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Triode versus Pentode

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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.


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.

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.
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.:smash: :smash:
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.

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

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.
Ricky said:
I want to design a Monobloc. 40 watts. Single ended. Would like to use 300B's or 845's.

I would say unrealistic as a first project. Se Amp CAD gives about 30 watts at 14% THD and 2.3 damping factor from an 845 SE. That performance requires a driver stage capable of 400 volts peak to peak low distortion swing and a 1200 volt B+ instant-death power supply. 300Bs are usually 8 watts, your requirement suggests five in parallel. It also means a complex driver.

You're best best is to start with tubelab's site and poke around some of his high power SE designs to get a handle on the effort required to make it work.
Ricky said:
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??

40Ws SE with 300Bs is very difficult - you would need 3-4 300Bs run in parallel which is a difficult load to drive. Check out this article for some ideas

The 845 requires high voltage and takes some experience to put together (think transmitter) correctly.

If you're a beginner, perhaps a SE KT88 amplifier would be a good starting point. Can make ~8Ws in triode mode and 12-15W in pentode/UL. A popular one to build is Michael Abdellah's

plenty of threads on DIY Audio on this circuit
Here is one of those 'here we go again' topics :)

Firstly, it is practically impossible to compare triodes and pentodes on equal ground unless we are talking truly basic characteristics. The reasons are:
1) no-one ever made a triode and a pentode which wired as a pentode, would be exactly the same as the true triode. It is very difficult even finding pentodes that triode-strapped look similar to popular triodes.
2) Wiring an amplifier for triode or pentode mode with the same tube (or for that matter ultralinear) rarely if ever takes into account that you actually need a different operating point, and by extension different OPT for the various modes. So, at best the amp is optimized for one of those modes!
3) Just like audio triodes, which were optimized for that purpose, so were audio pentodes. This also happens to mean that they are rarely optimized to work triode strapped. Some are better than others, and, in fact, sometimes you can find pentodes that are good for audio purposes triode strapped, but would not be a good choice for audio in their native pentode mode.
4) The grand majority of speakers are designed for voltage (or at least low impedance) drive. This immediately puts pentode mode at a disadvantage, because pentodes are inherently closer to current sources, whereas triodes are closer to voltage sources in nature, hence the latter are better suited to drive the typical speaker. In order to get a pure pentode mode amp (and most ultralinears) to behave as something approaching a voltage source, feedback is needed, which might not be used at all with triodes - adding a whole new dimension of comparison of the incomparable. In order to do a fair comparison, pentode mode amps should be driving speakers optimized for current mode drive.

Given that this pretty much spells out an apples vs oranges comparison even under the best of circumstances, making sweeping statements about 'which is better' somehow makes no sense, unless we start talking particular tubes, in designs optimized for their chosen mode of use and application.

I respectfully suggest that you go away and do some more research. As has been explained in previous posts, what you are contemplating is well beyond a first valve project.

Here are a few thoughts:

Single-ended designs produce a most specific sound signature. Try listening to some before jumping in. (I like it but some don't;) )

Single-ended amps are more complex that push-pull: Sounds paradoxical? No! For a reasonably performing SE amp, the power supply is more complex than the amp.

Paralleling output valves in SE amps is controversial; Some don't like it as it might cause loss of performance. You need better matched valves than push-pull.

To produce 40 watts SE with a single valve is difficult and expensive. I've done it, using frightening voltages and lots of dosh. I wanted it to drive my existing speakers. It would have been much more sensible to build a 5 or 10 watt amp and replace my speakers. I think I just did it because I could :rolleyes: but wouldn't do it again. (Wanna buy a nice amp??:cool: )
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1. Don't listen to dealers for tech. They don't know tech. They know popular trends and urban myths. Whatever fetches the $ is right, and written in stone for them. They normally don't fake it, they make believe.

2. All systems have certain interface variables. With 'system' I talk source to room. These have to be finely judged to lock. No single topology type is best and certainly not the only way to go.

3. Good 'poor man's DHT' you can make with 6V6 or EL84. See Tom Evans circuit ideas.
Ricky said:
Wouldn't it be better to use a pure triode?.

Sure it would. Unforch, the pentode revolution occured pretty quickly. This means a definite paucity of audio triode finals. You have such types as the 45 and 50, 2A3, and 300B. The 45 and 50 are pipsqueaks, and especially SE'd, are useful as headphone amps or only with uber-efficient speeks that can work with a watt or two.

There are also some TV vertical deflection power triodes, but they aren't much better than a 2A3, so far as output watts are concerned. Except for the 300B, there is nothing between the 45/50/2A3 and the big RF triode finals. Most of these need to operate in Class A2. (Exception: 845 -- Audio final designed for AM modulator useage and convenient sharing of the xmtr HV PS.)

That leaves trioding pentodes to fill that gap. Of course, some pentodes work well at this (a few -- 6W6 -- make better triodes than pentodes) and some don't. Unless and until someone starts up production of a brand new line of audio power triodes, this isn't likely to change.

Originally posted by 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??

Fugedaboudit! An OPT that can handle 40W, and do it single-ended is asking a great deal. I ditched a SE project with the 812 at much less power simply because Jack at Electraprint said that he could not guarantee that the requested OPT might not work right. As a gee-tah amp, sure, much less demanding. But I'm not going to pay those kind of $BUX on something whose performance SUX.

What an SET? Settle for tinywatts.
ilimzn said:
1) no-one ever made a triode and a pentode which wired as a pentode, would be exactly the same as the true triode. It is very difficult even finding pentodes that triode-strapped look similar to popular triodes.

Maybe true for the output valves, but for small signal, C3g, C3m, D3a, 7722, etc. all work very well as triodes -- better than many of the true triodes people tend to use.
Hi all.

Well, as a starting point, I have taken on board all that you have said but, at the same time, have made an integrated single-ended amp using ECC83's in the front and 4 6SN7's (8 triodes) in parrallel single end on the output stage using a 2400ohm output transformer. Scoped it and tested it. Using an 8.2ohm resistive load, 2volts RMS @ 1KHz input gives me 12 volts at the output. And triode does sound sweeter than pentode. 4 6SN7's easily sounds smoother than 2 EL34's.
Hi All.

Further to the above post, here's a piccy of the working chassis of the integrated.


  • se amp.jpg
    se amp.jpg
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