One of the output 4-65a was actually a military JAN-8165. Wanted to test a pair of them but in the rush of building the amp I ended up with a mixed of the two. Not an orthodox approach buy who cares! After playing the amp extensively for a couple of weeks I noticed that this valve droped anode current after 1hour or more of playing. Anode current could go down by 10-15mA. Perhaps it has to do with the pin oxidation, but I also suspected on the filament regulator due to the heavy current required on this. The regulator heatsink gets really hot as you can imagine so I was already blaming on the regulator FET before I even suspected on this output valve.
A simple test was to replace this valve by a new NOS 4-65a. I did that and surprised to find that the filament regulator wasn’t to blame. The 4-65a SE filament stayed rock solid at 100mA after 1h30m of fantastic music. Yes, the bass of this SE is unique. It exceeded everything I previously listened to. Is the 46 driver in filament bias? Not sure yet, but hey ho. What an amp!
I knew this amp would work for you, Ale, and that, in the end, you would be pleased that you had built it. It is likely that the amp holds still more mysteries for you to uncover and solve. However, you now understand that, down the road, whenever additional puzzles are encountered, it will be well worth the time and effort spent to solve them. Again, congratulations on building and amazing amp. -Edward
Thanks Edward for the kind words. I’m discovering this amp with proper listening time. I’m sure there will be tweaks and modifications in the future, but now I just need a lot of music time with him to uncover what it can provide. As you can imagine, it generates a lot of heat. The room gets very warm after playing a couple of hours, but it will be better in winter!
With this amp I confirmed why designing the driver to perform the best is so important. The 46 in filament bias can swing easily 200-250Vpp with less than 0.1% and runs on 30mA. It can therefore deal with the input/miller capacitance of the 4-65a (gm is 4mA/V in triode-mode) and the FET gyrator load in mu-follower mode can source all current required for A2 peaks when the 4-65a grid becomes a low impedance load. A hard task for anyone!
No cathode or coupling caps in the direct path albeit the gyrator load has a 1uF cap between the anode and the FET gate which is biased by a high impedance CCS (LND150) through a 4M7 resistor. I thought that this cap should have a minimal impact on the sound given its position in the circuit but read on DIYAudio forum that someone who built a similar amp got different results with different caps. He suggested trying different version so I got a pair of good quality Mundorf oil caps which I will try at some point. Any thoughts?
Ale