Ten of 4П1Л in parallel can produce 25 watt, in SE, needing cheap and transparent output transformer and relatively safe 250V anode voltage.
Yes, it looks like microphonics would be a problem with this design.
OK, I am happy to accept that the 300B has good linearity. This is not because it is a DHT, but because the grid and anode have similar shapes and the support rods are well away from the action. It ought to have been possible to make an IHT in a similar way, but perhaps cost was becoming more of an issue and NFB had been invented so nobody bothered. It is not necessarily the case that other DHTs would be similarly good.
OK, I am happy to accept that the 300B has good linearity. This is not because it is a DHT, but because the grid and anode have similar shapes and the support rods are well away from the action. It ought to have been possible to make an IHT in a similar way, but perhaps cost was becoming more of an issue and NFB had been invented so nobody bothered. It is not necessarily the case that other DHTs would be similarly good.
Well, nobody ever DID achieve a design of indirectly heated tube with 300B linearity. So it's merely wishful thinking to claim it is possible.
Conversely, many other superb DHTs which share the same grade of linearity - PX4, PX25, 45, 10Y, 3A/109. They measure and sound splendidly - but are hard to use compared to indirectly heateds.
But difficulties often accompany the best things of many kinds, and the filaments present trouble you could do without. But the rewards are commensurately great, in my view.
Pay your money, take your choice.
From:
A Study of the Output Power Obtained from Vacuum Tubes of Different Types
Pidgeon, H.A.; McNally, J.O. Proc. I.R.E, 1930, 18, 2. pp266-93
"
...Furthermore, repeater tubes must be uniform in their characteristics in order to be interchangeable; and they must maintain this uniformity throughout their useful life in order to keep
the transmission characteristics of the telephone lines within the limits required. Similar considerations apply to the quality of output.
...These requirements and other factors lead to more conservative tube design than is necessary for many other types of service. Particularly is this true with respect to the filament. More electron emission and, consequently, greater filament area are provided than is necessary in many other tube applications.
...Although [tetrodes] have been studied in this country, they have not come into general use, due chiefly, perhaps, to the following reasons. They are somewhat wasteful of space current, since the positive grid collects a considerable fraction of the total current. And for power amplifiers, with high-voltage plate batteries readily available, it has been found economical and more in accord with the general trend in tube design to meet the demand for increased output power by increasing the operating plate voltage rather than by increasing the plate current and, consequently, the electron supply from the filament, to the extent that would otherwise be necessary.
...Unfortunately, in the case of multiple grid tubes, particularly when one of them is held at a positive potential, there is no sufficiently,exact theoretical analysis yet available to permit the determination of the parameters of optimum tubes, as there is in the case of three electrode tubes.
...Flat type structures are the only ones well adapted to the use of filament having the characteristics desired for telephone use."
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It was optimized for different purposes. People needed powerful RF oscillators that can auto-bias themselves by grid-rectified oscillations. It was far from telephone communications.
No, the original question was:Rod Coleman said:
Wavebourn and I have attempted to answer this question. RC provided useful data. Others have provided anecdotes.cotdt said:
Merlin's input suggests they opted for a planar-like structure because that was the only one they could analyse and calculate at that time. (Later it was found that a concentric structure had the same advantages, but unfortunately this was often spoiled by grid supports etc.)
Judging by the literature, by the time of the 6L6 they had stopped using DHTs in repeaters- they didn't have the required bandwidth. Instead they used indirectly heated pentodes like the 175HQ, 5693 and SP12.
Who was encouraged? And by whom? I don't think I've seen a single DHT audio amp published after about 1940?
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The theoretical advantage of DHTs is that they are more linear across a wide range of anode and grid voltages.
The practical advantage of DHTs is exactly the same, and that this advantage applies especially to loudspeaker loads. I have demonstrated this with independent data that anyone can verify if they care to.
300Bs, PX25s and many other DHTs are more linear than any comparable device. If you don't think so, please show us some curves.
Wrap 6dB of feedback around that KT88 triode and its curves would probably beat the 300B, yet have the same drive requirements. Plus you could include the speaker in the feedback loop, if you wanted to.
I have in my possession a number of ex-British Post Office STC 3A/109s. These DHTs were especially designed for telephone repeaters, and the samples I have were made in 1968.
The Western Electric 300B was released in 1937, later than the release of the 6L6.
So, this claim is also quite unfounded.
I answered this point in an earlier post, above.
Repeatedly claiming that things are Not So because you haven't seen them highlights the weakness of your argument.
Rod, you keep asserting that DHTs are more linear. I now accept that some are (maybe all are, I don't know). Could you tell us why they are? It would be helpful if your answer included something which is true for DHT (because they are DHT) but not true for other valves. Otherwise we may conclude that early 1930s valves are more linear and it just happens to be coincidence that most of them are DHTs.
HA! I wondered when the feedback joker would present itself.
You may choose to employ feedback, or any other design technique you like, but it does not forward any theoretical advantage of the device.
Besides, if you are going to take a nonlinear device, and fix it with feedback, why bother with something so archaic, inefficient and fragile as an electron tube? A nice power MOSFET can be linearised with feedback just as well as a poor imitation of a DHT. Only you don't need a heater, or any of the bother of high voltages.
If you think that the outcome is equivalent in performance, you are welcome to it!
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