How much rig you need depends on how much rig the rest of the band has, and how much of it they actually use. 15W of all-tube amp can get painfully loud through the right speakers, and it wasn't until bands were trying to be heard over thousands of teenage girls screaming as loud as they could for the entire concert that amp makers even bothered with 30W. Amps got louder from there mainly because PA systems didn't have enough power to mike up a full mix of the entire band, a problem we left behind at least 30 years ago as the class D solid-state amp started packing high-wattage-high-efficiency amplification into relatively small packages.
So, first and foremost, if your guitarists are using more than about 30W worth of tube amp each, you need to sit down and discuss whether more is really necessary, and why. The solution is probably either to back everything off a bit (including taming the gorilla on the skins in the backline), or to invest in a PA that can cleanly amplify miked amps and direct sources at the proper volume.
All that said, bass wattage has to be calculated differently, for several reasons:
- Bass frequencies don't sound as loud as midrange and treble. At 90dB SPL, a 100Hz frequency sounds 12dB quieter than a 1kHz sound at the same SPL. To make them mix, the 100Hz signal needs approximately 20 times the power level of the 1kHz signal. That's a worst-case for many reasons, but it tells you what you're up against.
- The bass has to fill the low end, between about 40-600Hz, against every other instrument in the band. So that twenty-fold increase in power needs to compete with not just one guitar amp, but probably two, and the PA, and the drummer.
- Solid-state amplifiers are rated differently than tube amplifiers, because of the differences in amp circuitry, efficiency, and behavior at or near peak levels. Simply stated, tube amps today are basically built to be overdriven, while solid-state amps are built to avoid overdriving at all costs. Just 6dB of "headroom" to do so requires four times the peak power rating.
So, bass amps are rated up to and exceeding 1000W because this is what they're up against.
The formula I typically use, assuming you can't expect any PA help, is as follows:
- Add up the wattages of all the tube amps in the band. Divide this number by 2 if they commonly play on the clean channel and/or keep their master volumes below halfway.
- Add up the wattages of all solid-state amps besides yours (PA, separate keys amp etc). Divide this number by 4.
- If the drum kit isn't miked, take either number and divide it by the number of amps it represents (so if you have two guitar amps totalling 60W, figure another 30 as the drummer's "power").
- Add all the numbers from steps 1-3, and double it. You may even double again depending on brand and speaker count (fewer speakers need more watts).
So, simple working example, your band has a vocalist who plays rhythm, a lead guitarist, a drummer and you. The PA only supports the vocalist and a couple acoustic guitars when they're used. The guitarists both have 30W amps and they crank em pretty hard, while the PA rig is 300W. So, 30W for each guitar amp, another 30 for the drummer, and the PA is being used cleanly at about 75W to keep 6 dB of headroom, all means you should be looking at no less than 240W for your bass amp, and 500W honestly wouldn't be too much.
Now, that gets much harder when you say you want all-tube. You can buy amps the size of a dictionary with 750 watts peak rating. Tubes take up space, and they need even more space to ventilate they heat they generate. A single tube also has a power limit; EL34 and 6L6 tubes max out at about 50w per tube, so a 300W head needs 6 of em. Add in the input transformers (can't use a switching supply) and you're looking at a 40- to 80-pound amp head (that's
just the head; my old Peavey rig was a monster and it was 80 lbs for the 300W head and 4 10s). A class A amplifier at that wattage is also ridiculously inefficient, meaning the 300W head could easily draw over 1000W from the wall at peak levels (meaning if you're running anything else on a 15A breaker circuit you will likely pop the breaker), and even at minimum volume or with nothing plugged in, it could draw its rated wattage just to bias the tubes.
Long story short, bass players went solid-state for a reason. You
can get an all-tube head with enough oomph to be heard at practice, but you'll pay about $2000 for that practice amp, and it will definitely stay a practice amp as you will very soon find any excuse you can to avoid carting that beast to gigs.
If you want tubes, what you probably
really want is a tube preamp stage, which uses the old-fashioned tube transistor circuit to boost the input signal to working levels, before it's amplified to speaker power levels by a Class D power amp stage. That gets you most of the recognizable tone character of an all-tube amp - almost all of it if you're not planning on diming the master volume - at a fraction of the cost and weight.
