TUBE BIAS, socket to me!

thanks for providing that understanding, big help just knowing 'why' bias exists.

Is it fair to say, if it sounds great, don't mess with it, or why bother as some suggest?

Thanks and yes, if it ain't broke don't fix it. 

Technically, the bias setting really does make a difference. In practice however it is like an oil change, you will always swear it makes the car run better. 

By the way, the same concept of bias is used in tape recording. The little particles in the layer we want to magnetize to record on, they don't respond very well if all we do is feed them the audio frequency signal. They are kinda slow when asked to change from zero state. But if we excite them with a bias current, in this case 60kHz or higher, this puts them in a state where they respond much better to the music signal. Very similar or analogous to tube bias.  

Mine one time somehow got way out of spec, something like 2 or 3 times what it should be, and so I was anticipating this nice improvement after spending the few minutes to lug out the meter, get down on the floor twiddling and testing to dial them all in nice and perfect. And then... nuttin. Nada. Oh well. At least the car runs better now.

Elliot, I am not quite sure I understand your question, "Sonically, what are your experiences?"  Do I think biasing an output tube makes a difference?  That goes without saying; a power tube cannot operate without some chosen parameters: plate voltage, plate and/or cathode resistor values, grid bias, etc, that determine the electronic envelope in which the tube operates.  For every tube, there is a data sheet, usually available on-line and/or in published tube manuals.  These data show maxima and minima for plate voltage, bias, plate current, etc.  The tube has to be set up so its operation falls within these upper and lower limits.  And taken with the plate curves I mentioned earlier, you can "see" how different choices within that envelope of values will affect performance when the tube is fed an audio (AC) signal that will have a voltage swing.  I use exclusively Atma-sphere OTL power amplifiers, so of course no output transformer, and the power tubes are triodes.  In fact, in 45 years, I have never owned a transformer coupled tube amp, only OTLs driving ESL speakers. With Ralph's help, I built my amplifiers so I can set bias current for each of the output tubes separately.  This allows me to "match" the tubes for the way they are treated by the circuit.  But tubes themselves are unavoidably at least slightly heterogeneous in the way they individually respond to signal voltage.

If you consult with tube gurus, many of them have their own pet theories on where to set bias points for various power tubes, for best SQ and/or for most power, longest life, etc.  (Sometimes the settings for best SQ conflict with maximizing tube life.) By the way, the description of vacuum tube operation in my previous post is for triodes only.  So called because there are 3 nodes, plate, cathode, and grid.  Hence a "tri"-ode, the simplest type of audio tube.  Most power output tubes especially on transformer coupled tube amps, will be tetrodes or pentodes.  The extra nodes are additional grids that can further control the flow of electrons to the anode, but only the one grid receives the signal (with some very rare exceptions; for example some Berning amplifiers drive the screen grid); normally the one or two screen grids do some of the modulating of electron flow by virtue of the voltage supplied to them.  Small signal input tubes and phono and linestage tubes are nearly always triodes.

What are the benefits of proper settings (every 6 months) and what are the potential problems if simply ignored, (as I have done for 47 years)?

If the adjusters are simply set in the middle of their rotation, as I always do, and the thing sounds great, should I risk electrocution?

i would say that if the bias is well set at the start (when new quad of kt88’s are installed) then over time as the tubes age the bias will run a bit low for ideal sound but if you can’t hear a difference, then leave it be... the tubes will become under biased as they age and they will run cooler/easier and actually last marginally longer in use - once again, if you cannot sense a sonic penalty then leave well enough alone, just enjoy the music

middle of rotation on bias pots don’t mean much, as there are different sensitivities in the pots and more importantly different bias needs of the tubes set being driven... so best to have a tech read the correct bias voltage for your tubes set and set properly once, then go from there...

as an audio research tube amp user for years i have become kinda OCD on bias settings as there is much lore about failing power tubes and how bias resistors can be damaged in the amps when a power tube fails - so i am conditioned to check bias conditions fairly frequently (once a quarter, say) - but to be honest i have never let the bias run way off under or over the recommended setting, and so i cannot speak to whether the sonic penalty is noticeable... if anything i set the bias 5-10% below the recommended level to get a little more tube life, and i feel there is no sonic penalty i could hear with various arc amps running 6550/kt88's -- vt50, vt60, vt100, ref 75, ref 110 etc etc...

Well now,

I like the KT88’s more than the 6550’s, difference was obvious, as obvious as switching from Triod mode to preferrable ultra-linear mode.

I assumed all the difference was entirely in the tubes.

Seller told me he had it’s bias adjusted just a few months prior.

Now I’m thinking, it is possible seller had it biased for KT88’s, kept them, put factory 6550’s in, shipped to me.

IOW, I just got lucky, and the bias was part of the improvement.

How about I make myself a long wooden stick/screwdriver instead of plastic. Wood safe?