Very beautiful work! Very attractive.
Considering how much work you are putting into making these into an exceptionally attractive pair of monoblocks, it is worth mentioning that a LOT can be done to make the fidelity match the excellent appearances. (Unless you modified some of these issues?)
You are giving up quite a lot of fidelity by rebuilding to the original design schematic- Magnavox very much hobbled this amplifier to work within the constraints of the phonograph source and speaker it was paired with- rolling off the high end dramatically to limit dust noise from the ceramic phono cartridge, softening the harsh midrange, and giving up a lot of bass performance. To employ these as high fidelity stand alone amplifiers that work satisfactorily with modern line level sources and high fidelity speakers some aspects of the design really needs to be modernized- not only to ensure adequate fidelity, but to ensure adequate basic stability as well as transient performance with the wide range of possible external speakers, and even unloaded condition.
The bass performance is easy- the 6V6 cathode bypass capacitor C106D, 20uF is rather small, used by Magnavox because higher value capacitors were large and expensive, and also because the thin paper tiny magnet speaker they used could not handle the low frequencies that modern speakers can. With only 20uF across 270 ohms, the 3dB point is 30Hz, but 3dB is the half amplitude point so it starts to roll off below 60Hz. You can easily increase this capacitor- I typically use 100uF @ 100V at least, 220 is common, even 1000uF does not hurt, as long as you use a 50V capacitor or higher. The output transformers on these look pretty decent in size, so this one change could really add a lot to the bottom end.
The high end roll-off is more difficult. Using a global negative feedback loop with C103 (0.047uF for the BB) in series with R106, means that for bass frequencies, the amp is essentially running at full open loop gain. Normally this would result in an amplifier that was painfully sensitive, but the ceramic phono cartridges put out somewhat RIAA corrected audio at nearly line-level amplitude, so I'm guessing these monoblocks are not overly sensitive as is. With the C103 in series, global negative feedback only kicks in at higher frequencies, acting to then reduce the overall gain from mid-frequencies on up, meaning it will have a steep roll-off across the upper frequencies required to be considered anything near high fidelity. Additionally, there is a low pass (high cut) filter of R116 and C110 at the cathode of V101 with a 10kHz roll-off. These effects combined likely rolls off everything above midrange in an unsatifying manner. That is not necessarily a show stopper- My hearing rolls off at 15kHz, and I have immensly enjoyed germanium amplifiers that roll off badly above 8Khz, but in tube amps, the sparkling shimmering high frequencies are almost always a commentable aspect of perforance. It's a shame to leave that on the table.
To correct the high frequency response these various frequency limiters would have to be removed, the open loop gain of the amplifier characterized, and a proper global feedback network installed and tuned through measurements to give the desired final closed loop gain, flat bandwidth, and transient stability. This wheel has almost certainly already been invented. It would be worth seeing what others have done with tuning this on the AMP169- I'll poke about and see if I can find anything, otherwise it would be good to solicit feedback on suggested alterations. Without being able to do these measurements directly, it would be impossible to suggest the correct values.
View attachment 3699679
