Power tube saturation at various volume levels

[SactoBass]

It seems there are many TB'ers (myself included) that like the sound of power tubes right at the "brink" of saturation. But I also read alot of comments from TB'ers about having to crank the volume of the amp too high in order to reach that "sweet spot." For example, many TBer's say that in order to reach that spot with an Ampeg SVT, you have to play VERY loud, which, for some applications, may be too loud.

Other TBer's say that for them, a 100 watt tube head starts to break up too soon before reaching the desired volume.

So, at the risk of asking a dumb question, I wonder if it is possible to establish some kind of relationship between output wattage and the SPL at which the power tubes reach saturation? In other words, is there a chart that can be established that looks something like this:

25 watt = x SPL at power tube saturation
50 watt = y SPL at power tube saturation
100 watt = z SPL at pts
200 watt = (and so forth)
300 watt
400 watt

If such a relationship is indeed possible to establish, and assuming that the bassist knows what SPL he wants, he could then make a more informed decision on what power output to shop for when looking at all tube bass heads.

And yes, I know that the cab (number of speakers, sensitivity, etc) has alot to do with total SPL, and perhaps because of that, it may be impossible to create such a chart as I listed above. And differing power tube types also muddy up the waters. But assuming an identical speaker load, and assuming typical power tubes such as 6550's or KT88's, can such a chart be established, even if the SPL numbers are only relative to each other? Or do we have to just use the "keep trying different tube amps" until you find the one you like?

[T-Bird]

Hi.

Way too many variables to draw a chart like that. IMHO anyway.

BTW, it's not as much the power tubes than it is the OT that is responsible for the saturation. That's the reason 100W "bassman" type output section tube pulling doesn't quite work with guitar, but effectively transforms a 100W guitar (Fs 80Hz) amp into a 50W Bass (Fs 40Hz) amp.

Regards
Sam

[JimmyM]

it's as much about the cab as the wattage. a b-15 which does 25w rms will sound pretty goshdarn loud with an 810, but through a single 115 you can sit in the same room with it and not be too offended by the spl's. and of course, everyone's volume needs are different. a doom band may need 3 or 4 svt/810's just to get warmed up, but a classic rock band at your basic classic rock band gig where they run everything through the pa might find even the b-15/115 too loud when driven into saturation.

experience is the best teacher in this situation.

[anderbass]

Hey SactoBass, I took a different approach to this subject and modded my big tube amp to lift (shut down) pairs of power tubes, giving me the switchable option to hit that sweet spot at lower volume levels or still run at full power.
My mod Info's in this thread: SWITCHABLE MAX OUTPUT MOD, FOR a 300/w TUBE AMP.

[billfitzmaurice]

Originally Posted by anderbass

Hey SactoBass, I took a different approach to this subject and modded my big tube amp to lift (shut down) pairs of power tubes, giving me the switchable option to hit that sweet spot at lower volume levels or still run at full power.
My mod Info's in this thread: SWITCHABLE MAX OUTPUT MOD, FOR a 300/w TUBE AMP.

That seems a good idea, but if you cut power by half the difference in the sound is hardly noticeable; at least a 75% reduction is necessary to make a major difference. If you want to really get the over-driven effect a rig like the power soaks that guitar'd players use is the ticket.
As Jimmy said the other route is to use a speaker with less sensitivity and lower output capability, allowing the amp to be cranked higher without going too loud.

[AwkwardLoudness]

The tubes, bias, transformers, circuit, and speakers/cabs are all variables in addition to the amps RMS power. There's not a consistent pattern to predict SPL vs. watts among the many tubes amps out there. Also, not all amps will achieve power tube saturation without excessive pre-amp clipping.

This is one of the factors in gear acquisition syndrome, so try different amps and/or cabs to see what works. Like Bill said, use lower efficiency speakers/cab for more saturation at lower volume. An attenuator (like a Power Soak) between the amp and cab can tame the volume as well, but some people complain that attenuators negatively affect their tone. Not all attenuators are created equal, though, and the good ones somewhat expensive especially the ones able to handle high power. I think most attenuator users would agree that tone is least affected with the least attenuation. Too much volume drop from an attenuator causes the tone to be thin and gritty sounding typically.

[fu22ba55]

Originally Posted by JimmyM

experience is the best teacher in this situation.

The sweet spot really seems to be a moving target.

Here's hoping that I find it before I'm either too old to play anymore, or flat broke.

[Smurf-o-Deth]

You can always stick a boost in the effects loop to pound the power amp at lower input levels.

[fu22ba55]

Originally Posted by Smurf-o-Deth

You can always stick a boost in the effects loop to pound the power amp at lower input levels.

I tried this method with a few different pedals (blackstar valve boost, boss bass OD, green russian) and since my SVT is non-master, it just made the amp louder... (there's no real effects loop in my '76, so I was basically adding the pedal in-line between the preamp and power amp.) My power amp has no "volume control" so the boost from the pedal was just seen as me turning up the volume on the pre.

I imagine this unattainable ideal is more of an issue with us "non-master" folks.

[James Judson]

Exactly what I don't like about "tube" amps. They sound really good at one volume. Your best bet is let the FOH guy control your volume.

[JimmyM]

Originally Posted by James Judson

Exactly what I don't like about "tube" amps. They sound really good at one volume.

that's it? just one volume? well that certainly comes as news to me, bro.

[anderbass]

Originally Posted by billfitzmaurice

That seems a good idea, but if you cut power by half the difference in the sound is hardly noticeable; at least a 75% reduction is necessary to make a major difference.

I installed 5 lift switches so I can choose to lift as many as 10 of my 12 powertubes where I end up with a max clean output of only 34 watt. I can definitely hit the sweet spot at reasonable volumes with my 400+ at this setting.
(I plug an 8 ohm cab into the amps 2 ohm tap to help compensate for the output transformers mismatch at this setting)

[seamonkey]

But what about output transformer saturation? 34W into a transformer capable of 400W isn't going to send it into saturation?

[fu22ba55]

Originally Posted by anderbass

I really like the looks of this.

[JimmyM]

Originally Posted by seamonkey

But what about output transformer saturation? 34W into a transformer capable of 400W isn't going to send it into saturation?

ot saturation isn't the only way to do it, just like power tube saturation isn't the only way to do it.

[anderbass]

Originally Posted by fu22ba55

I really like the looks of this.

Thanks, I mounted that switch plate to pre-existing screw holes so the mod would be fully reversible.

[Randall]

I have never seen that on a 400 before. Very interesting. Of course you can buy various amp for this too.
50 watt Traynor yba-1
100 watt Ampeg V4B.
200 watt Mesa boogie D-180.
300 watt. Now i just need a svt

[SactoBass]

Wow! What a bunch of great posts! I hadn't considered output trannie saturation in my thought process. I was only thinking of power tube saturation when I wrote my original post. I guess it's a combination of both.

And as I suspected, there is no simple relationship between output wattage and SPL at the brink of power tube and OT trannie saturation. I guess that leaves us with the ol' "trial and error" process.

Thanks to all who posted in this thread.