Understanding Tube Power Ratings


What do I need to know about the power ratings for tube amplifiers?

With solid state models, I looked to see if the power output doubled from 8 ohm to 4 ohm as one sign.

What about tubes? Are 250 tube watts comparable to 250 solid state in terms of driving power? Please, I understand equal measurements should mean equal results, but am I missing something?

I'm looking to move from the Bryston 4B ST currently driving my Thiel CS 2.3s. I have a Rogue 99 magnum preamp and have the new Rogue Zeus on the short list. The Thiel need for high current had me shying away from tubes, but I am rethinking this.

Thanks!
johnmcalpin

Showing 1 response by gs5556

No problem. The Rogue 99 has a 23db gain and 30V p-p which is great for any power tube amp with the Thiels. I use a 3-watt 2A3 and a 26db/50Vpp CAT with my Thiels - and it works. And, no, you're not missing anything. Whether you have a tube or a SS at 250 watts, they both can, at full volume, blow out drivers which can only handle, say, 100 watts maximum.

As to why some will say that a "tube watt" is more than a "solid state watt" is not contraindicating the laws of physics. Tube amps have higher dynamic head room than SS amps. Headroom is the ability of an amp to push out more power than rated for a brief time, which comes in handy during transients. Each 3db of headroom doubles the peak power delivery. Tube amps have headroom ratings as high as 5 to 10 db whereas SS amps can have HR less than 1 db and 2 db is typical. The best SS have very low values (0.5 db and less) due to better power supplies.

If you look to SS amps for power doubling as load impedance is halved, then you run the risk of being baited by the sales brochure. Very few SS amps can double down to 4 ohms, much less to 2 ohms. While Ohm's law says this is inevitable, Ohms law also states that the voltage decreases with an increase in power (current). As the load is halved, the PS voltage drops and lessens its ability to keep up the current flow. The PS voltage and current regulators have to be of top quality and the transformer has to have plenty of reserve in order to double down. That's why a single $20,000 100 watt Class A monoblock will have a power supply transformer thats more than twice the size of a $3,999 250 Wpc and more elaborate voltage and current regulation stages. That's where all the money goes.