All about electronic valves and hi-fi
Brief post of a very long experiment. Unfortunately I’d have to dig out a long list of notes, tests and simulations to get the full thing out there. Unfortunately due to constraints, this will be brief but I hope it’d be entertaining.
Sadly, yesterday heard the sad news that our friend Jon Finlayson from the London Audio Circle passed away. Jon was an incredible person, a true music lover and passionate about life. He introduced me to most of the circle team members and enjoyed thoroughly the meeting at his place and listen to his electrostatic speakers and constantly evolving system.
I learnt such a great deal of stuff from him and will remember our conversations and how passionate and meticulous he was into doing things. In particular when I built my Starlight CD player and he helped me out with the DAC ladder resistor matching.
See you on the other side Jon.
The 811a amplifierThe last year of the pandemic allowed me to do some experiments whilst being more at home. I hooked up an 811a on the bench with the following circuit:
I reused my original D3a driver from the 300B SE Amplifier. That was an easy choice and speed up my building process. I added a PMOS driver for a “plate to grid” feedback. The feedback network was somehow tricky to trim as had limited resistors available but the combination of 2 39KΩ resistors and a 18KΩ resistor worked well. The follower helped with driving the positive grid current which is about 16mA on idle at 22.5V bias or so. The bias is actually adjusted with a positive regulator set to about 50V on the PMOS gate.
I ended up building a PCB for the PMOS driver, very simple but effective. I used an SMPS for the filament supply and was good enough for testing purposes.
The iron was some Lundahl gapped at 90mA which I had at hand, but can be anything you want.
Power? Oh yes, could extract 16W at about 1.5% and think measured 18W before clipping. Distortion at low power (e.g. 2W) was less than 0.4% so a nice beauty to listen to. I love the sound I got out of it, not my normal listening system and couldn’t move the whole Frankenstein down to where my speakers are with the kids around and 600V all over it (long are gone those days where 600V were everywhere in my music cabinet).
Long time ago I breadboarded the 1624 valve and liked the sound. The 807 is famous and I never ended up with a permanent system with this valve. Yet, I like it. And if you have them at hand, use them. Even for guitar amps are amazing.
