Triode vs. Pentode

Zaike .A very good question,and your guess is as good as mine here. The little CJ is a honey of amp and is always very engaging with the right speaker. I used a CJ mv52 for many years with the roger ls3/5a or the original proac response 2, sold it to a good friend whereby I do get to revisit it regularly and still very much adore the sound of it, it's like seeing a good friend after a long absence. What I find very intriguing regarding the sound of different output tubes and the output configuration in which their wired...... Have you ever heard the CJ with the KT77[beam power tube] output tubes in it? Quite the contrast from the el-34, and it elevates the amp to a whole higher league in every parameter! particularly the bass response. Sounded like a much more powerful amplifier than it's 45 watt spec would suggest. Unfortunately.... a quad of strong Genelex KT-77's will cost you more than the amplifier itself,and that's if your lucky enough to even find them. My VAC pa-90 monos can be configured to operate with 4 el-34's or 2 kt-88's in both ultra linear mode or triode mode with the flick of a switch.I much preferr the amplifier in triode with the el-34 tubes in it, however... when the kt-88 is employed I do prefer the ultra linear setting. The most intriguing thing about this is: Regardless of what mode I choose when the KT-77's are employed in the vacs, they raise the bar to a higher level regardless of power rating.Funny as it may sound.... when the amps are in triode with the kt-77, they sound more powerful than the 90 watts of ultra linear of the el-34. Go figure!

No, never heard KT-77's. I guess the only reason you don't use them all the time is you don't want to wear them out?

Speaking of C-J's and converting to triode, someone there once told me that the company - whose amps are available wired either for triode or Ultralinear, but permanently, without a switch - finds that around half of their customers who get their amps converted to triode eventually pay to get them converted back again to Ultralinear. As Trelja said, it can be tough to lose that power.

Which reminds me of today's listening session: with the mono's still down to 1/3 of their normal tube complement and in triode, I played a couple of albums at more room-filling volumes than last night. I wasn't as pleased with the sound - in a nutshell you could say it was 'slowed and rolled'. Not overly distorted, though I could push it there, but sleepy.

Then I reinstalled all the tubes, rebiased, and listened again. Problem solved - the speed returned, the full range returned, and fine detail reemerged. Wake up! Then later on, I really cranked things loud playing James Brown, and the bass got a little mushy and the soundstage a little smoggy, so I flipped over to tetrode, and whoop, there it was, clarity and authority that pulsed the room. Tetrode still has its purposes for the time being.

For those of you who have been tinkering with reduced tube complements and changing the operation of the amplifiers, this might have been said already but when you do that, you are altering the relationship between the tube and the transformer in a way that may not favor the reduced tube complement or triode operation.

This is because the transformer windings present a specific impedance to the tubes. If the tubes are wired for triode, their output impedance is usually lowered, and this may mean a mismatch between the tubes and the transformer: coloration. Ditto for removing tubes from the amp.

My point: this is not the best approach if you really want to audition the differences. Some triode/pentode switches, FWIW, are effective and others are not, so this issue gets tricky enough that you could be dealing with a red herring.

Just my 2 cents....

Atmasphere: Yes, I did speculate above on just this factor, and how it might have been a confounding variable in my audition results. Thanks for confirming that. I'm wondering what your take is, in light of how that impedance issue may have affected my listening impressions in these trials, on my conclusion that higher power may in effect be its own virtue, and not just for louder listening levels.

Your point about differences in the tubes' output impedance relative to the way they're wired makes me wonder exactly how VTL (or any non-OTL amp maker employing a mode switch) has chosen to 'optimize' their output transformer in the presence of such a switch. To judge by the fact that VTL eschews various output impedance taps for the speaker load in favor of utilizing the entire secondary and optimizing it for a middle-of-the-road 5 ohm nominal load, it might be reasonable to assume they've likewise optimized the primary for a value midway between the source impedances presented by tetrode and triode modes. This approach would seem to be a practical compromise, but it does imply some theoretical room for improvement were the mode switch simply to be dispensed with (particularly for those customers who spend the big bucks for the Reference models with the intention of taking advantage of the very high power to run them exclusively in triode mode).

Of course, such concerns could be of merely academic interest to a manufacturer of OTL amps, but the manufacturer of non-OTL's might argue that even a slightly-less-then-ideally-optimized output transformer will still present a more consistent load (meaning possibly less colored) to the output tubes than will a speaker. As they say, every choice in audio represents some kind of tradeoff...

Triode is better.