How does tube power scaling work?

jakester10

jakester10

New member
In light of the recent release of Marshall's Origin series, I'd like to ask the forum: How does power scaling work? Many tube amps these days offer the ability to switch from full power down to half or even 1/10 of the total output wattage. How does this work? How can you reduce the amount of power the tubes are pushing and still get the same tone? Does this effect the amount of power tube distortion you can get? Any and all insight would be greatly appreciated.
 
Re: How does tube power scaling work?

dystrust said:
That's how full power scaling works. For amps with a simple wattage switch (like my Marshall Mini Jubilee) I suspect that the power transformer has additional secondary tap(s). It essentially runs the power tubes at lower plate voltage which reduces total wattage and headroom along with it.
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What about pentode/triode switching?
 
Re: How does tube power scaling work?

Totally different, it runs the pentode power tubes as triodes, for a better explanation there is google. The original question was about power scaling, I think that is pretty much the only way to get a 90% reduction in power, not sure the power tubes would even work at a low enough plate voltage to do that (although, it has been well documented that I could be wrong).
 
Re: How does tube power scaling work?

Jacew said:
What about pentode/triode switching?
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Switching from pentode to triode does reduce wattage, but it substantially changes both the tone and feel of the power tubes. My Jubilee 2555 has pentode/triode switching and I never use it because 'low' power mode sounds like spongy, muddy crap. With the Mini OTOH, low power sounds almost identical to high power just a bit more spongy and less loud.
 
Re: How does tube power scaling work?

devastone said:
Totally different, it runs the pentode power tubes as triodes, for a better explanation there is google. The original question was about power scaling, I think that is pretty much the only way to get a 90% reduction in power, not sure the power tubes would even work at a low enough plate voltage to do that (although, it has been well documented that I could be wrong).
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They'll work surprisingly low as long as the bias current is correct. The reason power scaling works better than a variac is because London Power's solution only scales the plate voltage of the power tubes. A variac reaches its limit at around 90V input because running lower than that causes the heaters to brown out. A quirk or weakness of London Power's solution comes from always running the preamp tubes at full voltage. It won't brown out like a variac, but preamp signal strength remains constant while power section headroom decreases. With a Suhr Badger for example it can be necessary to turn down the preamp gain at lower wattage levels to avoid ridiculous amounts of poweramp distortion due to the reduced headroom.
 
Re: How does tube power scaling work?

Thanks for that explanation, I haven't tried it, so as noted, I may be wrong :kabong:
 
Re: How does tube power scaling work?

jakester10 said:
In light of the recent release of Marshall's Origin series, I'd like to ask the forum: How does power scaling work? Many tube amps these days offer the ability to switch from full power down to half or even 1/10 of the total output wattage. How does this work? How can you reduce the amount of power the tubes are pushing and still get the same tone? Does this effect the amount of power tube distortion you can get? Any and all insight would be greatly appreciated.
Click to expand...


Hey everybody,
Thanks for responding to my thread. I did a little research of my own with your posts helping to guide my search, and I found this article, which has some of the technical jargon that I was looking for.
 
Re: How does tube power scaling work?

dystrust said:
Switching from pentode to triode does reduce wattage, but it substantially changes both the tone and feel of the power tubes. My Jubilee 2555 has pentode/triode switching and I never use it because 'low' power mode sounds like spongy, muddy crap. With the Mini OTOH, low power sounds almost identical to high power just a bit more spongy and less loud.
Click to expand...

Yep. I wonder why the heck it's marketed as "half power" setting...

I agree with "low power mode" sounding spongy and undefined in TT too. It improved greatly, when I added separate cathode resistor for the 7 watt side though.
 
Re: How does tube power scaling work?

Another way I see this done, not sure if it's a method already described and marketed differently or another a whole new way... heck, I'm not even sure how they achieve it; but my Traynor does Class switching. 50 watts Class AB or 15 watts Class A. Volume is pretty much the same, but headroom is reduced. There's also less low end in Class A mode, but not by much. I assume the amp's ability to retain its low end is because Traynor use massive transformers.
 
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