Triode versus Pentode

[Ricky]

Hi all.

I have a question. Many people tell me that triodes sound better than pentodes and then tell me that they use EL34's or KT88's in triode mode. I'm now confused because EL34 and KT88 are not triodes. Wouldn't it be better to use a pure triode?.

Perhaps someone can educate me on the 'pro's and con's' of Triodes and Pentodes.

Cheers.

Ricky.

[Shoog]

Getting power triodes with reasonable gain and wattage is difficult and tends to be very expensive. The cheaper and easier path is to triode strap some high power pentode. The gain tends to be higher than with power triodes and this makes the circuit design easier.

It is my instinct that pure triodes will outperform triode strapped pentodes, but this is just from my very limited experience.

Shoog

[bigwill]

Quick run-down on each mode:

Pentode mode: Not very linear, high output impedance, lots of gain, highest power/efficiency, easy to drive


Ultralinear mode: Quite linear, almost as much power available as pentode mode medium output impedance, quite a lot of gain, easy to drive


Triode mode: Very linear, only about 1/3rd the power of pentode mode lowest output impedance, not much gain, harder / hard to drive due to high input capacitance and low gain. Triode strapped pentodes somewhat easier to drive than pure triodes.

Extra power can probably be claimed by driving the grids positive in triode mode though.

[dhaen]

The distortion spectra of each mode is quite different too.

[Ricky]

Hi all.

Firstly, I should like to thank everyone so far and future participants in this thread for the invalueble response. I have friends in the HiFi retail trade that sell these amps and the schools of thought are mixed. On the one hand, One dealer said that he expects, from a high end amplifier, between 15 and 40 watts, single ended design and NO pentodes. Won't entertain them in his shop. Another loves the sound of parallel push-pull EL84's in pentode mode. Says the sound is more convincing. This man's staple diet is Wilson Audio and Audio Research.

It does get confusing, doesn't it?

Ricky.

[quinnling]

Hi Ricky,

It would make sense to listen to the various types of amp with the different configuration. You can develop you own taste of what you like and don't like. For instance, I prefer amps with a pentode gain stage with a triode output stage. It can be a matter of taste, and the speakers you use.

Try it and see if you like it... that is the fun of this hobby.

Quinnling

[Sherman]

I built a single ended EL84 stereo amp wired in pentode mode. It produced about 4.5 watts and I thought it sounded OK. After a week or so I changed it to include a switch so that I could run it in triode mode or pentode mode (my OPTs didn't have UL taps so I couldn't wire that in also).

The first listen after the change was in pentode mode. Again it sounded OK. I powered off and switched to triode mode and played the same music again. Triode mode definitely sounded better.

I think comparing the same amp in pentode, triode and if your OPTs allow it UL is a good exercise and really highlights the differences. For hifi I think triode or UL is the way to go for SE amps but I have also built SE EL84 guitar amps and pentode mode works great there.

[Ricky]

Hi all.

Ok. Here's what I'd like to do. I want to design a Monobloc. 40 watts. Single ended. Would like to use 300B's or 845's.

What do you say??

[Ricky]

What you built there was basically a boutique amp.

[SY]

Ricky, when a statement begins with something like, "A friend told me that..." or "I've heard that...," it is 99% certain to be false.

If one method or topology were clearly superior to the others, everyone would use the same one. They don't. There are excellent (and classic) amps with pentode, triode, and UL output stages. And a few other output stage topologies as well. Each has their own design quirks and optimum uses; what's most important is understanding how to design an amp properly for the chosen output stage type.

Sweeping generalities like "pentodes are not linear" or "triodes have lousy bandwidth" are generally reflections of design inexperience.