Goodbye Jon
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 amplifier
The 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).
I loved the sound of 811A in A2 mode, I just local cathode feedback. I will send the schematic in a separate email.
The 811A amp was the prelude to the 40W 805 amp
thx
Thanks Sridhar, I’m sure the cathode feedback arrangement sounded great. Having the 6e5P as driver is also a nice choice from my experience.
Take care
Ale
Good to see you experimenting more with nFB Ale. It surely must be time to try the Blohbaum MTA design. If nothing else it is considerably simpler than your posted design. The bias is also self adjusting as the valves age–a big plus. At a recent listening session at mine with my Khorns the MTA UCL11 3 watt amp came out tops.
Go on….just do it…you won’t be disappointed…..
Rugby season has started in the Southern Hemisphere
Hey Tim
Sorry for late reply, I was down all week due to COVID. Thankfully, on the mend now.
I heard a few contraptions on Frank’s MTA which sounded nice. However, I’m on the path of exploring further the local nFB, not global or at least the least stages possible in the FB loop.
Not sure about complexity though, the MTA design will involve similar structure and if you want to drive effectively the positive current on the grid the follower will be needed. Anyhow, at some point would like to experiment and do some comparisons. unfortunately time is limited!
Enjoyed the Grand Slam last night and well done for France!
Cheers
Ale
Hi Ale,
I’ve just seen your reply.
The MTA circuit easily encompasses a cathode follower to drive A2 grid current.
Frank has several designs in Jan’s LA vol 8 showing how to do it.
It is worth the couple of euro to buy the article
Although not in LA Frank uses a CF to drive a 300B
Everyone (well all valve guys) says that gNFB sounds bad only because that is what they have read.
Most haven’t even tried gNFB
In my Sound Practice days I believed that too but I’ve found out that it is wrong.
Athough Frank uses gNFB (how much global do you get with a 470K resistor?) much is local FB and current FB.
By all means explore your own path but don’t ignore those who have gone before just because of the “dirty” global word.
t
Hey Tim
I’ve got nothing against gNFB. In fact, my last amp work was for a mono amplifier with the EF37a using Frank’s similar screen feedback arrangement like in “best pentode” driving a 6P36S and feedback loop into driver cathode. It was a daunting project as got stuck for a while with an LF oscillation issue due to pole interaction between the screen feedback and the cathode one plus the output stage. I had several exchanges with J C Morrison who helped me a lot and yet, the breadboard instability was never fully resolved. I managed to crank the poor 6P36S and get 7W at 0.3% with a lovely H2-H7 profile. sounded great.
I have all the LA issues and was sad to see that Jan decided to stop the project, I learned (and am still learning) so much by reading that publication!
Take care
Ale