Triode mode offers lesser power at an alledgedly better sound quality than Pentode. Hopefully Al will stop in to explain it better. Al...... :)
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Triodes are some of the most linear amplification known to man. Pentodes are not and generally need negative feedback correct to be useful for audio.
Good bass has nothing to do with the amp being triode of pentode; good bass results from having the correct relationship to the speaker (see: http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php and proper bandwidth to play bass in the first place. That is not an issue between tube types.
Atmasphere, there might be some issue since triodes have lower internal anode resistance leading to higher damping factor and perhaps perception of less bass.
Pentodes are less sensitive to supply voltage variations meaning that large change in supply voltage cause very small change in plate current. Triodes require better power supplies. I also read somewhere that pentodes are noisier.
Atmasphere points out exactly what my experience has been. With my Manley mono's, with speakers that don't need more than their 100 triode watts, triode always sounds better; including in the bass. Speakers (my Stax F81's) that suck up as much power as I can give them, pentode "sounds" better; but, only inasmuch as not sounding limited in volume and dynamics. As far as refinement, staging, and all that other good stuff, triode still sounds better; albeit at limited volume.
Not sure if it was clear to everyone that the question was not about pentodes vs. triodes, but was about pentodes operated as pentodes vs. pentodes operated as triodes.
The initial answer by John (Jmcgrogan2) pretty much said it all in a nutshell, IMO. It is common for triode mode operation to sacrifice something like 40% or 50% of the power capability of pentode mode operation, while providing some improvement in sound quality, mainly by providing an approximation of the greater linearity that a pure triode can provide in comparison to a pentode or other amplification device (as Atmasphere indicated).
See this Wikipedia writeup for a technical overview. Note the section toward the bottom headed "Triode-strapped pentode circuits." Basically, connecting the middle of the three grids of a pentode (the "screen grid") to the plate, usually through a low value resistor, causes the tube to exhibit triode-like characteristics. A triode being a tube having only one grid, corresponding to the "control grid" shown in the article.
The amplifiers I have had that could be switched between the two modes were two pairs of the classic vintage Marantz monoblocks, the Model 2 and the Model 9, which I owned during the 1990's. Using them in conjunction with 90 db speakers having easy to drive impedance characteristics, in both cases triode mode provided a considerably richer and more natural sound that was clearly preferable to pentode mode, as long as I didn't run out of power. Which in turn only happened on a limited number of recordings having exceptionally wide dynamic range, such as some of the Telarc symphonic recordings with their notorious bass drum beats.
Al, I wonder if you might be aware of the patent that was originally issued on the ultralinear technique?
To get around it, a good number of companies simply moved the tap. However, the original patent shows that if the tap is correctly placed, you get exact triode linearity with 90% of pentode power. Seems to me this would obviate having a switch.
The funny thing is, the patent is long expired, but so many companies did the work-around (which only approaches triode linearity) that many people (including designers) simply think that thats the way it is!
Thanks Ralph. No, I hadn't been aware of any of that. So by designing in accordance with the principles defined in the expired patent, it is possible to achieve triode linearity with a pentode, while at the same time reducing the power sacrifice to insignificance.
And the reason that approach is often not utilized is simply that designers are unaware of it.
Fascinating, but I'd have to say not surprising!
Atmasphere - I would be interested to know if the design if the output transformer will influence whether one way or other is preferable. With both Air Tight ATM3's ( 6L6's ) and Dynaco MkIII's ( EL34's, 6550's ) I prefer ultralinear to triode. In both instances the triode mode loses resolution and screws up the natural harmonic structure. Would this be that the trannies have been optimised for Ultralinear ? or possibly some types of tubes are not good in triode mode ?
Atmasphere, "the original patent shows that if the tap is correctly placed, you get exact triode linearity with 90% of pentode power."
The industry has chased this promise since the 1950s. If the patent Ralph referenced is the same as I'm thinking, it was filed by those who formed Dynakit; their own products (and I'm a devotee), which obviously had full use of the technology, certainly missed out on realizing the aforementioned claim.
At any rate, since the early 1960s, high-end audio more or less abandoned pentode operation. During "the age of progress", so many entered into building UL amplifiers for nothing more than to put forth ad copy deemed required to keep themselves current.
Most of the time, those in this hobby who mention pentode actually mean ultralinear...
I am guessing the less resolution in triode mode that Dover mentioned only occurs when the triode switched amp is playing a rather loud and complex passage.
Based on my limited experience, to get the best out of a triode amp you really need a very beefy driver stage that does not wimp out. UL has a bit more gain, so to do apples to apples comparison with the same volume you need to drive the gain stage a bit higher and therefore the driver stage would have to swing more voltage.
I have modded a few pcs of gear myself switching them from Pentode (Audio Research D115ii) or UL (Jolida 502) to triode and each time I ended up reconfiguring the driver stage with a much beefer tube such as 6H30 or ECC99 instead of 6FQ7 or 12AT7 and run a lot more current in the driver stage.
Hi Trelja, seems to me Dynaco was one of the companies that got around the patent by moving the tap.
Dover, The problem with going to triode mode on an amplifier like the Dyna MkIII is that the load impedance has to be set up differently- essentially you need a different output transformer if you are to really get the performance out of the tubes wired in triode. I got started in this business by doing mods on Dynaco gear; the above reason is why I don't wire their amps in triode.
Now an amplifier with a switch is hopefully a different situation- 'hopefully' in that the output transformer is optimized for best performance in both switch positions.
It depends entirely on how well the output transformer is designed. However if you caught some of my comments earlier about the actual correct tap point of the screen taps, if the transformer is really set up correctly there will be no need for triode mode at all, since a correct UL tap will have the same linearity.
IMO this is a bet that most manufacturers of UL output transformers have missed, on account of the fact that so many have for decades now put the tap in the wrong place in order to avoid patent infringement. As a result many designers today don't even *know* that there was even a patent- they think the wrong place for the tap is actually the right place :)
Thank you for the response on ultralinear vs triode. That confirms my thoughts. Fwiw I got the Dyna's in triode mode and converted them back to ultrulinear. It was so much better I never went back. Later on I converted them back from 6550's to EL34's which I preferred on vintage Tannoys. I dont use them but they are nice to keep around.
I am new to tubes, but my experience so far with my Prima Luna amps (a Dialogue 4 and a Dialogue 7) is similar to Dover's. In triode, in my system, the PL's seem to lose resolution and dynamics. And the bass becomes less defined, even with my speakers crossed over at 80Hz into a sub.
It must be system dependent, because I've heard the PL's several times at shows, and I always preferred triode. But here at home, ultralinear is the clear winner.