300b driver experiments

I’ve been enjoying and carefully listening my new 300B amplifier. I have to say that I love every bit of its sound, treble detail and strong bass. The amplifier is fast and can drive very well my speakers. I only discovered that due to my low level DAC, the gain of the D3a in triode is yet not enough to get it to maximum power. So, I hooked in my beloved 01a preamp. The overall gain is too much of course so had to place the volume control at the output of the 01a stage.

I think a gain of about 130-140 should be ok. Perhaps if I get around in adding the 6SF5 stage then it may be good enough.

So this got me thinking. Of course I have on my list 2 driver tests:

1. 4P1L pentode – yes, pentode!
2. 46/47 DHT with a SUT. I could do a 1:4 gain step up and drive it with the 01a. Need to double check the capacitive load if it’s too much or not for the 01a. The resulting gain will be around 150.

I’d like to move back to 2 stages though. So will try the above anyway, that’s the purpose of this latest modular amplifier build.

I immediately turned into the folded cascode (aka “shunt cascode”) topology which Rod Coleman introduced me to years ago. If you haven’t read about it, please read this post, this and this one.

Challenge is for me that if I’d want to add a 6e5p in here, will mean about 40mA or more of additional current, which would require a rework of the PS. This means more work.

I looked at the D3a or even the E810F instead. Challenge is that this topology is able to deliver higher gain than μ (which is what I need), however this is only possible at low distortion levels when the valve is operated at high current / gm. This means you need to operate the valve preferably at lower anode voltages and higher currents. This is counterproductive when you want many volts to swing. Therefore some of the frame-grid pentodes with good dissipation can work well. That is why the 6e5p is a good candidate.

I recently built a PCB with folded cascode and other small circuits for pentode drivers which I’m intending to use in phono stage experiments. The PCB is the same size as my HV LED voltage reference so can be stacked on top as per below:

Swinging 200Vpp at very low distortion is not easy. The D3a was a bit dissapointing on my tests so I moved onto the E810F. It has a higher gm so works better here. I tried the first setup like this:

E810F folded cascode. Not that great performance at 100Vpp

And the results aren’t as great as expected. Gain is great however, the 0.4% THD can be easily improved by many simple triode-connected contenders at 100Vpp.

If you want to do better, we need to operate the E810F at higher current, which turns out lower voltage to keep within the safe power dissipation levels. The below is a good compromise:

49dB driver stage! This can do 0.2% at 150Vpp but that’s about its maximum swing.

Gain turns out to be too much. If you want to lower R1, you need more current. Again means you need to watch for the ZTX558 dissipation limits or change the transistor. All of these means more current in the CCS which defeat my objective. Mind you that the attached is very similar to my phono stage and performs amazingly at low level signals. I’m currently working on a new phono stage and likely reuse this topology.

Nice to have been diverted my attention on this. You learn a lot from experimentation after simulating in Spice. No doubt. I will go back to my original plans now.

 

And yes, will make the time to do a proper writeup of the 300B amp. I need to get around to measure it in more detail. Even I still haven’t put in the EML300B valves! Still running with the chinese ones for testing.