Schematic and Final Comments
As promised, the schematic is posted with this thread.
For a project that started out as a simple development effort to ultimately build for real with a grandson in a few years, it quickly took on a life of its own, and as I have said, surprised me with the quality of reproduction it has returned. It is easy to see why these amplifiers have such a following, as they come off as the ultimate little engine that could.
Within the bounds of it's simplicity and non-high fidelity beginnings, it really is quite an impressive performer, being as impressive in the specs it produces, as what it delivers in the listening room. For those accustomed to the stock (or basically) stock performance of these amplifiers, the modified amplifier will show that we really do listen to specifications. It's just that with high caliber equipment, the differences are usually so small as to make discerning the difference more difficult. On this level however, the difference (for me anyway) between the stock and modified units represents a large quantum leap improvement. The physical appearance of the unit simply belies the sound it produces through capable speakers.
There will certainly be those who will want to experiment with the design, which is fine. But as experimentation relates to component values, every component value used was chosen for a specific reason to address a particular aspect of the design. Therefore, if your favorite grid stopper resistor for a 12AX7/6EU7 (for example) is a 240 ohm resistor, that's fine, but it will change the stability characteristics of the unit. Or maybe you think the output stage is running too hot. That's fine too, but power and distortion will suffer significantly. Maybe your favorite small signal audio tube is the 12AT7. Fine and dandy. But such a change will significantly lower open loop gain, and reduce circuit performance. Experimentation is great, as we all have our own preferences. But I would encourage those who wish to copy the design to listen to it first as offered, and let that sink in. Then try experimentation if you want.
For those who champion "voicing", that's fine too. But there was no effort of that kind involved with the development of the modification. This was a classic engineering exercise through and through, with appropriate components of today's typical good quality used to fill the various locations -- as surely as any manufacturer did with the components available to them back in the day for this type of exercise.
The resulting sound of the modification is high quality in nature, lacking any console "bloom", mid-bass kick, or rolled off high end. The unit presents a very uniform presentation across the audio bandwidth, with surprising bass capabilities, very smooth midrange, and an overall detail and is not hard to miss on material that is so recorded. Therefore, if you are not familiar with a truly flat presentation (that is, being more familiar with a colored sound presentation), you may want to use a preamp with controls to achieve the settings you favor. As for the unit itself, it is very uncolored, with a wonderfully balanced presentation throughout.
Finally, some may want to add a volume control which is fine, too. However, as with virtually all tube power amplifiers that use a triode input stage, the unit will perform best when driven from a constant low impedance source, which only an active preamplifier can provide (or if directly driven from a SS source).
Well, that's about that. This has been quite a departure from my usual projects. But I learn on each and every one of them, and this one was no different. I hope you have as much enjoyment out of this design as I am having!
Dave