Yes, that could be true. The numbers argue against it, though. For SS the possibility of crossover distortion complicates things, but for typical pentode/UL P-P the midlevel distortions are likely to be lower than SET (whether DHT or not). If distortion preferences are the issue then this is more likely for those technologies with larger distortions.Pano said:
Anyway, we have evidence in front of us that a low mu DHT has smaller distortion than a higher mu IHT (triode-connected tetrode). I don't argue with genuine figures/graphs. It does not clear up the issue of DH vs. low mu. I suspect the latter, but it just happens that DH (being early in time) correlates with low mu.
Such a claim withstands no scrutiny.
According to Jogi, the 6C33C indirectly heated triode has a mu as low as 2,7:
http://www.jogis-roehrenbude.de/Russian/6C33C/6C33C-B-6S33S-VExtendedDatasheetMB.pdf
And now look at how the curve compression sets in as low as 300V, and the mu variation at a similar voltages:
The 6C33C Tube
Same source of curves, making this a valid comparison.
If you don't accept my assertion about the general superiority of Directly Heated Triodes (for Speaker Driving, as I have qualified and demonstrated), here is your challenge. Please demonstrate an alternative device to compare to 300B or PX25 for an elliptical load line, representing a typical speaker. Please make sure it is a comparison on an equal footing - and comparable measurement conditions.
Just to clarify, I am not saying that low mu guarantees low distortion but it makes it more possible because the large spacing means that the electric fields produced by the grid and the anode can be the same shape when seen at the cathode - provided that the element shapes are similar. A high mu triode finds it much harder to achieve this, although it can be done.
Also, please consider why it is that Western Electric went to the trouble of designing the 300B in 1937.
At that time, the 6L6 was available, and much more efficient in terms of power output and cost. And yet this low-efficiency triode was used for decades in voice telecommunication.
They clearly did not accept the idea that indirectly heated triodes OR triode connected beam-tubes was equivalent, when driving a tough load.
Build yourself a SE 300B amplifier today, and compare for yourself. When properly implemented, one can only come to the same conclusion as WE, if yours ears be the judge. My examples of curves are an attempt at explaining what is plain to all who have made the listening comparison (given first class design work).
At that time, the 6L6 was available, and much more efficient in terms of power output and cost. And yet this low-efficiency triode was used for decades in voice telecommunication.
They clearly did not accept the idea that indirectly heated triodes OR triode connected beam-tubes was equivalent, when driving a tough load.
Build yourself a SE 300B amplifier today, and compare for yourself. When properly implemented, one can only come to the same conclusion as WE, if yours ears be the judge. My examples of curves are an attempt at explaining what is plain to all who have made the listening comparison (given first class design work).
On the surface of it, yes - that makes sense. Alas, the ear isn't so simple. Harmonics play a very large role in how we perceive sound. A well designed, well implemented DH-SET amp can have a harmonic structure that sounds transparent to the ear. It won't measure that way, but it will sound that way. Old news, I know, but it bears repeating.
I've often found that amps or line level devices that annoy me may measure with fairly low THD, at least >1 KHz. But they often have a plethora of high order harmonics, especially as the fundamental increases in frequency.
Certainly agree. I've found over the years that I like low mu devices best, but don't know why that would be. Maybe they tend to be more linear than hi mu. That could be the most significant difference to the sound.
No, it is saying that they spread fields differently, in terms of geometry. Not in the absolute quality, but on relative, when the voltage varies Edit: in typical working conditions.
Power pentodes like 6L6 were designed for the pulse duty, they needed to produce high currents, so their linearity may approach linearity of 300B only when it is driven too hard to dissipate anode power. I believe that their usage in audio amplifiers started from famous article of Otto Schade, but designers starting using them in audio amps omitted significant "small" addition: negative feedback proposed by Otto.
Otto H. Schade - Wikipedia, the free encyclopedia
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A bit of googling tells me that the 300B was designed (as the 300A - same innards, very slightly different base) several years before the 6L6 came out.Rod coleman said:
Wavebourn, can you clarify what you are saying? Is it small cathode (i.e. DHT) or low mu (wide cathode-grid spacing) which reduces island effect and so improves linearity?
Sure the 300A came first. But the design of the 300B, in 1937 cost time and money, and they still ended up with a highly inefficient (power output-wise) device.
Why did they bother fitting this low-efficiency device in voice repeaters, when the 6L6 was available? It is more expensive to apply, too, with a separate filament supply needed, where the 6L6 can share with the small signal tubes.
Similarly the Marconi Osram PX25 was encouraged to be applied to new designs of audio amplification right into the 1950s. The same difficulties are present: separate filament supply, low efficiency, high sensitivity to filament supply noise.
The answer is plain. The 300B and PX25 are more linear than any other device, when the load is reactive. Can you show us the curves for any alternative?
Why did they bother fitting this low-efficiency device in voice repeaters, when the 6L6 was available? It is more expensive to apply, too, with a separate filament supply needed, where the 6L6 can share with the small signal tubes.
Similarly the Marconi Osram PX25 was encouraged to be applied to new designs of audio amplification right into the 1950s. The same difficulties are present: separate filament supply, low efficiency, high sensitivity to filament supply noise.
The answer is plain. The 300B and PX25 are more linear than any other device, when the load is reactive. Can you show us the curves for any alternative?
Presumably due to long term reliability, not linearity.
Who was encouraged? And by whom? Did the king of audio lay down a decree?
The cathode is thin, but also, it is long (much longer than the tubular cathode of the indirectly heated tubes) - presenting a greater overall area to the grid.
This means the electric-field will spread more evenly, I suspect. Drawing it out will be instructive.
They rotated a locating pin on the base, to convert a 300A to a 300B. The reason was apparently so it could substitute for another valve. The paperwork probably cost more than the redesign effort.Rod Coleman said:
I'm not certain. Does the 300B have an essentially planar cathode i.e. made from a number of parallel and closely spaced filaments? If so, that would provide linearity when combined with a planar grid and a planar anode surface. Linearity comes from similar shape electrodes, as undisturbed as possible by support rods.Wavebourn said:
Sure. It seems to me one German designer was designing the best in the world triode, when his boss yelled, "Stop spending people's money for your toys, the Reich needs a pentode ASAP!"
Later they were captured by Allies from the East, and made 4П1Л tube. After success Allies from the East wanted to make similar, but more powerful, tube called ГУ-15, but failed to get German manufacturing tolerance. As the result, it is easier to parallel a pair of 4П1Л to get the same linear power. When working on my experiment I took tubes bought from several sellers, of different years of production, and they were very close to each other. I never saw such thing except a quartet of 6L6 manufactured by RCA in 1943. They were also as twin brothers. Rusted, in rotten boxes, but measured better than specs guaranteed. I sold them to somebody in Japan who wanted them, I did not dare to use that vintage rare tubes for a new design.
Why would a large-envelope and a fragile floppy filament, mounted on springs, last longer than a stubby little bottle with robust heater? I hear the sound of clutching at straws.
Just read the PX25 data sheet.
"...designed to supply a considerable undistorted volume".
Marconi-Osram made no such claim for their KT88.
I was thinking more along the lines of emission decay rather than physical failures. Repeaters run continuously, so as long as you get it going in the first place, the heater is unlikely to fail.
Sure, and longer electrodes that create more of uniform field give more linear results. I love also 6П15П tubes in triode mode. They are almost as linear as 4П1Л.
The principal characteristic of the Western Electric 300B is its linearity over a wide range of anode and grid voltages. This makes it ideal for driving difficult reactive loads, without feeedback - such as long transmission lines, and loudspeakers.
I have illustrated this property by comparing the curves with those of a comparable triode and beam power tube.
The superiority is perfectly obvious, merely by inspecting the curves.
If you don't think that linearity of the 300B is a big deal, please show us an alternative device of any kind, measured in comparable terms, driving a speaker load.
I have illustrated this property by comparing the curves with those of a comparable triode and beam power tube.
The superiority is perfectly obvious, merely by inspecting the curves.
If you don't think that linearity of the 300B is a big deal, please show us an alternative device of any kind, measured in comparable terms, driving a speaker load.
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