Introduction
The main challenge when implementing valve amplifiers using transmitting valves or valves which require a significant voltage swing (e.g. 300B, 45, etc.) is the driver. Getting the driver right is not easy. You’re asking for a single stage to swing 150 to 200Vpp at minimum distortion. There are some ways you can achieve this:
- Implementing 2 stage voltage amplification. Here is where we find a lot of bad designs and poor results. Sometimes the 300B gets a bad reputation due to a wimpy or poor driver. Many designs out there use 2 stages of 6SN7 for example. Nothing wrong about using the 6SN7, however when you cascade 2 stages the sound is muddled at low levels. Harmonic profiles may be encouraging but they simply don’t sound great.
- Implementing a high-mu driver stage. There are several high-mu drivers out there than can swing plenty of volts. 6Э5П, 6Э6П, 6j52P, 6j49p-DR, E280F, C3g, etc. They work well, specially if you couple them with a gyrator, you can achieve hi gain. If you opt for degenerating the cathode resistor, the gyrator still provides a low output impedance to avoid degrading it due to the degeneration resistor. I’m a big fan of this approach. The only disadvantage is that you need a buffer/line-stage capable of driving the Miller capacitance. I have a nice preamp/line stage so this isn’t a problem to me.
- Implementing a pentode driver. Pentode don’t suffer from Miller capacitance. However, you need to find the right driver, not all sound well in my experience. I like the 4P1L and C3g. You can use a gyrator load with pentodes as well. Some folks complain about the pentode harmonic signature. I think this is a question of personal taste.
- Implementing a shunt cascode driver. Hey, this is what this post is about! There are several benefits already discussed at length on this topology. If you need high gain and minimum capacitance load (e.g. Miller) as you have a DAC output for example, this is what you should look into. The Shunt Cascode operates the triode in a vertical load line (not horizontal like the CCS or gyrator).
Design
You should start by reading this extensive blog post. That will provide you with a lot of information around the shunt cascode and how it works. Back in 2013 I started playing with the 6Э5П in this topology. It was quite promising. Now, I have revisited and built this driver to see how it really performed.
The design is very similar to what we discussed back then. I shall proceed in describing the circuit, in particular the changes made. The driver is still the marvellous 6Э5П. There are few valves out there that I don’t like as much as I do with the 6Э5П. I measured the curves long time ago when I started with the curve tracer project. I also tested the 6Э5П and 6Э6П extensively. I do love the 6Э6П as well, it’s one of my favourite drivers.
The 6Э5П is biased at about 200V/30mA with a degeneration cathode resistor of 120Ω. As the gain of this stage isn’t dependent on the μ of the valve, then is good to do this to improve the linearity of the driver. M2 forms a CCS with Rmu. It provides the current to the 6Э5П as well as the current to the common base stage formed by Q1 and Q2. The gain of this stage is gm times R5. The gm is the valve’s transconductance The collector current of the MPSA92 is kept low to ensure distortion is minimised as well as its operated under SOA. D3 provides a protection to the darlington pair when is reversed biased.
The gain of this stage was measured to be x140 (or 43dB). That equals to a degenerated transconductance of 5mA/V with a cathode resistor of 120Ω and a gain resistor for 27kΩ.
