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Old 13th April 2007, 04:10 PM   #11
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Hi Simon,

Quote:
but this could point the way: http://www.vaughnaudio.com/carina-FAQ.html#Q14


Sorry, that sounds just like some maketing mumbo jumbo buzzwords collection - no hard facts given.

By all means, a screen grid tied to the anode just gets part of the anode, and no, neither Langmuir/Child theorem nor the electrons in space charge cloud do object to this.

Regards,

Tom
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Old 13th April 2007, 11:53 PM   #12
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I already tried to answer this in another forum, but I'll throw it in here too for people to argue about:

The reason, possibly the ONLY reason, why those antiquated DHTs from eighty + years ago are still popular and still produced, is because they are amazingly linear. Just have a look at the plate curves for a 300B or 2A3 and compare them with those of a triode-strapped pentode or beam tetrode, and you'll see what I mean - the difference is quite dramatic.
The consequence of this is that a triode-strapped pentode/tetrode produces more odd-order harmonic distortion. A DHT has plenty of second-order and a much lower amount of third-order, and that's about it. A triode-strapped pentode/tetrode produces also 5th and 7th order distortion, which can make it sound harsher. I don't believe the bass sound is any better or worse, but at the middle and treble frequencies the difference would be audible with a decent speaker and SET fans tend to be particular about this sort of thing!
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Old 14th April 2007, 05:38 AM   #13
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Hi ray_moth,

Quote:
Just have a look at the plate curves for a 300B or 2A3 and compare them with those of a triode-strapped pentode or beam tetrode, and you'll see what I mean - the difference is quite dramatic.
Just have a look at the plate curves for trioded EL83, 6V6, even 6L6 instead, I could name more. Too small? Just have a look at trioded 813.

For small signal tubes, f.e trioded EF86 and EF184 even rival, if not beat hands down, 6SN7 for linearity.

Compare them with some real triodes like, say, ECC81/12AU7 or 6AS7, I could name more, and you'll see what I mean - the difference is quite dramatic.

What was your point again?

Regards,

Tom

P.S.: Interestingly enough, when I held my "lecture" on triode-strapping pentodes in front of some real experts at the European Triode Festival ETF06 meeting, nobody stood up and said: "But real triodes inherently are more linear!". Instead, they had a good laugh when I (wink, wink) mentioned that I intentionally had worsened an example loadline plot of trioded EF184 so it wouldn´t upset the "real triode guys"
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Old 14th April 2007, 05:59 AM   #14
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Hey, maybe its that h4wt naked tungsten that makes the 813, 2A3, GK71 oh so nummy.
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Old 14th April 2007, 07:42 AM   #15
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Quote:
Originally posted by Tubes4e4


Compare them with some real triodes like, say, ECC81/12AU7 or 6AS7, I could name more, and you'll see what I mean - the difference is quite dramatic.

If I am correct, 6N13S I tried is a clone of 6AS7? Strapped 50L6 I've found more linear.
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Old 14th April 2007, 08:09 AM   #16
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Quote:
Originally posted by Tubes4e4
Compare them with some real triodes like, say, ECC81/12AU7 or 6AS7, I could name more, and you'll see what I mean - the difference is quite dramatic.
I don't think any of these are directly heated, which is what the question is about. It is not 6V6 v. 6AS7, it is 6V6 v. 45.
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Old 14th April 2007, 08:36 AM   #17
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Quote:
Compare them with some real triodes like, say, ECC81/12AU7 or 6AS7
Those are not particularly linear triodes but they're also not DHTs either, so that's beside the point. I would agree that a triode-strapped EF86 or 6AU6, for instance, would probably be more linear than any of those IDH triodes and probably better than 12AX7 too. However, these are small signal pentodes, not power tubes, so that is also beside the point.

I still maintain that 300B and 2A3 are more linear than triode-strapped power pentodes/tetrodes - that was my point.
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Old 14th April 2007, 08:59 AM   #18
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Hi dsavitsk,

Quote:
Originally posted by dsavitsk
I don't think any of these are directly heated, which is what the question is about. It is not 6V6 v. 6AS7, it is 6V6 v. 45.
6V6 ist not DH, so I don´t understand your point. I thought the point was more like "triode strapped" vs. "real triode"?

Anyway, have a look at triode strapped direct heated 813 pentode and then in turn show me a comparable real triode (DHT or not) of that class that does better regarding linearity.

Sadly, there are only very few direct heated pentodes of smaller power classes, which is natural when considering the time line of developments, and also when considering why direct heated emitter technology was superseeded at that time by indirect heater technology - what DH emitter afficionados like to ignore because it doesn´t fit in their argumentation. Instead, they just claim that DH is "the real thing" w/o giving hard facts, like for exampley how electrons are able to remember from what type of emitter they emanated, making them so proud that they sing the song with more musicality.

Please note that my point is not to claim that trioded pentodes are better than real triodes in general, but to show, that just repeating/exclaiming over and over "DHTs are superior in linearity to triode strapped pentodes" or just "real triodes are superior in linearity to triode strapped pentodes" doesn´t make a wrong statement true.

Regards,

Tom
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Old 14th April 2007, 09:15 AM   #19
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If you're comparing like with like, then look at directly heated tubes - something like the 2E22 and 1624 in triode. Then there's the 49 and the "almost triodes" like the 46. I personally haven't tried these - can anyone report? The 46 has a high reputation already, the others are less used.
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Old 14th April 2007, 12:56 PM   #20
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And heres my 2 cents:

Quote:
Originally posted by phn
A pentode strapped as a triode is a triode. It has the same characteristics as a triode. It behaves like a triode.

Quote:
Originally posted by Klimon


NOt really. Compare a default pseudo-triode with a pseude-triode with additional screen circuit to separate ac and dc paths -- not a subtle difference in sound

Simon

Hmm surely its just a DIFFERENT triode? Fact is, a triode strapped pentode or tetrode, is responding like a triode - it has triode like curves. Whether or not it is a GOOD triode is debatable. Like the 6AS7 is a triode but, not a linear one.

I've been looking at this as you probably guessed, and I'll say right now that I am definately NOT a fan of DHT's, but they are maddeningly linear on paper. Closest "pentode as a triode" that I could find is the trusty EL34. Which is what my SE amp is using. KT88 was OK but goes to pot at high signal swings

evidence for this claim is buried in a spreadsheet but here's the conclusions. I used a "typical" transformer impedance, measured the distance between the grid bias curves and took the standard deviation over a "typical" range of input swings. "Scores" were:

300B = 3
6v6 = 3.8
el34 = 4.4 (but 3.3 if you stay well away from clipping)
KT88 = 6.8
6AS7 = between 22 & 33 varies with transformer impedance

Ok I had to make some assumptions and therefore these are not definitive but it demonstrated to me that a correctly biased 300B is hard to beat.

Andy
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