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Paul S wrote:

Some time ago Romy threw me a fish in the form of two numbers, 280 and 185, meaning mV and V, respectivly, and I finally got around to chowing down.  I was getting annoyed with the sound this morning to the extent that I busted out the DMM, and I found the bias had drifted up considerably.  As I dialed it back I decided to just keep going, and I am glad I did. In my case, lower plate current and/or less power worked a treat.  It cleaned things up considerably and harmonics became better as well, including bass, with no loss in "articulation", placed in quotes because IMO bass "articulation" is not really present without proper harmonics.  In this case, bass was no more harmonically enriched than the rest of the spectrum, with the effect that sections were audible and interesting contributors in complex music.  Another beneift of the change was less HF hash, and in no way did the DEBZs lose power or "drive".  In fact, the net effect of less hash was more music and "drive" at any given volume setting. As I understand it, lowering plate current raises output impedance, so this is another move that "makes no sense", especially since the DEBZs are nominally 8R.  Still, the Lowthers certainly benefited from the change; but so did the big Audax.  The ribbons were the same, except they were dealing with less noise, so in that sense they benefited, too. Now (as usual...) I wonder how far I could go with this, at what point(s) the benefits would reverse.

First of all what do you do not controlling the plate current? Lamm drivers his out stage quite hard and it does require monitoring, or you easily jump to the too high currents with all know negative consequences.

Secondary, I do not believe that dropping the plate current to 250-280mA at the same (or higher) voltage might “cleaned things up considerably”. With lover current you should have in some instances deeper bass (because less power). All other things should not be changed too much. You might be very careful driving the ML2 at sub 35W or with one plate as the amp has global feedback and I detect that with dropping more then 3dB in feedback the amps become unstable, in fact you have no sensitivity in your speakers to go for lover plate power.

You might run across the ML2 taps, finding better harmonics but it is pretty much all the you can do with that amp. If you are not happy with harmonics of your channels with ML2.0 then it is absolutely undeniable evidence to me that you need to replace your speakers and do not try to twist ML2. The ML2 has stunning harmonic integrity, like no other amps I even seen, and they maintain the harmonics across VERY wide range of load. If you report that harmonic balance changes with dropping plate current from 300mA to 280mA then something is VERY wrong ether with your playback or with your perception. It is theoretically possible if you use ML2 for LF section and your bass damping is in “critical mode” but I very doubt that it is what it going on in your case.

Anyhow, you might review your findings… perhaps with other electricity. Do not mystify the output tube operation point. It is VERY simple to find the right operation parameters for your given load. You need to set your output 6C33C in the regime what at full power to your load the current and the voltage are clipping at the same time. Everything else is fantasies. I made countless experiments with that tube and find that with perfect semantic clipping the tube sound absolutely the best, not only this one but any other tube the I have tried.

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