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…
nice work, man! the screen voltage can greatly affect the total swing before clipping and the harmonic spectrum… with pentodes the gain generally increases with screen voltage, but so does odd order harmonic distortion. every pentode has a handful of sweet spots for every operating point, because of this it can be worth your while to feel around a bit. i find a good start for bigger swings is a screen voltage around half the plate voltage. but for preamp use i will go a bit closer to the plate for more gain/gm. the current source loading will always give a triode like spectrum to begin with, but as soon as the diode line comes into play there will be blood. you really hang on to the mu follower circuit. i appreciate that. i never saw it as elegant… or thought it was better than an optimized buffer. but yours has really evolved! that little bit of compliance is everything! keep up the great work!
Thanks JC, much appreciate your feedback and insight, you should be writing a book on this stuff!
I figured out whilst adjusting the screen voltage looking at the FFT the behaviour on the odd (H3 primarily) distortion. There is a sweet point indeed and a trade-off when looking for gain. This is the point where I think higher-gm pentodes will perform better than the EF37a. However, I’m in love with this pre-WWII pentode and used it a lot before. In this case the EF37a is running at 235V to give enough headroom (when operated with +400V at the CCS) whilst the screen voltage was reduced down to 112V so exactly as you say. I tuned this by looking the FFT plot at 200Vpp swing.
I need to experiment more, I guess as you say the topology is perfect for a preamp.
Have a Merry Christmas!