Fortunately I figured out a way around my problems by simply putting a patch cord between effects send/return. This allows the loop master to control the overall volume of the amp.

Sounds better this way too, presumably since the individual channel masters aren't sitting so close to zero now. 

Idea for low parts count amp (not tested):

This Line6 15W was a perfect amp to gut for this purpose. It's empty except the power transformer, which I could add back in later for line power, but battery is the first goal.

Now it's just a matter of the circuit. I would personally skip tubes for this, and just try to make a nice simple clean amp that is safe/easy to build. Maybe an opamp preamp/chipamp power amp? (The original Line6 used a little 5-pin TO-220 sized chipamp.)

I have a broken Line6 15W combo ready to be converted into a portable

I misspoke and meant to say to cut the power down to that of a 25W amp, not that the resistors would be 25W. Still something to be careful about.

My motivation for doing this is simply to make the volume control a little more usable. Right now it goes from "zero" to "loud" with a 1MM turn of the knob. It's loud or nothing basically.

Thanks for the tips. I guess using oversized resistors and maybe a fan wouldn't hurt. Will post my circuit for review when I get it planned out.

The amp is 100W/16 ohm cab, and is mounted in a rack so I'm thinking about putting the resistors on a faceplate and bolt it on the back above the amp. Will probably shoot for about 25W. Thanks for the tips, looks like those aluminum housed things would work really well.

Looking to make a power attenuator for my Dual Rectifier amp. After looking over some schematics, some things are not clear to me. I would like to use silicone wirewound resistors for cost reasons, if that's ok. Also, some circuits are purely resistive and others have bipolar capacitors in addition to the resistors. Do the capacitors matter enough to bother with? Thanks.

Sure, that would work. Search for 12v audio amplifier schematics and you'll find a lot of different things.

As Mr. Fahey indicated, a 9v battery wouldn't really cut it. However, old boom-boxes could run for awhile on eight D-cells. And there are rechargeable batteries with pretty high capacities now. So it's not a bad idea, especially for people wanting to build some sort of amp without getting into the dangers of line-powered equipment.

Finally finished this project. Ended up using a Dr. Boogey style preamp, but added channel switching and fx loops. It's all pretty basic but works. Turned out to be a reasonable fascimile of the original, but a little harsher sounding.

Decided against using the diodes in the power amp section, like Teemuk said the amp soft-clips anyway and sounds great cranked on its own. Seems more stable this way too.

Overall not bad for a SS amp, sometime I would like to redesign the power amp board and use a pcb for the preamp instead of rat-shack perfboards (and the rat's nest wiring job I did)

Edit: Drew up the power amp in its final state.

Fixed the preamp, apparently the direct-coupled buffer before the tone stack was screwing up the sound. So I cap-coupled the buffer, and altered it to include an input diode as in the other stages. Don't have time to fix the schematic(s) right now, unfortunately.

Has anyone tried FIMA clay for making knobs, etc?

The small size & lack of heatsink etc is amazing!

Been a while since I've been on...

Never finished my build, mainly couldn't get the preamp right. Still working on the power amp design though (fixed the VI limiter for starters).

Might explore using an output cap, only to prevent the slight bias shifts (which can occur with different loads or possibly extreme cold/hot) from putting DC onto the speaker. I don't believe it would ever be enough to matter, but cabinets are expensive, and maybe it's "just not right."

Saturation effects are largely responsible for the "transistor sound". Both BJTs and JFETs suffer from this problem: when overdriven they tend to latch up for more than half of the cycle. The signal becomes mutated beyond recognition. MOSFETs sound better because they don't have this problem. My own attempt at a simple emulation (Vulcan) completely eliminates saturation but has the side-effect of early clipping on one side.

The more complex emulations (designed for mass-repeatibility) can often be simplified. For example, the TransTube circuit sounds great with just four 2N5089's instead of the Darlington-connected pairs. I suspect that the Pritchard circuit could be done with a couple of transistors instead of the opamps, especially if several stages are to be used anyway. (You can actually make a TS9 this way but it will have low gain.)

Pushed output transformers are definately part of the "tube sound". This really shows up in professional tube amplifiers. The SVT Classic comes to mind, as well as a dimed JCM800. Raising the volume to 10 brings out a monster that wasn't there on 3.

Just some random thoughts...

With any technology there is an equation of cost, usability, and performance which is solved over time. Cost is the predominant factor, and it affects the audio industry like any other. From a practical standpoint I'd say "no", but a current amp might make a good DIY project.

Of course phrases like "the FATAL FLAWS of voltage drive" are always entertaining.

Something related here (kinda technical):
http://www.essex.ac.uk/csee/research/audio_lab/malcolmspubdocs/J12%20Distortion%20reduction%20MC%20current%20drive.pdf

Teemuk's book (starting at page 87) also offers a good introduction to the differences between the amplifier types.