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.
About 10 years ago I used this tetrode driver which I loved. The 24a. It has hungry filaments, albeit a lovely sound. I probably lost a few readers by now, but don’t care. The 24a into the 45 Single ended, was a great choice, no wonder why Torsten praised this valve.
I settled with the following driver which worked well for me. The 24a is carefully operated at 8mA and 80V on the screen to minimise distortion. This way you can get about 35.5dB (x60) gain stage with about 160Vpp output at 0.5% THD. Yes, more H3 component than a triode, but sounds really nice:
Simple and effective gain stage for a 2-stage amplifier. The output valve could be anything of your choice: 2A3, 45 or even 300B.
The hybrid mu-follower use R1 to set the gain of the tetrode stage (I covered this before many times) and the stable mu-output also feed the screen regulator (Reg 1). I made also a PCB for this regulator, so the driver footprint is very small.
About 5 years ago I built the first version of the Aa DHT preamp. It looked like this:
It’s always satisfying to see someone else enjoying the end result of their build work. Here’s a great example of Nash building his take on the 01a low gain DHT pre-amp:
Hi Ale, Its now over two months since I have been listening to your low gain version of the 01A preamp in concert with a Slagle AVC after it. Over a span of a few years I have built several preamps and I can honestly say that this one is substantially better than any of the others. As a classical music fan I value attributes like clarity, timbre, body, weight and speed and your design delivers! I would like to use the analogy of a superb cake with icing- the cake is the 01A pre and the icing is the AVC. Thank you for a great design! I have recently replaced RMu and the divider resistors with the TT Electronics PFC series SMD resistors mounted on a 1206 adapter. These are tantalum nitride resistors and they sound wonderful in this application. Further, I am getting interested in your 300B SE builds. Are you using a preamp before these? Please send me a link to your latest work in this area which you feel I should consider. Regards, Nash Bapasola in NJ
It’s been a long since I last posted here some project stuff. I’ve been busy with work and family. On my spare time, spent mostly listening to music, developing some synthesiser modules and playing music as well!
I missed ETF this year, sad to say. Work commitments made it impossible to travel to France. Hope that’s not the case next year as am keen to get back. I’ve seen some pictures and feedback and seems it was a blast despite the COVID restrictions.
On my exchange with Rod Coleman regarding this post, I asked him to send me a pair of his latest DHT filament regulator (version 9) to test them on my 801a preamp with a cathode bias resistor (degenerated or un-bypassed):
I took me a couple of years but eventually found the time to build the BOM and a simple documentation guide for the DHT LT supply PCB. I’ve been using this PCB for a long time on most of my DHT preamps and in my current 300B SE Amplifier.
It’s a 16 x 8 cm board which can be stacked for a stereo DHT stage. Enough flexible is provided for all sort of filtering combinations (either choke to cap input supplies).
Some may say is a killer, but I add a common-mode choke to suppress HF noise from mains. I design my supplies with a no-compromise thinking and best possible components (e.g. including split-bobbin main transformers), chokes and capacitors to deliver the lowest noise possible to the filament supply.
This board doesn’t replace the filament regulator (e.g. Rod Coleman regulator), it’s just the raw supply circuit needed before them.
I will sell the remainder of my stock of these lovely ENIG finished PCBs. You can order them here.
I added these PCB for sale. I made a small batch for my own use (you can see them in the 300B amplifier here) but am selling the rest of them.
A few years back I went down the path of exploring different DHT in pre-amp stages for sonic improvements on detail and dynamics. I explored all sorts of different DHTs as have posted in the blog and many more I never got around to write up about.
