Driving hard (Part I)

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.

The amplifier case study

The example amplifier was presented before, so I won’t discuss it again. The driver in this case is a triode-strapped pentode (unless is a triode itself). I’m omitting the screen and anode stoppers. The value of the degeneration cathode resistor (Rk) and the anode operating conditions (Vak/Ia) are discussed in each driver section. The overall diagram is like this. The same circuit topology can be used for a 45, 2A3, 50, 300B, 211, 845, 813, GK-71 and other single ended amps:

6Э6П-ДР (6e6p-dr) driver

You can search around my blog site for information on the 6e6p-dr. This was an amazing discovery years ago when I was using the 6e5p. This little brother is one of the best performers out there. Just look at the performance for 150 or 230Vpp.

With a gain of 30.5dB (x33.5) it will need 6Vpp input to take the GM70 at full tilt so will just be ok if you have an 2Vrms source. However, it can drive the 300B, 45, 2A3 to max power easily, so it’s my preferred choice for those amps.

6Z49P-DR driver

I was lucky enough years ago to discover this valve. A great one for a phono/RIAA stage, a microphone preamp, a guitar first stage preamp valve. It sounds beautiful and if you get the 6Z49P-DR with golden pins, much better. This is another super-linear valve when triode strapped. Lower gm and gain than the others, but very good anyway. I prefer this for a high gain stage in a phono stage or when the signal level is low. If your source is 2Vrms, you will drive in excess this stage (i.e. grid current)  if you’re not careful. Look at the bias point.

Yes, I’m running the valve above its maximum Vak, but at 200V it works fine. The gain is good but the valve will struggle to drive at full tilt the GM70. It does in the other hand a great job in driving a 300B, 2A3, 45, etc.

6Z52P driver

This is another marvellous Russian valve. You can read a lot about it on the web. What I found is that its parameters are all over the place, large variance. Also you need to connect both cathode pins (1 and 3) to avoid oscillation. It’s a hard beast to tame given the high gm it has. Stoppers everywhere or you will have a wild horse running around.

Also you need to test them at full swing for linearity. I found one of my batches to have high distortion at full swing. Here is what you get on an average valve:

It can do 200Vpp with 0.28% distortion but harmonics start to creep up more. This valve can definitely swing large volts and can drive the GM70 at full tilt with 3Vpp input given the gain of 67-70 with slight cathode degeneration. Only challenge is that you need to run it hot at 40mA.

With stacked power supplies you could drive the GM70 with DC coupling and be able to handle the slew rate current required by the GM70 grid. A bit complex, I’ve been there, have done that!

6C45P driver

Criticised by some, this is a lovely triode. Another wild beast with its 45mS transconductance.  I pushed it over the 150V and works fine:

It has a very nice harmonic decay with only H2 and H3 showing up at 200Vpp. With 32.5dB gain degenerated, can drive the GM70 just like the 6e6p-dr and a bit more.

Other drivers

I ran out of time today, and couldn’t cover some other great valves which are amongst my favourite ones:

1. 12HL7: can do easily 70Vrms output at 0.24%
2. 6e5p: similar to the 6e6p-dr, but not as good: 200Vpp @ 0.3%

I will continue in Part II.