Tube power stage vs Class-D (effective power)

[Paddy Morris]

Sorry if this subject has been done to death in the past. 

I have been using Class-D amps for quite a while now, and like them a lot.  But they don't really seem to appreciate being driven close to their limits.  So I've started looking at a few tube power stage amps, and a lot of the marketing blurb seems to say that they are 'more powerful' per given RMS wattage. 

 

I had a training in electronics (quite a looooong time ago) so I get that in a physical/electrical sense this has to be untrue, except for amplifiers where the manufacturers tell all sorts of fibs about the power output, and use terms like 'peak power', or 'music power', and avoid quoting an actual RMS or AES power rating.   But I was wondering if players had found that in the real world it does actually turn out to be the case that a tube output stage gives you more apparent loudness per specified RMS watt?  In particular, I was looking at this text from the Mesa-Boogie site.

"NOTE: Tube Amps sound as loud as Solid State Amps rated at several time their power (wattage) rating. Choose from 8 tubes producing 465 Tube-Watts (sounds like in excess of 900)"

It's a bold claim to make if it's just plain untrue.  So are they talking about the 'warmth' or harmonics, or saturation, or the soft clipping of the tubes at high power?  Or the fact that you can run the tubes much closer to their supply rails?

 

[mattpbass]

It’s certainly true that a valve amp of a quoted wattage will sound significantly louder than the equivalent solid state amp of the same quoted wattage. 

[skidder652003]

crank up a 300W ampeg SVT against a 300W class D and I assure you there will be a difference.

[Paddy Morris]

50 minutes ago, mattpbass said:

It’s certainly true that a valve amp of a quoted wattage will sound significantly louder than the equivalent solid state amp of the same quoted wattage. 

 

Thanks Matt.   Good to know.  But I wonder what the reason is, if it isn't actually the electrical power in watts. 

[Bill Fitzmaurice]

The reason is compression. The natural compression of valves reduces the level of transient peaks that cause high level distortion when the amp is at or near full power. It can be approximated with SS using a compressor. It can be very closely approximated with SS using a compressor plus DSP emulation.

[Paddy Morris]

Ah. Ok. That does make sense. Thanks Bill.  But so in that case if you drive the valve front-end of a hybrid amp quite hard, and get all that soft clipping and compression, is the class-D power stage ok?  Why would you need a valve power stage too?

Edited 17 hours ago by Paddy Morris

[Bill Fitzmaurice]

With a valve power stage the compression takes place within it, which gives a different result than when it takes before it. There's also a contribution made by the output section power supply, which doesn't happen with SS. For that matter the most sought after valve amps have valve rectification in the power supply, which gives a softer transition into clipping than SS rectification. The addition of SS diodes in place of a 5Y3 or 5U4 valve in Fender amps was one of the reasons why it was said that CBS ruined them, although in truth Fender started using them in some models before the CBS takeover.

[Paddy Morris]

Thanks. Sorry if this feels like valve amp principles 101, but I disappeared down a rabbit hole when I saw Mesa stating that valve power stages are 'more powerful' rather than just subjectively louder.

[Bill Fitzmaurice]

I'd be surprised to see Mesa saying that. Subjectively sounding louder yes, but not more powerful. A watt is still a watt.

[Bolo]

One aspect may be that RMS ratings must be given over undistorted signal, and most tube amps that are built for bass and especially guitar amplification are run way beyond clean.

[Phil Starr]

I don't know if this helps. As Bill has said it's all about the natural compression valve amps give you. The peaks aren't any bigger but you can bring the average level up and the overloaded peaks sound OK. Clipping of the waveform in most solid state amps is pretty unpleasant. Other factors are that a lot of the power claims are basically untrue. There is also the importance of a decent power supply to consider, a lot of switch mode amps also have switch mode power supplies which are specced for fairly low duty cycles. I'd expect Mesa valve amps to have a pretty decent power supply.