UL vs Pentode & Tube Life?

[oldflame]

One audio amp manufacturer was quoted, in a Stereophile interview, that properly implemented "pure" pentode operation was easier on the finals when compared to most UL designs. Lacking the in-depth knowledge of our AK experts, I'll ask: what are your opinions and results with different push-pull amps?

Is this another: "it depends" question? Does the total circuit design, regulation of current & voltage, transformer, feedback, or whatever-else, have more to do with tube life than output type? How might Mac's cathode-coupling fit in?

If this has been covered already, maybe this post could be merged or eliminated. Thank you for your thoughts.

 

[Mr Boochie]

It's prolly the Class in which the output stage is run, which affects tube life. Class A, AB, B.

More than whether the topology is pentode, triode, UL, or non-UL. Or whether it's push-pull or single-ended.

 

[kward]

What does the author mean by “easier on the finals?”

Maybe he was making a reference to the screen. Maybe he was making a reference to wearing out the cathode. Or maybe he was making a reference to total output power differences.

All else being equal I think the idle current would have the biggest single impact on the tube’s life expectancy, assuming life expectancy is a proxy for being “easier on the finals.”

 

[oldflame]

The article about Audio Note's Octive Jubilee Push-Pull SE Mono amp was in the 8/22 Stereophile edition. The claims were that non-pentode operation always stresses the tubes and leads to less life-time. Pentode output is also supposed to be less sensitive to load, have better phase response, and feedback works better. Each monoblock costs $40k, if that matters. For those $$$ long tube-life might be expected.

If the finals are operated conservatively and well within their parameters, wouldn't that be as important as the type of output tube coupling to its transformer? Is the screen-grid supposed to be the weak-link that Audio Note is suggesting? Seems like there are many successful UL designs getting good feedback from their circuits and listeners who own them.

 

[Mr Boochie]

The article about Audio Note's Octive Jubilee Push-Pull SE Mono amp was in the 8/22 Stereophile edition. The claims were that non-pentode operation always stresses the tubes and leads to less life-time. Pentode output is also supposed to be less sensitive to load, have better phase response, and feedback works better. Each monoblock costs $40k, if that matters. For those $$$ long tube-life might be expected.

If the finals are operated conservatively and well within their parameters, wouldn't that be as important as the type of output tube coupling to its transformer? Is the screen-grid supposed to be the weak-link that Audio Note is suggesting? Seems like there are many successful UL designs getting good feedback from their circuits and listeners who own them.

I think this Audio Note thing has a touch of advertising jive-talk.

 

[gadget73]

Every method is better than all of the others, depending on what you are selling. An expression from my grandma comes to mind "the fish man doesn't cry bad fish".

The UL / pentode thing is an "it depends" things. Some things will tend to eat tubes because of how the amp is built and operated, other things won't.

 

[opnly bafld]

I think this Audio Note thing has a touch of advertising jive-talk.

Mistake by previous poster calling it an Audio Note amplifier, it is an Octave Audio amplifier.

Octave Jubilee Mono SE monoblock power amplifier | Stereophile.com

When I was a young man, blind dates were always laced with anxiety. (Proms were even worse. Once, when I arrived in a rented tux and my father's prized dress gloves, my date's father ordered me to take out the trash.) More than half a century later, blind dates—now with review equipment—remain...

www.stereophile.com

"From the beginning, I've done true pentode designs rather than ultralinear or triode. Ultralinear was developed in the 1960s as a means to rival solid state performance. Ultralinear may give you a little more output power and lower distortion, but in my opinion, it kills the tube. The tube is always overstretched, and lifetime is reduced. It is one of the reasons why the tubes sometimes blow out."


When was UL developed???

Not that I think Audio Note is ultra-conservative in their often creative advertising jive-talk.

 

[trainbuftony]

Invented in the 30s
Developed in the late1940s
Popularized in the early 1950s
Considered out of date by the mid 1960s
A cycle which takes about 4.5 months in 2024.

 

[Mr Boochie]

Mistake by previous poster calling it an Audio Note amplifier, it is an Octave Audio amplifier.

Octave Jubilee Mono SE monoblock power amplifier | Stereophile.com

When I was a young man, blind dates were always laced with anxiety. (Proms were even worse. Once, when I arrived in a rented tux and my father's prized dress gloves, my date's father ordered me to take out the trash.) More than half a century later, blind dates—now with review equipment—remain...

www.stereophile.com

"From the beginning, I've done true pentode designs rather than ultralinear or triode. Ultralinear was developed in the 1960s as a means to rival solid state performance. Ultralinear may give you a little more output power and lower distortion, but in my opinion, it kills the tube. The tube is always overstretched, and lifetime is reduced. It is one of the reasons why the tubes sometimes blow out."


When was UL developed???

Not that I think Audio Note is ultra-conservative in their often creative advertising jive-talk.

I thought Audio Note was a mistake.

Ultralinear designs started a LONG time before the 1960s.

https://dynacotubeaudio.forumotion.com/t2267-history-and-development-of-the-ultralinear-amplifier

 

[opnly bafld]

Yes rhetorical question, I knew when UL was developed and was surprised that Stereophile didn't include a footnote explaining the mistake in the quote.

 

[GordonW]

Actually, UL is frequently EASIER on the screens, than pure pentode mode. This is important, because one of the MOST COMMON output tube failure modes, is screen failure!

Everything else equal- the screens will have LESS peak current in UL mode, than in pure pentode mode. This is because the screens are allowed to go down in voltage somewhat, as the plates drop in voltage (and as the plate current goes up), due to them being on a tap on the primary winding. This means there is never a situation where the plates are near zero volts, and the screens are still at full B+. This reduces the current through the screens, significantly.

In a 43% UL mode- the screen voltage would go down to 57% of the idle screen voltage, at max conduction. That means in a typical KT88 application- even with 500V on the plates- at max condution- where it really matters- the screens are only at 285V, when the plates reach zero volts. Most pentode designs using KT88s use FAR more than 285V on the screens- usually, at least 300V- to as much as 400V or more- on the screens is very typical. In this case, the UL amp will have SIGNIFICANTLY less screen current at full conduction!

In fact- there have been cases, where "pseudo UL" (low ratios of screen taps) have been used for the EXPRESS PURPOSE of allowing a tube to be used in a situation, where it couldn't have been in pentode mode- just because of screen limits. The Fender Bassman 70 is an example. 500V screen voltage on a 6L6GC wouldn't normally be allowable in a pentode-mode circuit- but the RCA manual specifically says that 500V screen voltage is actually OK, as long as the screen is connected to a tap on the primary, so that its voltage will drop to no more than 450V when the tube is at full conduction (plate is at zero volts, effectively). These amps have quite decent tube life, especially for something run that close to the design limits...

This is also the reason why a tube like an 807, can have 600V on the plates, but only 300V on the screens in pentode mode- but, is perfectly fine with 400V on the plates AND screens, in triode mode (plate strapped to screen). The extra 100V rating on the screens, is because the screens track the plate voltage... the screens are never at a higher voltage than the plate. This is why people can also get away with running 400V, or even higher, B+ on 807 UL designs- the screen is never seeing full voltage, at max conduction, there, either...

So, the whole thing that was quoted above, about pentode mode being "easier" on the tube- is actually completely backwards, in reality... it's the hardest possible use of the screens...

Regards,
Gordon.

 

[gadget73]

the 807 can also run happily in triode mode with a lot more than 300v on the screen for the same reason. No voltage difference from plate to screen.

 

[PakProtector]

This comes down to, 'it depends on the tube'. A 6LF6 is far different from a KT88. They both have areas of operation that would not stress one, but would melt the other.

Douglas

 

[dcgillespie]

The problem that can come in with UL is when you go chasing after low distortion numbers via elevated quiescent current levels. Basically, the higher the quiescent current with UL, the lower the distortion will be -- right up to the maximum Pd the tubes can withstand. Look at the Citation II as but one example of this. True UL operation is a narrow screen tap position where not only distortion, but power output and internal output impedance all come together theoretically to achieve the best (or nearly so) of both pentode and triode operation worlds. In my experience however, distortion is usually somewhat higher with UL than an equivalent pentode circuit will produce all else being equal. So if you then employ a notably elevated quiescent current to also achieve the lowest distortion levels, then the tubes will really take a hit. As a result, I think it's designs like the Cit II that have certainly helped brand UL operation as a tube eater. In the case of the Cit II, also a power transformer eater as well. With more blue collar designs like the Dynaco ST-70 however (sold before the Cit II was available), the amplifiers ended their useful life in the 70s due to rapidly changing technology -- but the original output tubes were most often still quite good.

On the other hand, proper designed pentode output stage will have the low distortion operating point achieved within a rather narrow range of quiescent current level, and always within the Pd rating of the tubes, so it can be perceived to be easier on the tubes. If the screens are handled correctly, distortion can also be quite low, but of course internal output impedance goes to the moon. Pic your poison.

Dave

 

[GordonW]

The problem that can come in with UL is when you go chasing after low distortion numbers via elevated quiescent current levels. Basically, the higher the quiescent current with UL, the lower the distortion will be -- right up to the maximum Pd the tubes can withstand. Look at the Citation II as but one example of this. True UL operation is a narrow screen tap position where not only distortion, but power output and internal output impedance all come together theoretically to achieve the best (or nearly so) of both pentode and triode operation worlds. In my experience however, distortion is usually somewhat higher with UL than an equivalent pentode circuit will produce all else being equal. So if you then employ a notably elevated quiescent current to also achieve the lowest distortion levels, then the tubes will really take a hit. As a result, I think it's designs like the Cit II that have certainly helped brand UL operation as a tube eater. In the case of the Cit II, also a power transformer eater as well. With more blue collar designs like the Dynaco ST-70 however (sold before the Cit II was available), the amplifiers ended their useful life in the 70s due to rapidly changing technology -- but the original output tubes were most often still quite good.

On the other hand, proper designed pentode output stage will have the low distortion operating point achieved within a rather narrow range of quiescent current level, and always within the Pd rating of the tubes, so it can be perceived to be easier on the tubes. If the screens are handled correctly, distortion can also be quite low, but of course internal output impedance goes to the moon. Pic your poison.

Dave

This is where continuously-operating auto-bias comes into play.

One reason why UL mode requires a lot of idle current, is that there is a desire, in order to keep distortion low, to always keep bias above a certain threshold. Oftentimes, even with fixed bias- this requires running the tube very hard, so that it never drops below the minimum bias needed.

OTOH, with something like the VAC iQ Autobias- that bias is continuously monitored and maintained- so that it remains above that threshold, without having ridiculous quiescent bias current. Tubes last longer, and low-distortion is maintained throughout the life of the tube. In fact, tubes are usable LONGER because this system will compensate for the normal wear of the tube- keeping bias in the optimal range longer, than is usually possible without running into runaway situations. This is partly because hand-in-hand with the iQ Autobias, is the VAC Sentry system- which will shut down B+ faster than a fuse can blow, if a fault is detected. So, the user can run the tubes, right up until the bias monitor tells you either that the tube is too weak, or that it's failed in another way. At this point, simply pull out the bad tube, install a new one, and go right on with it...

Almost all VAC amps are UL- and almost all of them with the iQ Autobias will test at about 0.05% distortion under normal conditions (until he output level is within 1dB of clipping)...

Regards,
Gordon.

 

[oldflame]

Thank you for correcting my AudioNote/Octave Audio booboo. I have excuses, but the list is too long to post.

Thank you, all, for the detailed and useful information regarding tube-technology -- I learned a few things. This all helped to confirm my suspicions of advertising jive talk.