We do our testing of preamp triode tubes with the Tube Imp tube tester specifically on their mutual conductance (or transconductance as some say), measured in the unit Gm. Mutual conductance is a common way of measuring, to be it simply, how much life is left in the tube, by measuring the change in plate current across the change in grid voltage. With operating characteristics from tube data widely available, we chart the readings according to what a specific tube type typically shows.
Every single tube type has their own operating characteristics; hence 12AU7’s Gm rating is different from a 12AX7’s Gm. Looking at different readings of Gms across different tube types may be difficult. So to put it simpler for you, we converted the readings into a percentile. It instantly allows the gauge of the tube’s lifespan to be so much easier. A reading of 92% simply means, theoretically, it has still up to 92% of lifespan.
The two percentage readings you see is because a preamp triode tube is actually comprised of 2 individual amplifying sections. So the reading is for one of each.
There are many talks and hearsays on a pair or quads of tubes got to be balanced/matched to sound the best. Matching only applies to the power tubes like EL34, 6L6GC, KT88, 6550 tube types; basically tubes at the output stage of the amp where current draw of the tubes are kept in similar range. Even so, amps with biasing features are totally not reliant on matched tubes to sound their best. Our tube amps like Yaqin MS-110B and Muzishare X5 have their biasing adjustment built into the amps, so unmatched power tubes are never a problem operating in such circumstances. What are the advantages then? Money is saved! Probably too much mark-up is done to tubes being sold just because they are matched. We are not saying that is not important, but if your tube amp has easy bias adjustment, there is certainly no need to spend an extra premium on power tubes being matched which we can only trust some seller to really do the matching.
With matching not applicable to preamp tubes, we then talk about them being balanced. As previously mentioned, a preamp triode tube has 2 amplifying sections in each tube. So a balanced preamp triode tube means the 2 readings within it is very close to each other. So how much of a variation is considered balanced? Well, the good news is this ‘balanced’ attribute of a preamp triode tube is actually not that a big issue in determining the good from the bad. Still, a range of not more than 15% difference between the two readings of a preamp tube is more than fine. And our tubes listed are definitely within that range.
In conclusion, do take precautionary note on getting tubes from sources that advertised their preamp triode tubes are matched, as there is no need for that as far as preamp triode tubes are concerned in audiophile devices. While balanced preamp triode tubes are desirable, if the extra premiums are paid just for them being balanced, the cheaper alternative with some slight variation is probably the better deal.