Triode mode in tube amp operation

polkfarmboy

I am getting some monoblocks that have a swich to triod mode but what does triod mode do?

nooshinjohn

It makes things go BOOM!


No experience with a triode amp, but I think triode is a more pure signal path, if I remember correctly.

ZLTFUL

I am being genuinely sincere and not in the least bit facetious.
Get your read on: http://en.wikipedia.org/wiki/Triode

Good solid background info on the topic.
Then for some more refined info as it directly applies to audio (low power) applications:
http://www.diyaudio.com/forums/tubes-valves/55231-difference-bewteen-ultralinear-triode-mode.html

polkfarmboy

ZLTFUL wrote: »

I am being genuinely sincere and not in the least bit facetious.

Yeah Iknow that, thanks for the links, the diy threads informative.

Joey_V

Triode is supposed to better than UL.

I think the idea is this:

SET > PP > Triode > UL

I think... someone needs to verify.

polkfarmboy

I am still reading but also trying to figure out if triode mode would endangering the amps by hooking them up to 3 ohm speakers. From what Iknow so far is that there would be less distortion but also lower output in power so hooking up 3ohm loads that is going to dip below 3 makes me nervou.

Joey_V

Yes, 1/2 the distortion for like what... 20% of the power?

F1nut

polkfarmboy wrote: »

I am still reading but also trying to figure out if triode mode would endangering the amps by hooking them up to 3 ohm speakers. From what Iknow so far is that there would be less distortion but also lower output in power so hooking up 3ohm loads that is going to dip below 3 makes me nervou.

Your concern would be with the efficiency of the speakers.

cnh

Good info, but I don't believe the "average" person is worried about the history of the invention and evolution of the tube. Too technical for most.

The REAL question being asked is what is the "difference". And it's all over the web if you google it. See this discussion on Audiogon, for example.

http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1391480097

Read through all the posts and it will open up some of the questions regarding power and Ultra-linear operation, feedback, and the design COSTs involved, tolerances, and so on.

A lot has to do with the design, electrons and noise and power, etc. I am not an engineer so to "pretend" I can discuss what is reviewed in the WIKI piece would be absurd. But I am CERTAIN that EVERYONE knows that, in most amps that have a switch for triode or pentode operation, the POWER (in wattage) usually doubles in the pentode mode and people disagree as to whether one is preferred over the other. A lot depends on the amp design itself. It would be "helpful" if the TUBE aficionados would leap in here and speak in simple English not technese!

The following from WIKI is also useful as shorthand (needless to say the triode came first, then the tetrode, then the pentode-which is the thrust of the reference above in terms of the technical density regarding the structure of the triode which you do NOT need to understand in order to appreciate what it means for sound, power, etc. As with most audio. For example, you don't have to read, understand (or perhaps even agree with) the WHITE PAPER on MIT cables in order to hear or appreciate what's happening because you should be able to "hear" it.

WIKI:

Comparisons with the triode[edit]

Example pentode current/voltage characteristics for differing control grid voltages.
Pentodes (and tetrodes) tend to have a much lower feedback capacitance, due to the screening effect of the second grid.
Pentodes tend to have a higher noise (partition noise),
Triodes have a lower internal anode resistance, and hence higher damping factor when used in audio output circuits, compared with pentodes, when negative feedback is absent. That also reduces the potential voltage amplification obtainable from a triode compared with a pentode of the same transconductance, and usually means a more efficient output stage can be made using pentodes, with a lower power drive signal.
Pentodes are almost unaffected by changes in supply voltage, and can thus operate with more poorly stabilized supplies than triodes. An examination of the characteristics shown here will show that the plate current hardly changes as plate voltage varies.
Pentodes and triodes (and tetrodes) do have essentially similar relationships between grid (one) input voltage and anode output current when the anode voltage is held constant, i.e. close to a square-law relationship.
Usage[edit]

The floor is now OPEN to the "experts" (Los Tubardos?); have at it.

polkfarmboy

So my speakers will just play low in triode and not explode the tubes?

Joey_V

`

polkfarmboy wrote: »

So my speakers will just play low in triode and not explode the tubes?

Explode the tubes?

F1nut

No Kevin, your tubes will not explode. However, if your speakers are not high efficiency, the resulting sound in triode will not be good.

Triode mode vs. ultra-linear could be compared to the results of solid state class A vs. class A/B.

cnh

And that's the way it's done folks! Sweet and simple and a great analogy by Jesse.

mhardy6647

Triodes are much more linear devices than pentodes -- albeit at the expense of operating efficiency. Power output of, e.g., a single-ended EL84 output stage with the tube operating in its "native" pentode mode is good for about 4 to 5 watts (at 10% THD); operated as a triode, it's good for about 1.5 to 2 watts (at 10% THD). In both cases, this would be with the tube operated at its "optimum" conditions for the respective modes.

It's a truism -- and just an opinion, on my part -- but a purpose-built triode power tube (e.g., a 2A3) will sound better (for whatever reason) than a tetrode, pentode, or beam-power tube (e.g., an EL84 or an EL34) operated as a triode.

As to tube primers, if the OP's never read it, I (still) don't know of a better place to start than this old PDF from Anthem: http://archive.anthemav.com/OldSitev1/pdf/taste.pdf

Knowledge is power; one may save lots of money by being a tad more informed about "how" stuff works (and "why") before buying into a copy-writer's hype -- or even - gasp - internet forum experts' opinons... my own included ;- )

EDIT: For the "linearity" reason mentioned above, many folks ("internet experts" included, of course!) feel that small signal chores in an amplifier (i.e., the voltage amplifier or "driver" stage) are better handled by triodes, too... but there are numerous fine amplifiers that use pentodes for voltage amps.

ZLTFUL

mhardy6647 wrote: »

Knowledge is power; one may save lots of money by being a tad more informed about "how" stuff works (and "why") before buying into a copy-writer's hype -- or even - gasp - internet forum experts' opinons... my own included ;- )

That's all I was trying to suggest.
To that end, most people don't know that even the designers of the ultra-linear amps at the "beginning" hated them but they were the latest marketing ploy at the time and were selling like hotcakes.
Those who don't know, don't know they don't know and all of that.

I forgot all about the Anthem article. Fantastic primer.

CNH, I wasn't trying to baffle anyone with the tech...just good to have a grasp on the concepts behind (I agree though that I should have specified the low power section as I am sure PF wouldn't really care about Lighthouse triodes hehe) the products you are buying and part of that basic knowledge is how the technology evolved. "Where it came from" if you will.

But in the end, CNH is right...Jesse's post is clear and concise.

mhardy6647

A little technology savvy is a good thing -- otherwise, folks do things like ditch CRT TVs for HDTV... umm...errrm... uhh...

The ultra-linear topology solves some problems while (arguably) creating others. Nothing's perfect. Well, except "class T" digital amplification, of course. Ahem.

;- )

Polkie2009

I remember back in the day Mesa Boogie tube guitar amplifiers were big on advertising their triode design, needless to say, probably many other guitar tube amp brands had a similar triode design too.

brgman

I always thought Mark O at Rogue described it clearly.

Triode/Ultralinear switch:
The output transformer of a tube amplifier acts as the electrical interface between
the very different impedances of the loudspeaker and the tube circuitry. There are
three fundamental ways that the output transformer can be connected to the tube
circuit.
1) Tetrode or Pentode connection – In this configuration, the highest power level
is achieved but at the expense of significantly higher distortion levels.
Practically speaking, this mode of operation is sonically inferior and has bean
disregarded in the Zeus design.
2) Triode connection – With this configuration, the lowest level of distortion is
achieved but at the expense of output power. In the case of the Zeus, its power
level in triode is high enough that it will likely provide plenty of power for
almost any speaker load.
3) Ultralinear operation – With this configuration, power levels near those of
tetrode mode are achieved with low distortion levels that approach those of
triode operation. For loudspeakers that demand very high levels of power,
ultralinear should provide excellent sound and plenty of power.
The triode/ultralinear switch allows the user to operate Zeus in either of the two
modes. In general, the sonic differences are subtle but for a given loudspeaker one
mode is likely to sound better than the other. There is a complex relationship
between the output transformer and the crossover network so try both and see
which one works best in your system. This switch can be operated while the
amplifier is playing but you will hear a small relay noise through the loudspeaker.

Joey_V

Ok so we got some real smart and knowledgeable people here.... how does PP and SET differ from triode/pentode?

cnh

Agreed but amps designed for Triodes have tighter parameters and need more stable and coherent power supplies, etc. So design IS important when choosing what tube you will use. It also, usually, makes amps designed for only triodes, not only lower power but more expensive in my experience. But there's a lot more to it all, as you suggest; as well as others above.

And I still disagree with Mark some; no, you do not need to know it all in order to appreciate the SOUND. If you did everyone here would need advanced degrees? lol As well as a Hawking-esque IQ? Oh, and they'd love puzzles, word games, chess and GO! All pretty useless for living one's life. Most creative geniuses don't need a gifted IQ to create a Picasso or a van Gogh, or a Fugue, unfortunately. Well, maybe that's a good thing because creativity and intelligence are so expansive and multi-dimensional that numbers fail to describe them. Humans? So DAMN DIVERSE! Confoundingly surprising!

SCompRacer

Joey_V wrote: »

how does PP and SET differ from triode/pentode?

Basically, Push Pull is more efficient in that each tube is amplifying ½ the signal running in A/B mode. You can get over/undershoot and added distortion due to output differences between the tubes. PP will tend to cancel out second order harmonics, everyone’s favorite.

SET’s operate in single ended Class A, usually not as powerful as PP, are more lush sounding but need highly efficient speakers, some even say horns, to do their magic.

YOMV....

Lots of choices...

Tube single-ended triode (SET)
Tube single-ended OTL (Output transformer less)
Tube push pull OTL (Output transformer less)
Tube push pull
Transistorized single-ended
Transistorized push pull
Transistorized digital
Single-ended hybrids
Push pull hybrids

EDIT I may still have some spectrum analyzer distortion graphs which showed the difference between ultra-linear mode and triode mode on a popular TAD-60 tube amp. If I find it/them I'll post it/them.

mhardy6647

DSkip wrote: »

Triode/Tetrode/Pentode is about the tube design and how many filaments it uses, not amp design. PP or SET, it doesn't change what a tube is.

Nope not filaments - just one of those per tube... well... OK, sometimes twofilaments that can be connected in series or parallel, such as in a 12AX7 (which actually has two 6V filaments). It does relate to the number of electrical elements in the tube -- specifically the number of grid elements -- that control the flow of electrodes from the cathode to the plate. A pentode has three grids, a tetrode has two, and a triode has one. The simplest vacuum tube amplifier is a triode -- a diode can only be used for rectification (converting AC to DC).


Now, that said, the important thing in the context of the OP's question is related to an amplifier that can operate pentode tubes in either of two modes -- as pentodes, using all three grids in a pentode tube, or as triodes, in essence ignoring the other two grids. The pentode is still a pentode, but electrically, in the circuit, it operates as a triode. It loses all of the benefits of being a pentode, and gains the benefits of being a triode.

mhardy6647

SCompRacer wrote: »

Joey_V wrote: »

how does PP and SET differ from triode/pentode?

Basically, Push Pull is more efficient in that each tube is amplifying ½ the signal running in A/B mode. You can get over/undershoot and added distortion due to output differences between the tubes. PP will tend to cancel out second order harmonics, everyone’s favorite.

SET’s operate in single ended Class A, usually not as powerful as PP, are more lush sounding but need highly efficient speakers, some even say horns, to do their magic.

Push pull is indeed more efficient, in the sense that, when the two output tubes or transistors (or two groups of paralleled output devices) are used for push pull amplification, the "push" devices (so to speak) are only actively amplifying half of the signal waveform (the "positive" half of the waveform) and electrically turned off (not actively passing signal, just idling) during the other half (negative half) of the waveform. Conversely, the "pull" devices are "off" during the positive half, and "on" during the negative half. This mode of operation is "pure Class B", and it has numerous advantages -- and one big disadvantage. That big disadvantage is called "crossover distortion"; a little "glitch" added to the signal at the crossover point when the "pushers" switch off and the "pullers" switch on.

Abrogation of that (rather nasty, actually) crossover distortion is the fundamental reason that most push-pull hifi amps operate in a form of "Class AB". In Class AB, neither the pushers nor the pullers are turned completely off when the other guys are doing the heavy lifting. They are biased to an intermediate state, which decreases crossover distortion. BUT it eliminates that crossover distortion.

As an aside, a single-ended amplifier (in which one tube, transistor, or one group of paralleled tubes or transistors) amplifies the whole waveform, the bias class is always Class A... the device(s) is/are biased "on" all the time; working all the time. This is the root of the low (electrical) efficiency of single-ended Class A amplification... or push/pull Class A amplification, for that matter.

YOMV....

Lots of choices...

Tube single-ended triode (SET)
Tube single-ended OTL (Output transformer less)
Tube push pull OTL (Output transformer less)
Tube push pull
Transistorized single-ended
Transistorized push pull
Transistorized digital
Single-ended hybrids
Push pull hybrids

EDIT I may still have some spectrum analyzer distortion graphs which showed the difference between ultra-linear mode and triode mode on a popular TAD-60 tube amp. If I find it/them I'll post it/them.

This is all cool but in principle -- when it comes to single-ended or push-pull amplifier design -- it really doesn't matter whether the amplifying devices are vacuum tubes or transistors.

PS - The comment about canceling out even order harmonics (although not just the second harmonic) in a push-pull circuit topology is true. The remaining odd-order harmonics are unpleasant sounding (sort of the kazoo effect), and the human ear is far less tolerant of a given level of even order distortion than the same amount of odd order distortion. The first order harmonic of a fundamental tone is exactly one octave higher -- this is why odd order harmonic distortion is often called euphonic (pleasant sounding) and is sought after, e.g., in guitar amplifiers... but it is still distortion and it should still be minimized in any amplifier that is considered a "hifi" amp :- )

mhardy6647

Check.

mhardy6647

Here are the elements of a tube shown on its schematic symbol -- this tube, the 12AX7, is a dual triode; it's got two complete and identical triodes inside of one glass element.



A tetrode has two grids -- a "screen grid"("S") in addition to the grid ("G") of a triode.
A pentode has three grids -- adding a "suppressor grid" ("Sup") to the other two grids.



Oh, one other thing about ultralinear topology. There's no way (AFAIK) to operate an amplifier using triode tubes using the ultralinear design. In an ultralinear amp, a signal is taken off of a tap on the output transformer and connected back to one of those "extra" grids on a tetrode or pentode. An ultralinear amplifier can be single ended or push pull, but it cannot use triodes as the power tubes -- there's literally no way to hook up the feedback loop from the transformer to the tube!

Here's a simple SEUL (single-ended ultralinear) amp circuit using a tetrode (KT-88, one of the "Kinkless Tetrodes"!) as the power amp tube. There's a tap 43% of the way along the primary winding of the output transformer (the "ultralinear tap") that is connected back to the screen grid of the KT88. In this particular design, there's also a switch installed in that connection -- with the switch closed, it's an SEUL amp; with the switch open, it is an (pseudo) SET amp... with the switch open the KT-88 thinks it is a triode and essentially operates as one.

source: http://diyaudioprojects.com/Tubes/KT88/



EDIT: stumbled across this little primer by Eric Barbour on "how tubes work" -- it's probably, in some respects, too much information for many folks, but, on the other hand, it's a pretty compact, one-stop shop for info on all the stuff this thread's brought up... with nice pictures, too!
http://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm



EDIT^2: stumbled on this, too... more serious but also a pretty concise yet comprehensive discussion by a fellow (Rod Elliott) who does know his stuff: http://sound.westhost.com/valves/design.html

vc69

I've always wondered where the term "valve" originated and why.

mhardy6647

vc69 wrote: »

I've always wondered where the term "valve" originated and why.

Actually, that's easy to answer.

Here's the deal. Electrons are negatively charged ("negative" because, when Ben Franklin had to guess which way current flowed in a circuit, he had a 50/50 chance of guessing right. He guessed wrong).

In a vacuum tube, electrons are boiled off of the negative "cathode" (or directly from the filament in a "direct heated" tube) inside an evacuated bulb (the "tube" part of a tube). Those electrons would just kind of float around in the vacuum if it weren't for the presence of another electrode, the "plate" (or anode), which is positively charged. Electrons are attracted to the plate. The simplest tube, a diode, has just the cathode and plate. An AC voltage applied to the cathode will be 'rectified' to (pulsating) DC by a rectifier.

In a triode, there is a third electrode -- the control grid. The grid has a small voltage (usually AC) applied to it. If that voltage is relatively negative, it repels the electrodes as they stream towards the plate. If that voltage is relatively positive, it attracts them towards the plate. In a hifi amp, the voltage applied to the grid is the AC music waveform! A small fluctuation in that grid voltage (you can think of it as the line-level signal from, e.g., a CD player or a DAC or a tuner or whatever) acts as a valve to control the flow of a large current from the cathode to the plate.

"Tube" describes what one of these gizmos looks like; how it's built. "Valve" describes what it does.

vc69

I had a feeling you might know @mhardy6647 . Thanks for the reply.

edit: Also, that's about the best explanation one could hope for. I learn a lot from you guys.

mhardy6647

I even think it's relatively accurate :-P

Of course, I failed to mention the most important part of all -- they look really cool...

DSC_0124 by mhardy6647, on Flickr

EDIT: Although this gets increasingly technical as it goes along, the first few paragraphs of this reference are, I'd opine, a pretty good "how it works" description: http://www.angelfire.com/electronic/funwithtubes/Basics_04_Triodes.html

I mention this primarily because it gives a pretty good explanation of the bias voltage that is (almost always) applied to a triode (or tetrode or pentode) tube. I kind of glibly avoided the topic of entirely :- ) but of course it's very important -- it's part and parcel of the "amplifier class" of operation chosen by the designer. Transistor amps use bias too, and for the same reasons.

ZLTFUL

Since we're on the topic...

https://www.youtube.com/watch?v=8n4WVRKkmww

https://www.youtube.com/watch?v=5WCWejeRR_s

mhardy6647

Nice!

I wonder if the OP's wandered back and looked at this thread -- rather different than (ahem) the status quo,