After a few exchanges with different people, it was time to make a few tweaks to the EF37a driver. Increasing the G3 voltage helps in squaring up the pentode curves as shown by few before. By increasing G3 to +12V or up to 20-25V can be beneficial in the long voltage swing of the pentode. There are a couple of interesting threads in DIYaudio to check out if you’re interested.
I should have fired up the eTracer and play with G3 biasing before modding the driver, but couldn’t help myself from doing this mod as it was very simple. The Screen voltage circuit I use has 2 identical versions of it in the PCB as it was designed for a stereo setup in mind. As I reused a PCB I built for other tests, I had a readily spare screen voltage regulator which could be easily tweaked to supply G3 at the levels needed:
To ensure I could deliver 10-20V range at G3 with same PCB, I operated the G3 grid from the G2 grid bias circuit which provides a 112V output. This actually gives me good adjust range on G3. Now the fun part was to experiment different G3 voltages looking at the FFT plot at maximum voltage swing (i.e. 200Vpp in this case).
It was interesting to see the following:
- Above 14-15V, the distortion decay profile was the best. Progressive reduction from H2 to H6. However, the distortion was higher. Overall THD up to 0.58%
- Reducing from 14V to 13V, you can get a predominantly higher H2 & H3, whilst H4-H6 reduces down to an even level. Overall THD is lowered
- Finally when you get down to 12V, things look probably the best in terms of overall THD. Odd harmonics get slightly lowered when even are a tad higher. You can see the profile below. Overall THD comes down to 0.47%. Nearly 0.1% improvement thanks to 1.3dB reduction in H2 and about 7-8dB on the rest of the harmonics. Good stuff.
This is now looking better. Mind you that the performance of this stage between 100Vpp and 150Vpp is extremely good with overall distortion below 0.2%. I’m just pushing it to the limit to see how it can be optimised.
Now need to move on to the rest of the mono amplifier. It will be fun to listen to this as well.
Hi Ale,
I am using EF37A input on an 801A A2 design, I think you’ve come across it before, will be undergoing a final bench test soon before the build.
I’m interested in adding some positive voltage to g3 to improve H2/H3 ratio during prototype. I had thought a simple way might be to add a battery between g3 and the cathode.
I tried it out with a common 9V battery in my own etracer here: https://www.diyaudio.com/forums/tubes-valves/357559-corona-ultra-low-distortion-a2-dht-se-amp-prototype-post6592457.html
This is with 105Vg2. As I said in the post, current draw was 5µA. Improvement seems most significant at 0 to -2Vg1 with lower grid voltages developing a “tetrode kink”. Will be interesting to see the effect in circuit.
Do you see any pitfalls to using a battery, other than longevity? Thanks.
Hi! I found it easy to derive g3 dc from the screen supply. I see no issues with the battery approach other than the inconvenience. I didn’t see much improvement in practice I have to say, I think it’s more relevant for an output stage (e.g. EL52, GU-50, etc.).
I battled for some time with LF instability of this final amp. I used the EF37a as a transconductance driver into a 6P36S. Then feedback from anode to cathode of the EF37a (SiC diode plus resistor). Performance and sound was excellent, however couldn’t get around fixing the pole issues which led to the instability. Annoyingly at 1.5 to 3Hz peak, hard to track down. I ran out of time and postponed the project for later.
Very interesting and learning exercise though, had JC Morrison exchanging and advising a lot with this amp. It was fun and hope I can get around it at some point.
Cheers
Ale
Hi Ale – thanks for getting back. Yes, understood, after tracing the curves I do have to wonder if there will be an improvement given my load and bias point, but never hurts to try. I did briefly implement JC Morrison’s g2 DC feedback circuit by your suggestion. It worked but had stability issues in my circuit, ended up going a different direction so did not get to troubleshooting. Hope you find time to revisit your circuit, good luck and thanks for your input.
Please don’t talk to me about “old school” design with all of the new school shit in the circuit. I’m 81 years and every time I get to your website I’m learning I need: a capacitance multiplier, or a source follower, or a fixed … something. I’m about ready to say Ef it and please give me a simple 801A/VT-62 circuit with a Tango NC-20 IT to drive a 300B..
Hi Robert, no point in reinventing the wheel for me. Only interesting in exploring other avenues. you don’t need to do what I say for sure. You are welcome to unsubscribe and build a traditional known design. Cheers, Ale