5881.... my new favorite power tube

[ALBATROS1234]

I bought a used 5881 from a tube dealer which has a broken off key pin. It's a brown base g.e.  boy does this thing sound sweet. The only other power tube I have that sounds close is this slightly used 1950s national union 6v6. I have a couple questions.

First how does an older brown base 5881 compare tonally to 6L6 types. I know it's basically the same tube but ruggedized. Some things I read say they are just a 6L6 built more sturdily others say it sounds noticaably different with ita own unique character. I like it so much I am thinking about getting more and/or maybe other 6L6 types to compare it against.

Next I am wondering about wattage. I know that watts are power used To simplify it the heat given off by the tube ad it uses energy. When calculating amp wattage I used the formula where I take kathode voltage divided by kathode resistors tested value to yield current the subtract kathode voltage from plate voltage and multiply by current to get watts at idle . Well when I was running other tubes in that slot and calculated it's virtually the same wattage but the 5881 was much louder than 6f6,6k6,6v6 so does that mean the 5881 is just a more efficient tube. I get that I was maxing out plate voltage on the 6f6 and 6k6 in this circuit so they had no more to give and if I put more voltage on the plate of the 5881 it with drastically create more power and get closer to max plate dissipation but was just wondering why such drastic volume difference in the same circuit under the same conditions.

Next is it safe to operate it with the missing guide pin long term or is there a fix? Besides taking another octal which is shite and sawing of her guide pin and neatly epoxying it on my 5881 base.

Boy this is a sweet sounding tube.

[sluckey]

https://www.tubedepot.com/products/octal-tube-keyway-replacement

[ALBATROS1234]

Thanks sluckey my man. Looks like a plan needs some 9 pin sockets anyway for this pair of nod rca  6em5s I got a great deal on. Thinking valco clone.

Btw what a difference a day makes today the national union 6v6 had me floored . It has higher gain than the 5881 but they both have such amazing overtones. So silky

[92Volts]

I have an amp, AX84 Lead 2 preamp with single-ended output. A vintage 6CA7 sounds amazing, 6V6s have too much treble, 6L6s have too much bass.

It depends on the circuit. Mine is a "weak" driver for the output tube because signal goes through tone controls and master volume first. So it's sensitive to differences in the output. I've found a tube that's perfect for this amp, but other amps might not show much difference between tubes at all.

Next I am wondering about wattage. I know that watts are power used To simplify it the heat given off by the tube ad it uses energy. When calculating amp wattage I used the formula where I take kathode voltage divided by kathode resistors tested value to yield current the subtract kathode voltage from plate voltage and multiply by current to get watts at idle . Well when I was running other tubes in that slot and calculated it's virtually the same wattage but the 5881 was much louder than 6f6,6k6,6v6 so does that mean the 5881 is just a more efficient tube.

It is more powerful.

Tube power is less-than-ideal because a tube can't draw increasing current as voltage drops towards 0. If the tube doesn't operate down to 0% of available voltage, this means the load/output won't experience 100% of the available voltage.

Valvewizard has a page which goes into more detail: http://www.valvewizard.co.uk/se.html

And on their push-pull page http://www.valvewizard.co.uk/pp.html there is an estimate for power: P = 2 * (HT-50)^2 / Rload

That estimate assumes about 50v is unused, but that page examined EL84 data and found they can do "better" at 35v.

So is it more efficient? Yes, in terms of output power. But the EL84 filament draws more than the similar 6V6, and the 5881 filament draws nearly twice as much as a 6V6. This allows higher maximum current and better performance. So it may work better, but that performance isn't "free"