When everything was going to plan…
This build became one of the quickest and eventually the most painful from all, perhaps not really. However, it was very challenging in the end. I will tell you why in more detail. Yet, it has been a fantastic learning experience.
The amplifier building experience was going swiftly. For someone who struggles to find some free time to work on the projects, the lockdown gave several nights of work. I rigourosly made progress and everything was working to plan, until the final test was made. I individually tested every stage and part of the amplifier and everything worked just fine.
However, when I made my final test and brought up the output HT via a VARIAC, I found that the amplifier driver stage / source follower oscillated! Argh, the worst nightmare came true. Very unpredictable behaviour and hard to pin down. Spent few nights without success. After several attempts to find the root cause, I reached out for help to Rod Coleman. Rod has always provided tons of advice and his experience is great. He’s also very helpful and ready to support where he can. That makes him so valuable in these situations.
The real culprit
Although there was a LF oscillation which made the -200V bias raw supply go crazy. It was puzzling to see the circuit to operate well when operated under a bench supply. We suspected on stray inductance and capacitances playing around and turning the SF into an oscillator.
So ended up adding extra bypass capacitors, changed the Source Follower tail CCS for a resistor load instead. Still after desperate changes and modifications, I was unable to isolate the source of oscillation.
Looking at the passive -200V supply. I ended up testing leakage currents to earth and found that I had the -200V at +80V from earth! Dissecting the supply, I sadly found the Mains transformer with a leakage short into the copper screen between secondary and primary which is connected to earth. Phew!
Luckily I had a similar transformer around, which made me compromise a bit the supply and voltage levels, which still deliver what I want, and voila. The amplifier was now working like a charm!
The amplifier is dead quiet, amazing. Flat response up to 80kHz which is hard to achieve without these OPTs. The hybrid mu-follower plus the source followers have a bandwidth of 3Mhz. At 37dB gain is a fantastic stage, but a devil one if you don’t take care of taming any source of potential wild oscillation due to the high gain and bandwidth.
I will post diagrams soon, for now will enjoy the sound of it after so much work.
I’m delighted with the sound so far. Strong bass, detail and transparency of treble and overall sound image is perfect. Glad to see the 300B back on my system.
This isn’t a build I’d recommend to get into, unless you’re prepared to live with fixed bias and ton of iron, etc. However, you need to take care of driving a 300B if you want to get the best sound out of it. That doesn’t come cheap and there is no free lunch of course!
Holy cow that is pretty! Looking forward to your schematic! Congratulations!
waiting for an overview of the schemetics and all the measurements……
It will take me a bit of time to go through proper measurements on THD, etc. For now I can say that it sounds fantastic.
Have a look at the attached frequency response with the oscilloscope. Nice to see it up to 80kHz!
Hi Ale,
Good to see your 300B project is doing well!
I’m also happy that the HF response is matching my measurements.
As a sick perfectionist, 80kHz is not a top HF bandwidth for a 300B SE output transformer, but this is the best overall performance I can get from this core.
I was wondering about the idea, if you would shoot-out different OPTs on your amplifier one day?
Best regards!
Hi Alexander
Yes, I’m very very happy with the results and the sound of this amplifier. I still need to do further measurements when I find the time.
At the moment my priorities include experimentation with the driver stage.
As you know, I have a few OPTs which could fit here from Amorphous Core MM to Lundahl’s. It may take me some time before could do this.
If you find in future a better core to improve this OPT, I’d be happy to try it myself.
Excellent job again, well done for the OPTs.
Cheers, Ale
Hi Ale,
Thanks, I will be waiting!
It will be worth testing vs the MMs.
I guess it’s not worth to improve this OPT further. It needs a bigger core, which is more expensive. Soundwise, materials are more important for the sound than HF response.
Thanks!
Best regards!
In one of your earlier posts named “300B Amps: an early breadboard” you had D1,D2,F1,R2 and you had mentioned: “… Should the fuse blow, R2 will force the valve to nearly cut-off and protect the valve and OPT.”
I don’t see that in your current 300B schematics. Are you not using it?
Hi Valentin, yes I’m using them and thankfully they’ve never come to action. For whatever reason I forgot to include them in the diagram. Although they are not mandatory, they are there for peace of mind.
Best, Ale
Thanks Ale, peace of mind sounds very good for my build as well. Thanks for clarifying.