some tubes will bias cold and some will bias medium, some hot. I just ask for the ones that will bias hot.
I don't get this about tubes...you mean to say there are tubes that will bias cold, medium or hot? Or it has something to do with the amp??? Got confused here.
- Well, we already know that the tolerances of power tubes vary widely, even with tubes from the same batch. Given the same plate voltage and screen grid current, one tube can pass a larger cathode current than the next. This means that one tube is biasing hotter than the next.
- Then, let us look at guitar tube amplifiers. The most common examples can be divided into two groups: adjustable bias and fixed bias. For amps with adjustable bias, there is commonly only one bias adjust potentiometer/trimmer to adjust the bias for all power tubes, i.e. you cannot bias the tubes individually. Fixed bias, well, as the term suggest: the bias is fixed at a certain point, you cannot adjust the bias at all.
O.K., so, for the reasons stated above, tube suppliers like Groove Tubes, etc., 'match' each sets of tubes. This means that, say, in a set of 4 power tubes, all four power tubes will bias closely with each other. Meaning, if you adjust the bias and measure the cathode current of one tube at 35mA, you can rest assured that all four tubes in your matched set will measure a cathode current of 35mA (or relatively close within tolerance). Now, let us say that you replaced one tube out of the matched set with another tube that is not matched with your set, the unknown tube could bias at 25mA (colder) or at 45mA (hotter) and not at the 35mA that the other tubes are biased at.
So now we know why we have to 'match' a set of tubes. Let us look at fixed bias amps. If we took a matched set of four power tubes, we know that all four tubes will bias the same. But, what is it biasing at? This is the reason why tube companies like Groove Tubes (again), Mesa Boogie, etc., grade their tubes in addition to matching them. GT for example, has 10 grades, subdivided into three: Blue (hot), White (medium), and Red (cold). This means that each set of tubes will bias the same in a fixed bias amp, but a Blue set of tubes will bias hotter than a Red set of tubes.
If your amp has adjustable bias, you don't really need to worry about whether a set of tubes will bias hot, medium or cold. Regardless of which matched set you get, you can adjust the bias to your preference anyway. This is more for fixed bias amp owners to consider. Like me, my amp has fixed bias, so when it comes replacement time I'd just ask for a matched set that bias hot.
