On the last post, I shared this great circuit. Now, with the Holiday season and being locked down I somehow find the time to build it. A very quick process as it’s all modular. I’ve got all PCBs that can be interconnected like LEGO, that’s what I have always in mind when I design a new PCB.
I had already built a Screen Bias PCB, which was fully populated for a stereo setup. I’m only using one side of the board. A bit of a waste, but the board was ready available. On the back you can see the Source Follower. On top of the Screen Bias board there is the output coupling capacitor. I added two 4-pin turret boards on each side to facilitate the connections between the two daughter PCBs. The one on the top (right) holds the EF37a and under the octal socket you have 2 4-pin turret boards with the SiC diodes and the feedback resistors. On the back there is space for a filament regulator (although I’m planning to run this one with AC), the mini CCS board and connecting turrets:
The previous picture I think explains it all. Here is the assembled unit:
Let’s see how this little unit behaves. I was surprised how stable it is given it has 2 CCS fighting with each other (the CCS anode load and the pentode). The screen feedback works really well to stabilise the unit and loading the mu-output is pretty handy.
Here is a snapshot on its performance:
The stage can do below 0.3% for 100Vpp. Mainly H2 dominated and the higher harmonics only appear when you’re pushing it to over 150Vpp.
Nice to see it can do nearly 50kHz with a gain of slightly over 40dB. I think it’s a very nice stage.
Yes, there are better pentodes than the EF37a, but I love this one!
More to be done…