Some vacuum tubes have two model numbers stamped on them. For example, I have a 6LF6/6LX6 tube and I also have a 6LF6/6MH6 tube. I also have 6LF6 tubes and 6LX6 tubes with only one of the model numbers on them. Looking at the datasheets, the 6LX6 is the same as the 6LF6, but the max plate power is only 30W on the 6LX6 versus 40W on the 6LF6. Meanwhile, the 6MH6 has the same specs as the 6LF6 except for the 6MH6 having a 38.5W max plate power and a whopping 500mA max DC cathode current (the 6LF6 has only a max of 400mA csthode current).

If a tube has two model numbers on it, is the specs the maximum of both tube models or the minimum? For example, does a “6LF6/6MH6” stamped tube have a max plate power of 40W and a max cathode current of 500mA, or is it 38.5W and 400mA? My guess is it is the max because the tube has the heat dissipation fins that a tube stamped woth only 6LX6 does not have.
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It's usually the specs of the more powerful (newer) tube. The idea is to make life easier on service techs that didn't bring their substitution book on a house call.

As a rule, you could use the higher-rated version in place of the lower-rated one but not vice versa, as that could risk overstressing it (the lower-rated one).

It's usually the specs of the more powerful (newer) tube. The idea is to make life easier on service techs that didn't bring their substitution book on a house call.

As a rule, you could use the higher-rated version in place of the lower-rated one but not vice versa, as that could risk overstressing it (the lower-rated one).

But that is precisely why I am confused. The 6MH6 is both the newer tube model and it has equal or higher ratings than the 6LF6 for everything EXCEPT for max plate power, where the 6LF6 is rated for 40 watts and the 6MH6 is rated for 38.5 watts. Should I assume that a tube marked with 6LF6/6MH6, the plate power is 40 watts?

If it's labeled for both it should work in both applications...If it didn't warranty claims would have sunk them.

The difference between 38.5 and 40 watts dissipation is irrelevant, besides, no sane designer would design for a tube to operate at its maximum rating. Tube life would be poor.

If it's labeled for both it should work in both applications...If it didn't warranty claims would have sunk them.

This makes the most sense to me.


The difference between 38.5 and 40 watts dissipation is irrelevant, besides, no sane designer would design for a tube to operate at its maximum rating. Tube life would be poor.

Agreed the difference is small, but while operating at the max rating would not be done, "derating" that is operating at some fraction of the max rating is how designing works, so any difference in the max rating means something because the derating fraction changes.

Tube life expectancy depends mostly on how much derating is used for max cathode current and max plate power. Looking at different 6LF6 tubes, it appears as though the 6LF6/6MH6 labeled tubes have the plate design for 40W (example being the heat dissipating fins at the top), and while I can't see the cathodes, I am going to guess that they are the 500mA current rated cathodes, which should make the tube last much much longer than a vanilla 6LF6 that has cathodes rated for 400mA.

I'd also like to remark that 500mA DC cathode current is incredibly impressive! Industrial vacuum tubes for radio and TV broadcast towers top out at 400mA DC cathode current, though they do have much higher plate power ratings than 40W. Non-solid state TVs had some impressive components inside them, and this 6MH6 horizontal sweep tube is a good example.