This is a topic that rears its head periodically on vacuum tube forums and discussions. Just a couple of days ago I was reading a forum thread on preamps and someone threw out the suggestion of using a triode strapped 6V6 as a preamp tube. Now this is nothing new. I’ve been hearing this suggestion, on and off, for at least ten years. But using a triode strapped 6v6 (or any beam power tube) as a voltage amplifier is a poor idea which simply doesn’t hold up to technical scrutiny.
Compared to true power triodes, the triode strapped beam power tubes just don’t stand up as voltage amplifiers. The only place where this is not the case is Miller capacitance. The additional screening provided by the screen grid keeps grid to plate capacitance fairly low; about 0.7pf for the 6V6 and about 0.4 for the 6AQ5. At low gain levels this keeps Miller capacitance low. If I were looking for a power tube driver at RF frequencies, the 6V6 would be on my short list. But as an audio gain stage it is clearly inferior to many other options.
This idea of the triode strapped 6V6 as a preamp is superficially understandable. Designers looking for lower gain and lower output impedance are drawn to the tube. Especially those hoping to be able to drive both vacuum tube and solid state amplifiers. And at first glance, it looks like it should fit the bill nicely. Where problems really start to arise is when you attempt to use it to drive low impedance loads.
To illustrate these differences I formulated two complete low gain designs and attempted to keep the operational characteristics as close as reasonably possible. One of these is for the 6V6 (triode strapped) obviously. For the other I chose a small low µ triode that is both cheap and plentiful, the 12B4.
First I present the 6V6 gain stage design.
There is a lot going on in the load line design. This is a design with B+ set at 300v and a load resistor of 10kΩ. It uses a 500Ω cathode bias resistor and is biased at Vp=133v, Ip=16.5mA, Eco = -8.3v. The first set of data contained in the blue box is for the design with a 100kΩ output resistor.
The data listed shows the gain to be about 17dB with distortion at about 2% for a 3.5v-rms input. The stage output impedance is about 2.2kΩ. However as I said above, it’s driving low impedance loads where problems occur. The orange box contains the operational data for the same stage with an effective output resistance of 10kΩ. This is to simulate driving a typical solid state amplifier. Here things aren’t so well behaved.
With the low impedance load the distortion jumps to 4.5% for the same input. Not only this but the low frequency performance gets worse. The gain also drops to a little over 15dB with this load. Still, if looking at the stage in isolation I can understand how someone might want to try it. However, before going off and building a prototype, maybe we should look at the other design.
This design is for a small power triode, the 12B4. This is one of those television tubes designed primarily as a vertical deflection amplifier. Not only does it have a lower µ and plate resistance than the triode strapped 6V6, it is a very robust tube. Here is the load line design.
As before, this is a design with B+ set at 300v and it uses the same load resistor of 10kΩ. It uses a 2kΩ cathode bias resistor and is biased at Vp=158v, Ip=11.75mA, Eco = -23.7v. Also as before, the first set of data contained in the blue box is for the design with a 100kΩ output resistor.
So let’s explore how this triode stacks up against the 6V6. First off gain is about 14dB so about 3dB lower than with the 6V6. This may or may not be a concern as usually preamps have too much verses too little gain. This stage has much lower distortion being about 40% lower than the 6V6. The output impedance of this stage is also about 35% lower than the 6V6 stage, 1.46kΩ vs 2.21kΩ. So at first glance, this stage should be much better at driving low impedance loads.
Shifting to the case with the 10kΩ load resistor some things become apparent. Distortion increases as before going to 7.3% with a 7.1v-rms input, however this is still lower than the 6V6 stage. It is important to remember that the figures for the 6V6 stage were quoted at a 5v peak swing on the input. The numbers on the 12B4 stage are for a 10v peak input swing. To directly compare the distortion numbers one must either double the 6v6 numbers or halve the 12B4 numbers.
The loss in gain is only about 1dB going from a high impedance to low impedance load. This means that the stage behaves much closer across differing loads than the 6V6 stage. As expected, the Miller capacitance of the triode is significantly greater than the 6V6 however even the triode levels are not a problem for audio frequencies.
The last thing I would like to address is how the stages compare in overdrive characteristics. The 6V6 stage will go into grid conduction at about 8.3v. This will be abrupt and generate lots of odd order distortion. The 12B4 stage will go cutoff before ever seeing grid conduction. Compression will be more gradual and will add even order distortion. It can also reasonable handle twice the input voltage swing than the 6V6 stage. This is a significantly more desirable overdrive characteristic than the grid conduction case. It could be argued that one could increase the 6V6 bias voltage to get lower on the load line but this invites added distortion on a fixed load line. The the stage distortion is already almost twice the 12B4 stage.
Conclusion
I am not suggesting that you can’t or shouldn’t build a low gain 6V6 stage. Instead I am suggesting that given a set of requirements, there are better ways to build the stage and better tubes with which it can be accomplished. My hope is that this simple example will illustrate this fact.
I usually attempt to stress making good design decisions before attempting to build something, and that’s all I’m doing here. It’s easy to just throw out some idea in a forum or email discussion. But it’s not that much more difficult to look at some alternatives and potentially arrive at a better performing and a well thought out design. I’m just suggesting taking that small additional step before moving forward.
As always, questions and comments are welcome.