will I get less wattage for a simple sub of 5881?

[Doug H]

My project plexi/bassman style amp is 2 el34s running at 385v into a 4.2k OT. I'm guessing one could rate the output of the power section in the low 30s, maybe even around 30 watts.

Once I get sic of the EH EL34s, I want to move to more of a 6l6 style PT, probably a JJ 6ca7, I'd have to dig up the notes I made to be sure, but I also spent some time looking at and am intrigued by 5881s.

I know I probably won't be able to "tell" specifically, but will just swapping in a pair of 5881s knock a few more watts off just due to the nature of the tube itself? I see a lot of amps with low output running these tubes. Or they are just better suited to lower voltage b+?

[Haze13]

From a LOUDNESS point of view, you will not hear a difference.
Tubes works this way: They say Give me the voltage in the input and I (The tube) will amplify it! Some tubes need different voltage to get the SAME output!
Example: 2a3 power triode needs 40Vp-p to give 2-3 watts, El84 pentode only 9Vp-p. These of course are very different tubes but that's what is going on in reality, only in guitar world we are using Pentodes only (What a shame).
The voltage that a power tube needs is given by a Phase Inverter and you don't change his gain (May be just a little bit beacuse of NFB being affected beacuse of the tube swap). That's the reason why big bottle tubes (KT88, 6550) do not clip as soon as a small ones, because they need more voltage to get to that thing!
SO! Will you get less wattage - YES, will you be able to open a preamp more - I don't think so...

[Doug H]

the cool thing is then, while they might be compared to 6l6s, 5881s are typically reported to have less headroom than even an el34, so while the wattage will be very close, there might be less overall clean thump, less perceived volume, maybe?

[Haze13]

5881 - 19 watts, el34 - 25 watts, 6l6gc - 30 watts.
More power the tube can handle, the more you can "open" it and bigger headroom you will have. IN THEORY! There is an explanation to every thing! No Magic! What a shame...

[Doug H]

I'll hoping to hear a new flavor of magic when I stick a couple 5881s in my amp

[Haze13]

Just don't burn your tubes or you can loose the OT in the process

[Carbia]

5581 has a transconductance of around 5000
6L6GC has a transconductance of around 6000
EL34 has a transconductance of around 10.000

So yes, with the same input voltage, the 5881 is less loud

[Doug H]

Just don't burn your tubes or you can loose the OT in the process

wouldn't matter, I go through OTs like other guitarists go through strings. It would just give me a reason to get something in the 6k primary range.

[Doug H]

5581 has a transconductance of around 5000
6L6GC has a transconductance of around 6000
EL34 has a transconductance of around 10.000

So yes, with the same input voltage, the 5881 is less loud

Now I'm curious. What is transconductance?

[Carbia]

"Transconductance (for transfer conductance), also infrequently called mutual conductance, is the electrical characteristic relating the current through the output of a device per the voltage across the input of a device"

I think is clear the definition from the wiki

[Carbia]

But anyway, if you take a look at some datasheets at "typical operation" boxes, you find that running a single tube in class A at 250v both in plate and screen grid and -14v on control grid, the 5881 delivers 6,7w with a 2,5k primary and the EL34 10w with a 2k primary.
The higher the primary, higher the output, so you can imagine the difference in power output of 5881 and EL34

[Doug H]

I always thought output went down each way from the sweet spot. Meaning all other things equal and standard, I'd get less output with an 8k primary on a pair of el34s than with a 4k primary.

[Carbia]

yes, it's not a linear ecuation.

But the datasheets use to show the appropiate loads, so, you can get a good idea.

I've tried 5881's in a Plexi and is less loud than the EL34's for sure.

[Haze13]

Just to save some time....
LOADLINE SIMULATOR!!!!
http://www.trioda.com/tools/triode.html

[Haze13]

For 5881 you can use 6l6GC... More or less same result