED but that is probably not audible relative to the 12AT7's intrinsic noise.
The optimum voltage remains 2.5 volts, as I designed it.
As to the best choice, try them and see. It's neither expensive nor difficult! Personally I'd use the resistor with a 45 - simpler, more reliable, and closer to the optimum voltage. The LED(s) will be quieter, but I think the margin will be insignificant - a small fraction of a decibel. But that's just me, and I haven't done the experiment.
