Inspiration for milliAmp started with the following AX84 thread in which Chris Hurley asked if anyone wanted weigh in on a new beginner project. Chris's query generated a bunch of suggestions and comments. As the discussion emerged, the following criteria began to surface:
Chris proposed that it be simple to build and suggested a single-tube project.
It needed to be relatively low-cost. Pain Devine suggested that he'd like to see tube and transformers come in around $20 US. That would make the entry barrier for beginners much more acceptable. Chris and Jamie Anderson, both elaborated on this requirement.
A number of folks mentioned Bob Stewart's single-tube Spud as a good example of the level of complexity folks had in mind. Brian was the first to recall this amp and later dug up the BOM for the build.
Dean Owens was the first to mention the 12DW7 and noted that JJ was currently producing that tube, so we wouldn't have to rely on surplus supplies.
At some point during the discussion, I started studying the curves of the 12DW7 began playing around with load lines. That lead to my Single 12DW7 Proposal. Basically, I tried to incorporate the ideas that folks on the board had expressed. The design is relatively simple: using triode 1 of the 12DW7 in as high-gain a stage as practical, I drive the second triode on a deliberately steep 5K load-line. This steep load-line effectively narrows the input signal swing that the second triode can take before the onset of clipping. I've also asymmetrically biased the output stage at -2.5 volts to futher limit the voltage swing necessary before clipping. These are values that we may need to adjust in the prototype.
Many of you contributed other ideas that didn't directly end up in the design but influenced my thinking. Therefore, I'd like to thank and recognize all of you thread participants for your contributions to this circuit. Frankly, I am indebted to all of the participants on www.ax84.com for suggesting ideas or teaching me where to fish in the sea of tube amp design lore.
There was much discussion of whether to have any controls on this amp at all beyond the Herzog line-level control. Eventually, I settled on a design with a gain control between the two stages. I've also designed several variations that include tone controls and one that eliminates the gain control. You can see these variations on page two of the PDF schematic file. They include:
Additionally, I wanted this amp to serve as a stepping stone to the P1. In keeping with that desire, I've designed an alternate power supply that uses the Hammond 269EX power transformer. One could start with a P1 or HO transformer and a chassis punched for either, but build the milliAmp as a starter. Frankly, I'm not sure how much easier it would be to build the milliAmp than it would be to build a P1, but the several expressed a desire to be able to follow such a build progression. Besides the 269EX power supply, I've also included a schematic of supply that I'm building into the prototype. This last one is primarily for reference; I doubt many people are sitting on a Triad R-29A power transformer that they'd like to use.
Feel free to build an amp from any of these schematics, but do so are your own risk and please take appropriate safety measures. The power supply in this design is sufficiently powerful to kill the careless builder. For safety guidelines consider the following document: Tube amp building and use safety. at Drifter Amps. Be forewarned, however, that no amount of documentation can protect you if it isn't properly applied.
Pain Devine has published bills of materials for the milliAmp sourcing the transformers, tube and miscellaneous hardware from Antique Electronics and the other components from Mouser. He's got the price down in the $75 range. You can see those documents here:
I also constructed a bill-of-materials for the simplest of these, sourcing all parts from Antique Electronic Supply. Pain's is a better list and with his permission, I'm going to make a few refinements, tie in the part numbers from the schematic and incorporate it into mine. But that will have to wait until I have some more time.
Note these PSUD files are little out of date. With the bias changes and such, milliAmp now draws closer to 16 mA, so the dropping resistors and the loads need to be adjusted accordingly. I'll do that sometime soon.
The power supply for all of these relies on a relatively inexpensive power transformer from AES. Because it's secondary voltage is only 115 volts, I elected to use a voltage doubler. Arguably, I could have used a bridge rectifier and let the voltage run a bit lower. I decided I want to run the first triode with a relatively high plate voltage (and corresponding preamp gain) in the hopes that we might be able to gets some clipping out of the power stage. Prototyping it up will tell whether this approach has merit. The 250-255 volts on the plate of power stage stage is about as high a voltage I can reasonably go with the 5K load line and still keep the power dissipation within the design maximum limits.
For the initial prototype, I have on hand a 230 volt transformer that I'm going to use. So I've also designed a power supply version around that unit. Ultimately, I or someone will have to build the doubler version to test it out. At the outset, however, I'm going to limit my cost and use as many parts on hand as possible. After all this design might not work out as expected.
For those of you who like to tinker with Duncan's Power Supply Designer, you can click on the links below to download the PSUD net lists:
The doubler supply:
The bridge supply: