My experience is that when an amp is designed with a switch which literally alters the circuit, there is some compromise in one of the circuits. If the amp is setup primarily for Triode, the tetrode/pentode circuit won't sound as good as a pure circuit designed that way. Ultralinear seems to be an exception.
Generally I prefer triode couple amps. I am currently running Art Audio Opus 4s with KT120s (also runs KT88s) and I love them. They don't offer a switch. Tom offers a Triode couple (45w) or Pentode coupled (75w) amp but no switch because the switch forces a compromise of one of the circuits.
The McIntosh MC275 is a pure pentode circuit for either KT88 or 6550 tubes. It sounds as good as many triode coupled amps I have heard using the same tubes.
Alternatively, I have had amps in the past and found that the Triode circuit would sound great and although it delivered more power, pentode/tetrode would sound muddy. I thought it was an inferior circuit yet you hear the MC275 and that is clearly not the case. It is the switching process.
Ultralinear seems to be an exception. Tubes in Ultralinear sound...well, linear. Tight bass and extended highs. Sounds SS to me frequently. I find this unappealing as I purchase tube amps to sound like tube amps. If they sound SS, then all I am getting is an amp that costs more with tons of maintenance.
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answer to your question is totally dependent on speaker and its load to the amp
triode mode reduces output 3 db which is substantial on some lower sensitivity speakers and also changes the power tubes' 'leverage' into driving the speaker
so as is the case in many of these questions, you can really only try for yourself and see |
DPHP user here. Check out the KT77 is you like the EL34 sound. To my ears they are a little more dynamic overall. Look at KT88 for more on the bottom end at the expense of less midrange magic. I have been using the old RCA 6l6G coke bottle in my PL. They are a lower power(different from modern 6L6). They are a close enough combination of EL34 mid and KT88 dynamics. One of the few NOS American power tubes still available for a reasonable cost. https://www.ebay.com/itm/RCA-6L6G-Smoked-Glass-Tube-NOS-foil-twin-D-getter-copper-grid-posts-code-35...They have a "dry"sound which to me is more convincing compared to the "lush" EL34 tone. I prefer the triode setting most listening sessions. A flat recording sometimes sounds better in UL. You are aware of the 2 innermost preamp gain tube also will fine tune the sound? Subtle, but another tweaky option. Be an early adopter and try the KT170 https://primalunablog.com/2020/10/30/tung-sol-kt170/They should be like a supercharged KT150, which already is a powerful tube. I haven't tried them in my amp yet. 8=$$$$ |
I use a non PL amp triode wired pushpull in class A running a quad of KT-77's and it sounds fantastic.
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i would concur with others that the modern russian kt77 labelled gold lion genelax (gold and red box) is pretty much the best sounding current el34 variant
good linearity and drive with a fair dose of tubey magic |
I have a PL Dialogue integrated. Hands down the KT150 is my favorite and my change of pace the 7581A. I am a Streamer using the Roon interface. I like Ultralinear for Radio and Triode for everything else. I have heard others mention they prefer the EL34-7581A tubes with the PL HP power tube array. It’s hard for me to imagine going to any other power tubes since I seem to have found exactly the sound I’m looking for |
Thanks for chiming in. Like so many other things, it seems the devil's in the details. Someday I'll know better than to think I can come up with a simple rule of thumb without more exceptions than rule.
I suppose when you start getting into tube rolling and mode of operation the changes are subtle enough that everything else has as much or more influence. I'm surprised no one has mentioned changing modes with different cables yet.
KT77 were already next on my list of tubes to try. I got the 7581A because a lot of their described attributes were also appealing to me, and they are about $100/set cheaper than the KT77. For my speakers and listening habits I definitely don't need more power and I don't feel I'm missing any punch. I keep trying things and muddling through because "You can't get what you want, till you know what you want." |
Triodes are more linear and have a lower output impedance. If you can get them to make the same power and if you have two amps, one optimized for tetrodes or pentodes, and the other for triodes, the triode amp will sound better hands down. This is assuming that the build quality and topologies are otherwise identical.
'Better' in this case means lower distortion, so more neutral (since the ear interprets all forms of distortion as tonality), bandwidth will be the same, and output impedance will be lower, so again more neutral with more speakers.
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My Dennis Had Firebottle HO (it's a HO...says right there...) is a single ended pentode amp at about 12 to 17 watts (HO is "High Output") depending on tubes. Although Atmasphere's frequent comments regarding SET (Or SEP in my case) distortion make sense, after using various tube and SS amps over decades, my Firebottle never sounds harsh or distorted or anything besides beautiful through high efficiency speakers with all sorts of music at any level I can tolerate. That said, Gold Lion reissue KT77s seem to stick around in this amp longer than KT120s, 150s, 7581As, KT88s (GL or Sovtek), or anything else now living in my crowded tube drawer. |
I only have two amps that are switchable AES Six-pacs and a Prima Luna. I prefer both in ultralinear mode. I have Gold Lyons KT-88s in the Prima Luna and it sounds great with better extension on the top end. |
FWIW if ultra-linear is set up correctly, the result should be a pentode or tetrode power tube operating with the same linearity as a triode but with about 90% of the power it could make as a pentode or tetrode.
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@atmasphere always nice to get answers from an expert. They never really covered vacuum tubes in the few EE courses required for an ME degree, and those are only a faint memory. From further reading I gathered that the ratio of the screen-grid and plate can vary quite a bit to find the sweet spot in ultralinear and would be dependent on the tube type. So with tube rolling on an amp like mine (Primaluna DPHP) would running in triode mode be more consistent when comparing different tube types?
What about tube life? Hypothetically, if you adjusted for the same output voltage, would one mode be significantly harder on the tube than the other? |
"FWIW if ultra-linear is set up correctly, the result should be a pentode or tetrode power tube operating with the same linearity as a triode but with about 90% of the power it could make as a pentode or tetrode." Bless Blumlein, Hafler and Keroes's hearts!
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would running in triode mode be more consistent when comparing different tube types?
What
about tube life? Hypothetically, if you adjusted for the same output
voltage, would one mode be significantly harder on the tube than the
other? The patent suggests that the ratio is independent of the tube. Ultralinear has nothing to do with tube life either. Plate voltage will if the proper bias point for that plate voltage isn't observed. Bless Blumlein, Hafler and Keroes's hearts!
And I guess Acro Sound too. |
