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:

1. 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.
2. 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.
3. 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. 
4. 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. 

 

There are a few considerations to bear in mind when you set yourself to implement this circuit. This is a high-gain stage, so you need short and tight connections. Ferrite beads as well as stopper resistors. Otherwise this thing will oscillate like in a rave party.  You can see the build pictures for reference further down.

As you can see there is a delicate balance between the collector current and the output bias point to ensure maximum swing at large output levels. The stage was designed to

The cascode reference voltage is crucial. You ought to get this as stable as possible. The preferred option is a shunt regulator. The current consumption is minimal so it’s easy to implement. 

I opted for a quick and dirty approach to test the circuit using components I had at hand. This isn’t a regulator, it’s a voltage reference without feedback or regulation.

Let me explain briefly how it works. The reference voltage is provided through a resistor formed by R9 and R14. The CCS formed by M1 and RSET provides a stable current to generate the reference voltage. This voltage is fed into a PNP emitter follower (Q4) which has a cascoded configuration with Q5 to avoid stressing the follower. Q5 is fed from a 100V reference formed by R14. The emitter follower has a CCS on the tail (instead of a resistor) formed by Q3 and R12. R12 sets the CCS current which is about 4mA. Rset sets the reference voltage for the shunt cascode stage. It provides a stable voltage reference with minimum ripple thanks to the filtering of C3 and the capacitance multiplier effect of Q4. PSRR is about 100dB according to the simulation. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

If you want a more stable and sophisticated approach, you should opt for a shunt regulator or a series feedback regulator as well. 

The stability of this stage is dependent on both the CCS as well as the reference voltage on the shunt darlington pair. 

Build

Again, I was lucky enough to get 3 hours free of work on Friday and build this circuit on a prototype PCB. Here is how it looks like:

As said earlier, with a high-gain circuit like this, it’s imperative that connections are short, layout is well planned. I added ferrite beads to grids, anodes and collectors. This helps preventing any unwanted oscillations. 

If your target gain is considerable, then avoid point to point wiring. It will be a recipe for failure. Layout of this stage is critical, so a PCB is recommended.

Testing

It’s always a real challenge to test a high gain stage breadboard. Building this cascode stage isn’t straight forward and I wouldn’t recommend it to any beginner. Avoid this if you don’t know really what you’re doing.

You will have to set the reference voltage to 200V and then proceed to trim the CCS current whilst looking at the oscilloscope. Drive the stage hard to make sure you’re not clipping the output. Given the high gain, it’s crucial to set the collector voltage to the right level to avoid clipping when driving it at 150 or 200Vpp. 

Here’s a taste of the stage at 55V/155Vpp. Distortion is really low. The artefacts between 15kHz and 20kHz are from my bench. Probably the LED lighting system which is amplified greatly with the high-gain stage:

Funny enough, comparing results with Rod Coleman, it seems we both arrived to same distortion performance from different approaches to the same stage. I like that.

Should I go down this route?

In my opinion, unless you’re not looking to drive your amp with a single stage without having any buffer, then probably don’t go this route. The driver is more complex than other ones and requires some skills to build this (and get it stabilised as well). If you are flexible to accommodate a line-stage / buffer then Miller isn’t a problem to you. In that case, you can opt for some other alternatives.

Here is what I have tried myself with success.

My first and personally preferred option is the lovely 6j49p-DR.  

Look at the performance of this valve at 200Vpp:

Amazing, and it sounds brilliant too. I have recommended it to a friend who used it as fist stage of a guitar amp and he was delighted with. 

Secondly, albeit with lower gain, the triode-strapped 12HL7. This valve has generated a lot of noise in the DIYAudio community, and its reputation is well justified. Again, the performance at 200Vpp is better than the shunt cascode with a simple gyrator load:
One of my other favourite drivers it the brother of the 6Э5П. The 6Э6П has a nice H2 profile when driven at a full tilt:

As you can see, carefully chosen triode-strapped pentodes can do this job. You need to consider the Miller capacitance though, so the ability to drive the stage is crucial to avoid HF rolloff. 

What’s next?

I was abusing the stage until I accidentally touched the DN2540 tab with a crocodile clip and fried the FET. Well, shit happens as they say. I need to replace the CCS MOSFET before I can do further work. 

Surely this driver is on my list of test with the 300B SE amp.

Hope you enjoyed this post.

cheers

ale