This makes it possible to maintain stock voltages and adjust those with the knob, OR you can run a high voltage drop, taking it from 45 volts to around 100 volts.  This drops the bias current from around 170mA down to around 120mA.  Of course the power also drops, but that is kind of the point.  It gives you an opportunity to run your amp at a lower power which is usually all that is needed during normal listening.  

This would make the tubes last considerably longer, perhaps twice as long, and creates less heat for those trying to stay cool in hot climates.

The sound is considerably more lush, more natural, perhaps a bit more bass heavy so if people are to like it or hate it will probably depend on speakers more than anything else.

Steve, can you explain this? I feel like my 300b operates like this. It is more lush and the bass more solid and defined when the volume knobs are at 25% than 60-80% where I’ve always historically run my tube amps. This seems counter-intuitive to me. Generally with tube guitar amps you gotta crank ‘em to get that tone.

My non-technical brain has always assumed that the lower the frequency the more energy required to drive it, so that's where it throws me off that lower voltage blooms the lows.  Is it just that the lows seem more present because the highs are being compressed?  Not trying to derail this thread, just trying to understand it better.  

On the contrary, I don't feel like running my Sarah higher makes the highs more present, the mid range just gets a little obnoxious.  Everything gets more balanced when Sarah's volume knobs are turned down. Could be tube choices and room treatment too.

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I hope Steve will respond at some point, but this is in a similar vain to a discussion Will and I were having in a different thread, so I'll give it a shot. But this is very much JMO/FWIW and all the other disclaimers.

I think a big part of the confusion is that in the context of tube amps "power" has several (very) different meanings. There is the power the tubes are dissipating with no music playing, the "operating point" of the tubes. Then there is the maximum power the amp can deliver into your speakers (without distortion), which is also determined by the operating point, but unless you listen at or near clipping it isn't a factor in your life so lets not worry about it any more. And finally there is the power your amp is delivering "right now" as you play music which is determined by where you set the volume knob.

So when Steve says "...This drops the bias current from around 170mA down to around 120mA.  Of course the power also drops, but that is kind of the point.  It gives you an opportunity to run your amp at a lower power which is usually all that is needed during normal listening..." I think he is talking about the power the tubes are using with no music playing, aka the operating point. Without getting too far down the rabbit hole of how amps actually amp, when you listen to music you can think of the music signal swinging around the operating point. The actual volume (aka power) at which you listen is determined by the size of the swing, not the point to which it is referenced. And the size of the swing is set by your volume knob. Hopefully that is less confusing than it seems to me.

The operating point also affects the frequency response of the amp. It is also why changing the VR tubes makes your amp sound different: the tube will amplify different frequencies different amounts, like harmonic distortion. So different operating points will make the amp sound different because the ratio of e.g. high to low frequency is different. When an amp designer talks about "voicing the amp" she or he means, among other things, determining the operating point that delivers the sound they want.

Hope this helps.

I thought you were querying me about my experience with the low gain input on the SEWE300B.

I love this amp. It can be so many different amps with different tube complements and different gain structures. And all of them sound fantastic!

Steve,

Will it work with Rachael (SE34I.5 version) as well?

Best,

ChrisC

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Yes, it is similar but in the case of the UFO25, you can tune the output stage independently of the input stage and visa versa.