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!
After enjoying the 4-65a SE amplifier for many months, I couldn’t resist myself from upgrading the output stage to the 814s. I just needed changing sockets and filament raw supply transformers to fit the requirements of this lovely transmitting valve. Needless to say, my recent tests on 814s were very encouraging. The 814 seemed to perform much better than the 4-65a in delivering 10W of class A2 sound at half the distortion levels. This to me, was only worth trying.
