Is the max current spec in power tube data sheet RMS or peak?

[Alan0354]

I am looking at EL34 that specified max current I_p = 150mA. Is this RMS or peak current? the reason I asked is a lot of the guitar amp use +B>440V with grounded cathode. That is full 440V across the tube. Assume the lowest voltage when swinging negative is 100V. So the Vp=340V. In Marshall and other amps, the primary of the OPT from anode to anode is about 3.4K to 4K. Let's take the lowest current and use 4K primary. So the load seen by each tube is 4K/4=1K. If you swing 340V down through a 1K resistor, the peak current is 340mA!!! That is way above 150mA spec. Even if you calculate RMS of 340mA, it's 0.7 X340mA=238mA.

In order for the tube to survive, it must mean 150mA RMS. Then you can look at the complete cycle. The current described above is only the negative half cycle. The current is lower or even cut off in the +ve half cycle. So if you average the whole cycle, it's going to be below 150mA.

Am I correct?

Thanks

[es345]

From my understanding its the average cathode current, which fits to my measurements with that tube in PP configs. My (simplified) design aid estimation excel results are in the same ballpark based on average calculation.

Edit typo: changed from anode to cathode

[J M Fahey]

As always, "the devil is in the details"
If you are considering plate dissipation, it's average current , because the plate has a certain thermal inertia and will average dissipation along a certain time.

There's also the max current you can safely pull from that cathode.
Notice that higher power tubes generally have higher cathode current/dissipation.

Without dissecting them, I can guess that higher current means larger cathode surface, which in due time requires higher heating power.

Besides a "single number" which is written in the datasheet and which I guess is a suggested safe, long life parameter, we have a more detailed picture in the datasheet itself.

There we have all the combinations, including some which allow much higher currents, which has been routinely used (and abused) by Guitar amp designers to pull out the last watt possible.

That some of them abuse tubes, does not seem to be a big concern.

FWIW our friend Stan already commented that in Russia he serviced tube amps used by commercial Musicians (think party/wedding/restaurant) with the original tubes, still working perfectly, 20/30 years old.

When he measured them, none used "Fendery" 430/450V supplies, doubly so in the screens but, say, "by the book" 360V on plates, 300V something on screens, pulling, say, 25W from a pair of 6L6 .... working within very conservative limits yielded very long life.

Just like with Humans

Back to tubes: you want to know exact current you can pull? ... go look at the datasheet curves, don't overthink a generic reference single number.

[Alan0354]

I guess I can look at the track record of Marshall that use over 450V quiescent voltage, so the peak current is over 340mA. So I guess I can use that as guide line. Then use power dissipation to see the power limit.

[Gregg]

450V is considered "normal" voltage for 40-50W class AB guitar amps. However it's not uncommon to see 500-520V at idle in some amps and at high volumes (and when amp is overdriven) then current limit is easily exceeded.

[jazbo8]

I guess I can look at the track record of Marshall that use over 450V quiescent voltage, so the peak current is over 340mA. So I guess I can use that as guide line. Then use power dissipation to see the power limit.

Randall Aiken has already shown that to operate in such condition results in a short tube life, hence the legend of the "tube eater".