300B/845 Amp: Driver Options

For more than 6 months I’ve been playing with a lot of satisfaction my 300B amplifier with the 47 driver. Since then, I’ve made 2 modifications:

1. Major one was the introduction of my Open-Baffle (OB) speakers. These are the “Liionidas” based on the LiiAudio C-15 and W-15 drivers. I need to do a proper write-up of these, but have to admit their sound has taken my system to another level. They are simply superb. Detail and dynamics are beyond expectations. Nothing less to be expected from the designer (Leon Huijgen) :

1. Second modification was minor, I removed the Zobel network from the input LL7903 to minimise loading to the source. Also, after several tests I found that the impact in the HF resonant peak is minor as the source has some resistance (below 100Ω) which is beneficial. So the 300B amplifier looks like this now:

I could live forever with this 2-stage system now. It’s capable of driving the OB speakers without an issue. Although for larger rooms you better go with +10W and the 845 in the cooking process is much better suited. I don’t play too loud and the 300B doesn’t let me down at all when playing it loud, I’m in love with it.

Yet, I’m missing the thoriated tungsten sound of the 801a/VT25 or 01a in my system. Recently I posted about this low gain DHT preamp stage based on the 01a, which can be a good option, however introduces another stage.

I’ve been testing on the bench a couple of drivers based on 801a and VT25. They are different valves but not too far in terms of sound in my opinion. Here are the two flavours:

First one is old school design with 1:1 interstage transformer (custom made) and LL7903 in 1:8 SUT to deliver the 35dB of gain I was aiming at. The drawback of this design is the need for a negative supply. However I have already one for the output stage (either 300B or 845) so easy to reuse. The benefit is that the HT supply can be as low as 250V. I’m adding a capacitor multiplier to dial the right anode voltage and provide a low noise HT rail due to the poorer PSR of the transformer loaded driver.

The performance is magnificent. See below test for an RCA 801a delivering 200Vpp at 0.1% THD. The harmonic profile is very nice from H2 to H5:

Now, here is the VT-25 driver based on my previous designs:

The hybrid mu-follower uses a 1kΩ cathode resistor instead of filament bias. This reduces heat dissipation and raw PS voltage requirements, making it easier to implement. Soundwise after my similar implementations with the 46 and 47 isn’t compromised at all. By running it at 15mA and 265V (15V bias) the stage can deliver 200Vpp at low distortion (below 0.15%). The gyrator PCB is the mini version and overall driver physical footprint is quite small. Drawback of this design is the need for a higher HT (400V instead of 250V), however it’s significantly cheaper unless you have a very good IT at hand to reuse.

Both are great options and will be tested side by side. Very likely I will implement the latter one in my 300B amplifier as it’s easier to swap the current 47 driver and introduce this one. Also, can adjust it for the 801a and compare the differences. Good fun!