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In Reply to: The Convention Wisdom / Thought Is.... posted by drlowmu on April 28, 2007 at 08:42:15:
Not always. I meter everything and have consistently found that most tubes run 'high and cold' compared to classical op points show more of the least objectionable 2nd harmonic distortion and greatly reduced distortion compnents above the 3rd or 4th. It's almost certainly the reason for sounding less strained. The effect is easy to see on a spectrum analyzer. Within reason a side benefit in output stages is the increased grid bias means greater plate swing and increased output, which can be traded off with a higher turns ratio OPT for any even greater reduction of upper distortion components. Meters are the reason I end up running oddball points like a 6CW5 SE at 260 B+ and ~35ma into a 5k OPT instead of the classical ~150 VDC at 75ma. And yes, along with reducing the DCR in the PS, to my ears it results in terrific sounding amp.
Follow Ups:
> ...a side benefit in output stages is the increased grid bias means greater plate swing and increased outputCan you provide more details regarding your meaning on this point? Increasing the grid bias moves the tube further toward cutoff. That limits the positive-going swing at the anode. More bias = less zero-signal current = less swing = less power.
> ...a side benefit in output stages is the increased grid bias means greater plate swing and increased outputAgree that this is not entirely true. All output stages produce their max power within a range of bias voltages which is dependant on load, HT and topography (lower impedance narrows the range as does higher HT voltage). 0V is generally the limitation for Class A1 but obviously not so for A2. More negative bias will produce greater power in PP amps but only at the expense of driving deeper towards Class B.
Sorry about that, the statement was a little too general. It was in comparison to low and hot, where unless the driver is designed to supply grid current it's usually the limiting factor.
Thermionically addicted.
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