Triode versus Ultralinear/Pentode

I have an Audio Space AS-3i with a triode/ultralinear switch on the front panel. This is my story...

I've used both EL34s and KT66s in this amp. With the EL34s there was more power available, of course. I ran the amp all the time in triode mode because I found the sound sweeter and more detailed. With KT66 tubes the difference between triode and ultralinear was less obvious. I used ultralinear for the extra power. FInally, though, it began to appear that triode was the nicer one even with KT66s.

Having a switch on the front panel is really useful. There are some discs--rockers, full orchestras--which get a needed boost in impact with the extra power of ultralinear mode. But in the end it's a bedroom system and I do most listening in triode.

I've never listened to a pure triode designed amp so this is definitely a FWIW.

I have three amps were you can select triode/ultralinear on the fly. One, (a Cary) in triode sound much the same as two other's in ultralinear. In ultralinear it becomes slightly hard. All have low negative feed back.

With the speakers (3 different pairs)I have used with these three amps, best described as relatively neutral 3 way cone designs with a gentle impedence curve, unltralinear produces what I consider natural and 'linear'. When put into triode the mid-range power seems to deminish and the center sound stage recesses. A very relaxed sound but on a lot of music it sounds a bit thin.

I suspect triode might be a mode that would sound best used with more aggressive speakers (no shortage of those in the high end).

I do like to use triode for background music however. Apart from it being 'relaxed' at about half power the tubes and amp run cooler and I suspect last a lot longer.

Switching an amplifier from pentode to triode can be a bit tricky inside a single amplifier. The reason is that triodes tend to have a lower impedance than pentodes and so will want to operate at a different impedance on the output transformer.

That is tricky to set up and a lot of amplifier manufacturers 'fudge' that parameter a bit.

Its also interesting to observe the behavior of a pentode wired in triode as opposed to a real triode- it becomes obvious that real triodes have better performance. It is often difficult to demonstrate this fact because there are so many variables that get introduced, but- if you are thinking that triodes might have something to offer, you would be right. One of them is *not* power :)

You use triodes when the best sound possible is your goal. They offer lower distortion meaning little or no feedback is required, and lower output impedance which means less turns on the primary of the output transformer (that translates to: less distortion and more bandwidth and *that* translates to 'more detail, better highs and lows') or maybe no transformer at all.

The trick is like it always is- get the right speaker to take advantage of those advantages. There are high power triode amplifiers but they tend to be rare.

No. With my Mesa Baron, Triode is preferred for acoustic and smaller scale music, but adding some pentode helps a great deal with rock, orchestral, some jazz, piano and even solo guitar sometimes. and it solidifies imaging a bit. I suspect it's as we've always heard: it's all in the implementation. Unless you listen only to chamber music, there are times when pure triode is subobtimal.

Lloydc, your amplifier might be a good example of what I meant- that even though there is some advantage of the 'triode mode', that mode is not optimized in the amplifier in order to allow for the switching in the first place. Were it optimized, there would be no trade-offs in sonic performance, only power.