Pentodes, Triodes & "Fake" Triodes

by Steve Deckert
June 2000
 

The most dangerous thing you can do if you're really serious about conjuring up a good hi-fi system is to draw assumptions. In audio I have found that the popular opinion is usually never correct when it boils right down to it.

Many people ask what the difference is between Pentodes or Triodes, and what's the difference between a "real" triode like the 300B and a Pentode that's wired in triode... always followed with the question, which is better?

If only there was a definitive answer, but there is not. I've heard all three sound wonderful and terrible based on the following variables:

The circuit, the power supply design, the types of capacitors used, the quality of your source and cables, speakers, room acoustics, quality of your hearing, and subjective personal preference. These variables can be applied to every question in audio ending in the phase "which is better?".

A true triode tube has advantages and disadvantages like anything else in this game, as does a pentode, output transistors etc. Which is better depends on which one will compliment the variables in the most desirable way.

A large factor as to which is better is really the issue of negative feedback, used to keep solid state amplifiers and pentode tube amps stable and low in distortion. Triodes have a different operating character in that they do not require negative feedback resulting in a usually more transparent sound with greater clarity and better imaging.

Lifting the negative feedback off most pentode amplifiers yields a power curve that rises with impedance. Since most speakers have an impedance that climbs with frequency, the pentode will put out more current as the frequency increases. The result of this is a thin dry ultra fast sound with lots of spank. A monster that makes some recordings sound interesting for the first time, and most everything else sound like crap.

Pentodes are far more efficient than triodes and with the same bottom line can usually produce 5 times more power. That makes pentodes and solid state transistors a partially monetary issue. So, as an example, if you have speakers that are marketed for handling large amounts of power with low distortion, you can bet they require large amounts of power to reach their potential. In such a case, with say a well damped room, and some easy rather laid back soft dome tweeters, a Pentode may sound overall better because of the extra power and dryer signature.

On the other side, if you've gotten more sensitive (sensible) speakers you would know that they are higher in fidelity, more expensive, and harder to find than normal speakers. But in such a case, the higher sensitivity also means higher transient speed, much more resolving power with the potential to unmask layers and layers of additional detail and crucial harmonics that make the difference between real and recorded sound. In a system like this, more pressure is on your source and cables again because of the much higher resolving power of the speakers. You can and will hear everything that's wrong with any one part of your system and or recordings. This is high fidelity. In this world, the difference between negative feedback and no negative feedback amplifiers is as obvious as listening to your stereo with your fingers in yours ears and then taking them out.

Also in this world, because of the resolving power of your speakers and the higher quality supporting gear required, bells and whistles become setbacks from a sonic standpoint, and the shorter your signal path is the less chance of contamination. Suddenly we want simple circuits with part counts in the single digits and this is where tube amps typically (but not always) excel. Within the tube camp, the Triode will almost always sound far superior and far more natural with the correct timbres and harmonics. In the inefficient system it may and probably would sound dull, soft, and be in a state of constant clipping which because triodes clip so gracefully would be interpreted as a lack of clarity until such a point where the poor thing blasts off and sticks in your ceiling with smoke coming out of its ass.

In my opinion, if a man wishes to set up a personal listening room in his home with the serious intent of creating a perfect 3 dimensional soundscape with perfect frequency balance the top of the audiophile food chain is single ended triodes and either expensive high quality full range horn speakers, or electrostatic or planer magnetic panel speakers. And I'll confirm a secret that you've probably heard before... You've never experienced PUNCH or dynamics until you've sat inside this combination. The detail and speed is breathtaking and if you compared it to huge conventional speakers of you choice, boasting multiple woofers, massive solid state amps, and even a subwoofer with it CRANKED you wouldn't be close.

Many of us, with the restrictions of family and budget are content to just have the pollution free sound at lower levels on smaller more common speakers.

One more "which is better".... (Same answer coming) A REAL triode or a pentode wired as a triode? Right now, especially after the newfound popularity of the 300B and perverted prices of some, it is popular to be in the "real triodes are better" camp and snub you nose up at anything else. Fact is, the reasons covered above and the differences between pentodes and triodes are unchanged regardless of whether or not you use a true triode or wire up a pentode as one. So, if we eliminate the politics from the equation we're left with only the resulting sound. So, which "sounds better"... same answer- same variables as discussed above.

Is there is difference in sound between wiring a pentode as a triode and a true triode, yes. Is there is difference in one brand or type of triode and another, yes. All tubes sound a little different, so by the shear fact that a pentode wired as a triode and a real triode are different tubes you are certain to hear differences. Which is better?... Here's what I've found:

In a pentode such as the EL34 where the screens are not tied together internally, it is possible to wire it as a "true triode". It is the only pentode I know of that is. The other ones have internal connections on the screens that can't be changed so you end up with a "Pseudo triode" as I've heard it called. The "Pseudo" triode will retain some of it's pentode characteristics and be higher in efficiency generating slightly more power from the same bottom line. On the EL34 you can wire it both ways and observe the differences. In fact we've got a product coming out soon that has this feature built in allowing you to use your choice of two sockets, one wired in true triode and one in a form of "pseudo triode" .

I will say that when comparing a real triode with a pentode, even an EL34 wired as a triode that there are certainly more opportunities for complications and or artifacts in the sound with pentodes simply because of the higher complexity of the tube design. Those extra grids inside the pentode could cause complications if directly connected to the plate as though they weren't suppose to be there. They would certainly alter the emissions path and flow of electrons to the plate.

In the project I'm working on that has both options you can hear a clear difference between the two, and because of this I have explored the possibilities that exist with the extra grids available in a pentode to a point where I have found that you can do some wonderful things with them. By controlling the resistance between them you can manipulate or stage the electrons and I imagine create subtle phase relationships that would be frequency dependent. If you think about it, this is the very heart of the single ended amplifier, the output device itself, and with the pentodes extra grids you can put yourself right inside the emissions path between cathode and anode... that's where the action is.

I have found the sonic relationship between making changes to the resistance between the plate and each grid is a tool for sculpting out a perfect sound stage when doing the final voicing on an amplifier. In the case of our little project, the pure triode mode sounds real good, typically up front and personal. But in the other configuration the soundstage simply LOCKS-IN deep and wide, and the shimmering trails of high frequency harmonics are much longer with a silkier tone and far better balanced within the rest of the soundstage. Size and space relationships within the soundstage become clearer.

There is also something else to consider with all of this tech talk, and that is the amount of space between the cathode and anode (plate) of a given tube type. One of the secrets of our Zen Triode amp is the EL84/6BQ5 tubes small glass envelope. The plate and cathode are very close together. Having grids in-between the two and properly wired close the gap even more. There is less gas in the tube which may or may not effect things, but in any case the sonic result is greater speed and potentially wider frequency response.

Many people consider the EL34 to be the big brother of the EL84, but there is an obvious difference in speed that has to be dealt with during the design stage of a good amplifier. In the case of our SE84B amp, the tube is so fast that a tube rectifier was required to slow down the power supply enough to effect a musical vs. anal result. On our new project using EL34's, we've been spoiled and are used to the speed of it's little brother and were unable to effect the same sonic result with tube rectification from the same bottom line. In this case, the tube is a little slower so the power supply needs to be a little faster. Doing this gives results that bring the signatures of the two amps much closer together, even though the circuits are completely different.

But just when you start to assume that smaller tubes are faster in all cases, or make any assumptions about "what is better" in relation to audio configurations you're just hopping on a Mary-go-round which stops where it starts. Understand that every stereo is unique, just like no two humans are exactly the same. Even identical stereos are different because of the rooms they're in are different. And even if they were identical stereos in identical rooms, no two sets of ears would interpret it the same. You will never know for sure what is better sounding between any given set of choices in audio until you try them for yourself and see.

We have chosen to use popular (but overlooked by the high end industry) tubes that we use because they are cost effective and readily available. I firmly believe that the art of amplifier design has very little to do with the tubes you use. But that's only because so many of our Zen amp customers have sold their triode amps with the high dollar valves in favor of a tube costing around 5 bucks. This magic little amp also uses what is considered to be by most assumption makers "the worse sounding tube rectifier there is, a 5Y3GT!" But there again... there's a rude lesson in it all somewhere.