Help me understand wattage reduction switches

[Gregg Hermetech]

Hi all,

Something that I've been wondering about recently that I don't fully understand, but know I'll get a good answer here! The kit maker for my 5F1 clone sent me info on how to add a Wattage Reduction Switch to my amp, that goes from 4w (standard) to 2w or 0.4w. This apparently cuts the loudness of the amp significantly, enabling you to get breakup at lower volumes in the room. I've also seen these switches on many commercial amps over the last few years (I am presuming they are doing exactly the same thing). It seems a very simple mod so I am thinking of adding it to mine.

He describes it thus:

Have a look at the attached image, it shows how to use the 3 taps of the mains transformer's HV winding with a 3-way (break before make) rotary switch to create the 0.4/2/4W switchable amp. You can try it without the switch, just by moving the wire on the solder lug. E.G. 0V and 190V would give 2W.

What is not clear to me, is why or how this works, and what the negatives, as well as the positives might be. I keep thinking of questions like:

Does that mean you get more preamp tube or power amp tube breakup at lower levels, or both?
Doesn't reducing power throughout the whole amp like that that put the bias on the tubes at the wrong place, possibly negatively affecting tone?
All the amp measurements are likely to be completely different, right?
Is there any chance of damaging things running at these lower wattages?
Is this the same as the "starved plate" designs people talk about, which are supposed to sacrifice tone?

I'll add more later if I think of them. Thanks in advance for any help!

[martin manning]

What is not clear to me, is why or how this works, and what the negatives, as well as the positives might be. I keep thinking of questions like:

[1] Does that mean you get more preamp tube or power amp tube breakup at lower levels, or both?
[2] Doesn't reducing power throughout the whole amp like that that put the bias on the tubes at the wrong place, possibly negatively affecting tone?
[3] All the amp measurements are likely to be completely different, right?
[4] Is there any chance of damaging things running at these lower wattages?
[5] Is this the same as the "starved plate" designs people talk about, which are supposed to sacrifice tone?

[1] Both. The signal voltage becomes relatively larger with respect to all of the other voltages, reducing headroom.
[2] In cathode biased stages things stay relatively in proportion. Fixed bias stages need a separate bias scaling circuit.
[3] Yes, but they remain more or less in proportion.
[4] No.
[5] Yes, but in use the voltages are not usually reduced to the same extreme.

Most people will perceive reduced power sound as being different from full power sound. Search topics like Power Scaling, and VVR for more information.

[Gregg Hermetech]

Thanks so much Martin! I knew I could rely on this place for some clear and experienced answers.

So, it seems that most of my fears are ungrounded. I think the key penny that hadn't dropped for me was this:

The signal voltage becomes relatively larger with respect to all of the other voltages, reducing headroom

That totally makes sense, everything scales to a lower power, except the actual input signal, brilliant.

I'll go ahead and order the bits to do the mod!