Introduction

I’m a firm believer that if you don’t share, you don’t get back and learn. What the point of not sharing what you’ve learned? I asked myself the question again yesterday, just to push me a bit further. Joys of Easter break is that I have the time to sit down and write. At least for a little bit.

Here is the result of my quest of the years to find the best drivers for a SE amplifier. I’d been looking and experimenting with them in terms of best linearity at large volt swings (I mean large when I say 200Vpp), harmonic profile and most importantly the sound contribution.

Why should you bother? Well if you are in the DHT space (otherwise don’t bother reading further) and, unless you are building a 4P1L amplifier, the majority of the output valves require large volt swing. You also need good headroom. Therefore if the driver is clumsy, it will ruin your expensive project. Again, one of the reasons why people claim that their 300B sound bad. Achieving a driver which can perform 200Vpp effectively with minimal distortion and a decaying harmonic content isn’t a simple task.

In one of my recent post, I blogged an example of the GM70 amplifier. Look at the curves below and the demand to get all of the juice:

Yes, you can load it with a steeper load and use a 6KΩ instead of 10KΩ to get more power, but you still need the same volts to get the full swing.

The Driver Topology

I tried it (mostly) all over the years. Transformer coupled, choke loaded, resistor loaded. However, in my experience the best is the gyrator load. You may have a different view,  and so you may: well, it’s a free world and I’m not expecting you to agree with me. If you are prepared to accept my point for view, then you can continue reading this post 🙂

The hybrid mu-follower (aka gyrator load) is a very effective topology for a driver. You need sufficient volts at the supply, but that’s not generally a problem. You will need at least 25-50V more than the largest voltage swing. Most of the valves I will review below have a good compromise operating point at about 200V. For a 200Vpp or 250Vpp headroom, this means you need 200V+250V/2+50V = 375V. MOSFETs can work at this level and providing you put them the right heatsink size we’re on business!

Initially some years ago I explored the use of LEDs, diodes and particularly SiC diodes to bias the valves. However, I found later that a bit of cathode degeneration by placing and (unbypassed) resistor was a good choice. This linearise the valve a bit and won’t impact the output impedance of the driver. However, if the resistor is within a reasonable value (smaller than 300R I found in practice), the impact on the Frequency Response (FR) is manageable and also the reduction in gain of the stage.

I will present in this post my favourite contenders for the best drivers. These are:

1. 6e6p-dr: the brother of the famous 6e5p, but a real performer on his own
2. 6z49p-dr: an amazing pentode.
3. 6z52p: high-gm and mu pentode. A replacement for the E810F or D3a for some. This is a pentode that works brilliantly on all fronts. Even at low voltages (thanks JC Labs)
4. 6C45p: this little triode monkey oscillates widely, but can be tamed and works well, so don’t be afraid.
5. 6e5p:  this famous Russian beauty doesn’t need introduction
6. 12HL7: the sleeper and best discovery last year.

In all cases I found the sweet spot with fixed bias which allows me to dial-in the right operating point in conjunction with the gyrator setting point. Once the best performance was measured (and listened) it was replaced with an equivalent resistor and re-tested. A tedious job, but worth the efforts.

A note on oscillation

These valves have mostly high gm and gain. You’ve been warned. Don’t even attempt to build with them without special attention on the building aspects. It will oscillate, believe me. You should add grid, anode and screen stoppers. I prefer nice ferrite beads added straight to the socket pins. Continue reading “Driving hard (Part I)”