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.