Tube hardness/bias question

[daz]

If you take 2 sets of 6v6 of the same exact brand/model and one set draws 5ma more, will there be a tonal difference if you bias each set the same? If so what would the difference be tonally ?

[stokes]

If you are biasing at +/- 70% it shouldnt make much difference. I often use "matched" power tubes differ by 5ma, i.e. one is 30ma and the other 35ma.

[daz]

I was wondering because i have 2 pairs of 6v6. One pair is matched and the other pair draws a lot more current but are not matched. If i bias the matched pair at 22ma then put the unmatched pair in, one draws 30ma and the other 26 or 27. They sound a lot different even when i put them in and bias them to an average of 22ma like i bias the other pair. and i was wondering if what i am hearing is the mismatch or the fact they are "harder" as suppliers call it when tubes draw a lot. Oh, and both pairs are JJ 6v6S.

[Chuck H]

Helmholtz may step in to correct me, but...

A difference of 30mA and 27mA -isn't a gross mismatch at all. But it may be having an affect on tone. I believe any difference in current between two tubes in a push pull pair will increase second harmonic distortion. And...

If your other pair is drawing 22mA each tube and the mismatched pair is drawing 26 to 30mA per tube (with the same grid voltage? I think this is what you meant?) this would very simply be more current. In most cases more current should translate to slightly sooner breakup and softer attack in overdrive. Slightly more compressed tone. This could also affect the EQ a little relative to performance dynamics. Conversely the lower current and more tightly matched pair may have a lower level of second harmonic distortion, break up a little later and a tighter "feel". Perhaps better/cleaner bass performance.

When you say "if you bias each set the same?" it could mean that current was measured and the the bias was adjusted for the same current from either set of tubes OR that the bias setting in the amp remains constant and the tubes, and their current characteristics, are the only variable. I think you mean the latter since you stipulated the current for either pair. One matched and drawing 22mA per tube and the other mismatched and drawing 26/30mA per tube respectively.

[daz]

What i meant is i bias the low pair at 22ma. When i insert the hotter pair the meter reads about 26 and 30 so i then bias them to where the low one is about 20 and the high one about 24 so they average 22ma as i keep the other pair at. Thats where i am comparing the sound, with the hot pair at 20 and 24. Lets just say for the sake of argument the hotter pair were matched and after pulling the colder pair and to insert the hotter pair they draw 30 each. Now i set the bias to 22. Would they sound different than the colder pair biased to 22? Thats the actual answer i need because that will tell me if what i hear if the mismatch or the hotter tubes. In any case i hear the hotter pair as sounding more broken up and in a sort of slightly rattier way, and this is at low home volume. My feeling is it's the mismatch and not the hotter tubes but then i wanna know if there should e any difference between low drawing tubes and the opposite when each is biased to the same MA.

[uneumann]

Lets just say for the sake of argument the hotter pair were matched and after pulling the colder pair and to insert the hotter pair they draw 30 each. Now i set the bias to 22. Would they sound different than the colder pair biased to 22? Thats the actual answer i need because that will tell me if what i hear if the mismatch or the hotter tubes. In any case i hear the hotter pair as sounding more broken up and in a sort of slightly rattier way, and this is at low home volume. My feeling is it's the mismatch and not the hotter tubes but then i wanna know if there should e any difference between low drawing tubes and the opposite when each is biased to the same MA.

Thanks - question clarity helps... so if both pair are matched and healthy ideal tubes, the main difference is that the hot pair needs a larger bias voltage (assuming fixed bias). A larger bias means slightly more headroom before the output stage starts drawing grid current and breaking up. That's making the (ideal) assumption that all other factors are the same between the hot and cold pair. That answers your theoretical question. But it's unlikely to be the rule and consistent in practice. Tubes vary in many way from bottle to bottle. The bias differences you see are only one factor. AC parameters (gm, rp, mu) can also vary and they will impact the results too. In practice there is no simple answer to this question. The only answer you can count on is that one pair of tubes will likely sound/feel different from another in some slight way. How much different and whether you find the change better or worse is a dice roll.

All of that ignores the preamp and PI in a given amp. They can influence the results too. Say the PI is slightly distorting when it pushes the cold tubes to grid current and clipping. The hot tubes take a bigger signal so the PI will distort even more before the output stage does. Again, there is no simple answer. All the parts interact so your experience on your amp may be difficult to explain without a lot more measurement, testing, and analysis.

[daz]

Thanks - question clarity helps... so if both pair are matched and healthy ideal tubes, the main difference is that the hot pair needs a larger bias voltage (assuming fixed bias). A larger bias means slightly more headroom before the output stage starts drawing grid current and breaking up. That's making the (ideal) assumption that all other factors are the same between the hot and cold pair. That answers your theoretical question. But it's unlikely to be the rule and consistent in practice. Tubes vary in many way from bottle to bottle. The bias differences you see are only one factor. AC parameters (gm, rp, mu) can also vary and they will impact the results too. In practice there is no simple answer to this question. The only answer you can count on is that one pair of tubes will likely sound/feel different from another in some slight way. How much different and whether you find the change better or worse is a dice roll.

All of that ignores the preamp and PI in a given amp. They can influence the results too. Say the PI is slightly distorting when it pushes the cold tubes to grid current and clipping. The hot tubes take a bigger signal so the PI will distort even more before the output stage does. Again, there is no simple answer. All the parts interact so your experience on your amp may be difficult to explain without a lot more measurement, testing, and analysis.

Thanks. Even more reason to believe what i was haring is the mismatch.

[kg]

They sound a lot different even when i put them in and bias them to an average of 22ma like i bias the other pair. and i was wondering if what i am hearing is the mismatch or the fact they are "harder" as suppliers call it when tubes draw a lot.

in historical parlance a "harder" tube is one with a more perfect vacuum (they literally pulled a harder vacuum before sealing the envelope). this has a direct effect on some merits of the tube such as g1 ionization current and ultimate longevity. a good hard tube can last 50 years+ of use easily. the cost of pulling vacuum goes up exponentially with hardness.

a "hotter" tube would be one that pulls more cathode current for a given operating point. if rebiased to the same current they are capable of greater g1 swing before clipping, and if the rest of the circuit is up to the task can generate greater output power vs cooler tubes.

idk how suppliers are using the terms these days, but that was the way they used to be defined!