Scaling Wattage

[Cpfromng]

Hello All,

I've been reading this forum for a long time (since Jr. High and I'm now almost 22) and I'm now finally deciding to go ahead and take the plunge into designing/building my own circuits. I may do it as a small business, but I don't want to make it my main thing. Perhaps something to help raise funds for an audio technology venture I'm working on getting off the ground.

I'm just wondering a few things as my circuit design nears completion. I already have my primary preamp, overdrive and effects loop pretty much ready to go. I found an old Peavey schematic that had some nice instructions for making a variable ohm switch. What I can't seem to figure out for the life of me is how to make a wattage switching feature. In my head, it looks like having three separate power amps and a toggle to determine which one(s) get voltage driven through them. Part of me is tempted to take apart my friend's Hughes and Kettner just to figure out how the fuck they do it and then make some changes so I'm not infringing any patents!! (If even applicable... It may be a basic feature since I've seen it on a few amps).

That would bring me to this question: how on Earth do you scale a power supply up/down to meet a specific wattage rating? In specific, I want 25W, 50W and 100W operation modes.

I would also like to know if anyone on here has figured out how to make something similar to the tube monitoring systems found on modern boutique monsters like the H&K, Diezel, etc. I plan to use two or three different tube brands and possibly an NOS 12AD7 in the v1 position just to give my guitarist some extra headroom. Having the option of variable wattage will help this aim further, as will a few of my other features. Basically, this amp is all about getting as many different tones as possible with only one channel so it's easy to play live rather than having to do stupid stuff like changing channels mid-song or even worse, carrying multiple amps around... I gave this thing a little Arduino in the FX Loop so that it can be MIDI-interactive or full-analog.

Last thing: noise gates. How do they work and what does a basic schematic look like for one? I know, I'm a tube. Laugh, throw fruit, do what you must.

Just for those who were curious, my circuit is an original design based on all my favorite things about Dumble and all my favorite things about Trainwreck with a few features here and there inspired by various amp makers (most of whom I've named here).

Thanks for all your help, and hopefully I'm not the MOST annoying new member of this community...

[gingertube]

Power Scaling is achieved by a variable high voltage supply applied to the output tubes, anode connections via the output transformer centre tap and screen connections.
For a cthode biased output stage that is all that is required.
For a fixed bias amp you need a tracking bias supply regulator too such that as you dial down the anode and screen voltagees you dial down the bias voltages too.

Some "limited" scalling amps just adjust the output tube screen voltages which sort of works but not as well.

Some amps just scale the voltage to the Phase Inverter but I've never liked any of them.

1/2 and 1/4 power amps can be done just by switching out 2 of 4 output tubes (for a quad output tube design)
and/or switching output tubes beteen pentode mode and triode mode. You don't actually get 1/2 and 1/4 power but close.

Cheers,
Ian

[TUBEDUDE]

Not to curb your enthusiasm, but It may be easier in the long run to build something simpler first. Slinging solder will tell you important things about design you won't learn from schematics and theory. Not to mention that troubleshooting a build of a new design is more complex with the added factors of basic construction errors.
Your build will be better designed as a result of what you learn of wire routing, component spacing and position etc. Many things that seem small, but make big differences in builds that are more complicated, or have higher gain.

[Cpfromng]

Tubedude, I'm not concerned. My guitarist has a physics degree and my other partner has a mathematics degree with an emphasis on electrical engineering. Between the three of us, I'm sure we'll figure it out when it comes to the actual build stage. I also don't have time for that approach. I need to take this thing to the studio in just a few months. That gives me one month to build a prototype and then around 5 months to tweak it to my liking.

We also plan to build a "mock" circuit using cheapo components before we do the actual build. That way, we have a sandbox to play in so we know everything we need to about our designs by the time we go to build the real McCoy with all the super high-end components.

Thank you Ian for the very informative tip. Those are all three interesting methods and I think I'll probably have to mull it over a bit as to what I want to do. Right now, that first option seems to be the design I like. We'll see...

[xtian]

Just wanted to chime in. I've built VVR (voltage scaling with MOSFET) into several amps. I have also played several amps with half-power switches. I want you to remember that the relationship of amp output power to perceived volume is logarithmic. 100 watts is twice as loud as...not 50, but 10 watts. And 10 watts is twice as loud as 1 watt. I think you will be dissatisfied with 100/50/25 watt options. Far better to have 100/10/1. And the easy, tested way to do this is with a MOSFET voltage scaler.

[Cpfromng]

Just wanted to chime in. I've built VVR (voltage scaling with MOSFET) into several amps. I have also played several amps with half-power switches. I want you to remember that the relationship of amp output power to perceived volume is logarithmic. 100 watts is twice as loud as...not 50, but 10 watts. And 10 watts is twice as loud as 1 watt. I think you will be dissatisfied with 100/50/25 watt options. Far better to have 100/10/1. And the easy, tested way to do this is with a MOSFET voltage scaler.

I am aware that the perceived volume to Watt scale operates logarithmically due to interaction between air pressure and the cochlea. The wattage rating has a lot more to do with the amount of clean volume I have available vs. the amount of actual volume available. There's DEFINITELY a difference in breakup between those three wattages. What I may do (now that you've got me thinking) is a 5/30/100W. That makes the most sense for my particular design, methinks. After all, Trainwreck-inspired circuitry does tend to run quite loud for it's given wattage...

[Guy77]

Another good option for changing clean headroom is to change the power section from cathode biased to fixed biased, this is not too difficult to implement just takes some extra wiring and a DPDT switch. Also setting up negative feedback so that it can be switched on and off is a good option. JMTC. Keep us posted on your build!

Cheers
Guy

[TUBEDUDE]

Sounds like you have a team for backup. A deadline will keep you on target. You sound super stoked so good luck. And as you know there's help available here as well. Be sure to post your progress. At the finish line try to post sound samples and gut shots.

[Cpfromng]

Great advice, guys!

I will be building a scaled-down version for a friend of mine before I do this monster because he just HAD to have me build him an amp as soon as I was doing them, so I will build the "simple" version for him as a proof of concept and then once I've got it down, go back and build my own.

I will definitely keep everyone posted!

[southpawed]

I wonder how it all worked out.

[Mac Dillard]

I wonder too: Even if they got it designed there is a huge difference in building an amp and marketing and selling an amp.

[TUBEDUDE]

I'm sure they found that out. That sounded like some big plans and a short timeline for not having actually built something before.

[Phil_S]

The sound of unbridled youthful enthusiasm. It comes with notes of invincibility and naivete that only older folks can hear.