Vari-Watt Power Scaling

[Diablo]

Anyone have any familiarity with adding this Vari-Watt to a fixed bias amp? Sonic qualities? Does it work as claimed? Reliable? All comments welcome.

http://www.skipzcircuits.com/products.html

[daz]

There are several people offering versions of this and a lot of people seem to like it. I tried it in 2 amps and for me i wasn't thrilled with it at all. It's ok, but i found i preferred master volume tone for very low volume playing. guess it just depends on the amp. But one thing i feel is going to be a big factor is whether your amp is one that gets it's gain mainly from the preamp like a JCM or such, or a crank it up to get the goods type of amp. If it's the latter you may like it because those are the people that seem most satisfied with it. I still have it in one amp and i also have a post PI master and a pre PI master. I have been experimenting with them all by them selves and in ever combination. And i really believe that in a high gain preamp style amp a master work just fine. Also, so crazy low volumes like 2am bedroom volume, these power scaling circuits don't work well because even at the lowest setting they are too loud for that.You may get conflicting opinions, but that is mine. And i think if i had to give my opinion in a single sentence it would be "might be good if you use it in a amp designed to get it's distortion from the output section, but not the preamp gain type of amp".

[Diablo]

Thanks DAZ. I have three amps that are potential candidates for the Vari-Watt.
1. Ampeg V4
2. Tweed Bassman clone
2. Blackface Deluxe Reverb clone

None of these has a master volume, and I tried a resistive load power attenuator on the V4 and wasn't all that pleased with the tone.

I assume all these amps get their distortion from the power tubes, so the Vari-Watt might be a good match for any of them, right? Also, does the Vari-Watt work with tube rectifiers as well as solid state rectifiers? I'd like to see instructions or a circuit diagram so I can understand how it works, but I think I have to buy it first in order to see a schematic.

[daz]

I'm not familiar with the variwatt, but the VVR version i use by Dana (who hangs out at most of these forums) works with tube rectifiers and i don't see what any of them wouldn't. one thing to note tho is that the VVR at least has two versions, and one of them only works with cathode biased amps. the other one has a design that tracks the bias so it can be used with fixed bias amps. So make sure any amp you put it in is cathode biased unless this variwatt has the ability to track the bias voltage to go up and down along with the B+ voltage as the knob is turned.

[Ptron]

Originally Posted by Diablo

Anyone have any familiarity with adding this Vari-Watt to a fixed bias amp? Sonic qualities? Does it work as claimed? Reliable? All comments welcome.

http://www.skipzcircuits.com/products.html

I wonder how they work? The variable power methods I know of use high power mosfets..sometimes several of them for bigger amps. And the simpler circuits, such as the VVR Daz speaks of, require special "mil spec" pots because they have full B+ accross them. Those circuits look pretty simple and have neither of those things so either they work some other way or yikes.

[Diablo]

This fellow from Twister Amps uses them in a few of his designs, so they must work OK for him.

http://twisteramps.com/DUSTDEVIL.html