OK folks here's the update.
First, the amp is easy to build and to get working. Mine went together in an evening of about 2 hours work and then the next morning. I had music by lunchtime. The three stage scheme (on the website) for bringing up the amp worked just fine, although the voltages came up correctly from the start. I was able make the current adjustments easily enough. After some break in I readjusted the currents and they have been stable since then.
I've done quite a bit of tube rolling between 12au7s and 6dj8s and their variants. The front end works as designed, balancing the plate voltages reasonably closely depending on the matching of he triodes. The worst mismatch that I have seen is with one of my EI Elite 12au7s which shows a 5V differenc ein plate voltage between the two halves. All of the other tubes are within a few volts, a few within less than one volt.
The amp sounds very good to me and is well broken in. It gets hot, but not unacceptably so. The tube rolling has been great fun with a very simple jumper swap to go from one tube type to another.
Here are the two things that have surfaced in the prototypes:
1. Hum - my build shows some audible hum depending on where I put the toroid in the back of the box and exactly how I position it (that is, rotating it changes the hum). Other builds (some in a single box, one in a two box build) are not showing hum. To clarify this, full volume exhibits a small amount of hum in one of these builds, but nothing until about 85% of the travel.
I am able to rotate the toroid until I don't get any hum, but I have not fully cased the amp yet.
The peculiar feature of this in my case is that the hum only appears when the amp is in the Hammond enclosure. When it is on the workbench, I can move the toroid very close the board with no induced hum. But when the transformer and board are slotted into the case I get the hum. This seems to be related to the transformer inducing currents in the case that are affecting the tubes. I think.
On option is, of course, a two box solution with the transformer in a separate box. But, then again, single box builds seem to be working ok with respect to hum.
The result here is, therefore, inconclusive. So we don't know what your mileage may be on this.
2. Dropouts - the built in epsilon 12 is dropping out on some of the builds. This appears to be a sensitivity to line (mains) voltage variations and not the music. In some of the builds when there is a transient change of the AC line voltage this transient makes its way (I can explain later how) to the output as a few hundred mV of DC that trips the e12. Eventually the servo responds and resets the output offset to 0VDC.
But, during this time the music is interrupted.
The appearance of the issue is also inconclusive. That is, my build has never dropped out over many many hours of operation with my entire house running (Air conditioning, washer/dryer, etc.). OTOH, one of the builds seems to show more of this issue (in another part of the country). There is a third build that has traveled around the country with no dropouts and a fourth that is in Canada that has had a few, but not many.
Thus, this item is also inconclusive and we don't know what will happen in any specific build or location. We are also suspecting that a number of other amps may exhibit this behavior but just don't have an e12 to detect it. But, we really don't know for sure that this is the case.
These two issues have caused the prototype team to delay a release of the amp until a few more are built and tested. My position as the originator is that I don't want you guys to spend money and time on this unless it is very solid.
And, actually, it is pretty solid with just some concerns that are giving us a bit of a pause.
We would appreciate any of your thoughts.
Dr. Cavalli gained notoriety with his first DIY amplifier projects. His success has blossomed into Cavalli Audio, a world leader in amplifier design.