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Old 29th November 2012, 02:44 AM   #11
Pano is offline Pano  United States
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I thought the advantage was linearity. And consistency of that linearity throughout a large range of frequencies and powers.

DHTs are not the only microphonic tubes around, so I doubt that's the big selling point.
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Old 29th November 2012, 02:46 AM   #12
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Quote:
Originally Posted by 6L6 View Post
When they were designed, the best minds in science and engineering were working on them, and they are designed for the audio range.

The designers had to work with materials and (much more important in my mind) geometry to get them distortion free.
Nah.

Indirectly heated cathodes were not invented yet.

The DHTs were designed for RF down to even DC operation.

The higher quality tubes (although some question that) that were DHT, like the 300B, came after the thoriated tungsten cathode was invented. Tubes like the 300B were intended for *telephone* service by Bell Labs, working for AT&T which at the time had the phone service monopoly.

Screen grids naturally came after triodes were created.

I think this is pretty much correct.

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Last edited by bear; 29th November 2012 at 02:53 AM.
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Old 29th November 2012, 02:58 AM   #13
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Are DHTs more linear? I can't tell by looking at the curves.
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Old 29th November 2012, 03:29 AM   #14
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They're older, perform more poorly, and cost much, much more.

Normally, those would be disadvantages. Go figure.

Linearity of lower-gain tubes, like 300B and 6F6, isn't too bad, and I think that's because the cathode-grid distance is large relative to the wire diameters and grid spacings. These parameters yield low gain and perveance (a 300B takes, what, about the same filament power as a 6L6, but the 6L6 has higher gain and much higher peak current). The distortion is very near the theoretical value of the Child-Langmuir model, whereas tubes with "tighter" construction have more pronounced nonlinearity.

It's noteworthy that, although high gain tubes (e.g., frame grid pentodes) have higher distortion, the ratio of distortion to gain is much more favorable than these old, low gain, relatively linear tubes. If you want to make a low distortion amplifier, high gain is absolutely always the correct answer.

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Old 29th November 2012, 03:35 AM   #15
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Quote:
Originally Posted by wa2ise View Post
I would have thought that the filament being heated with a few volts of AC would cause problems with 60Hz artifacts getting into the audio. Various nonlinearities not cancelling out even if you manage to balance the hum out.

There's some filament tubes intended for portable battery operated radios, like the 3S4 or 3V4, that were designed for DC on the filaments, and the filament pins were labeled as to which gets the positive filament supply and which gets the negative side.
DHTs were originally meant to be powered by lead acid batteries, and therefore designed to be DC powered. You're right, AC powering generates distortion even if hum is cancelled out. I have a switch on my amp to switch between AC and DC, and the AC is slightly warmer while the DC is slightly cleaner, but I'm not sure I would be able to tell the difference in a blind test. It's subtle.

A DHT like 300B to me sounds better than KT88 in both tone and detail, even though the KT88 has better specs. I wonder why this is so.
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Old 29th November 2012, 03:57 AM   #16
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The harmonics are different, that's why.

Also the KT88 is a screen grid type tube, which makes it a pentode, and that means a different set of curves, which means a different spectrum of distortion. That and the aforementioned "microphonics" of any DHT.

Also, you probably will never want to run a pentode without feedback... which is a very different animal than ZFB.

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Old 29th November 2012, 04:03 AM   #17
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Quote:
Originally Posted by cotdt View Post
Are DHTs more linear? I can't tell by looking at the curves.
No, in general they're not, compared with unipotential cathode triodes (or triode-connected pentodes). And in the case of power tubes, the drive requirements pretty much guarantee that you'll lose linearity elsewhere as well. I'll adopt Nelson Pass's term for Lowther loudspeakers- "fetish objects."
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Old 29th November 2012, 04:05 AM   #18
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Quote:
Originally Posted by bear View Post
I think this is pretty much correct.
It is. My first experience with DHTs was a high powered RF amp with four V70D/8005s in parallel. Great performer on the 10 meter band.
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Old 29th November 2012, 04:14 AM   #19
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Originally Posted by cotdt View Post
DHTs were originally meant to be powered by lead acid batteries, and therefore designed to be DC powered. You're right, AC powering generates distortion even if hum is cancelled out. I have a switch on my amp to switch between AC and DC, and the AC is slightly warmer while the DC is slightly cleaner, but I'm not sure I would be able to tell the difference in a blind test. It's subtle.

A DHT like 300B to me sounds better than KT88 in both tone and detail, even though the KT88 has better specs. I wonder why this is so.
Not true... the 45 triode was specifically designed for large output power in AC operated receivers per the original spec sheet (all manufacturers) and all specified ratings specifically state that an AC filament is used for the specs... same holds true for the 2A3 and the WE 300b, albeit WE notes bias changes for operating the filament on DC.

You do have some AC induced artifacts from the filament, but good (45) tubes and good design will minimize it. I've got sets of 45 triodes that manage upwards of 90dB s/n (ref 1-watt) with AC filaments. Granted, not all DHTs can be quiet enough with AC, but even that seems more subjective as some folks appear elated with ~1mv of output noise.

Your comments on AC vs DC via switching your amp makes no sense to me, if you can clearly articulate the difference between them, how can you claim you couldn't tell in a blind test??

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Old 29th November 2012, 05:04 AM   #20
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Microphonics. People like that extra little "ting!" in their music.
What is the standard for measuring microphonics and where can the data for commonly used tubes be obtained?

John
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