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Old 28th October 2007, 09:43 PM   #1
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Default Best Sounding Output Tube

Hi,

Looking for opinions - power aside but all other things being equal, is there a noticeable difference between different types of output tubes. I am not thinking about different makes of the same tube but for example the KT88 compared with the EL34 - this comparison in particular interests me as I have a EL34 power amp and I am currently building a KT88. Apart form the difference in power, should I expect to notice any difference in the sound quality? I realise that this is a difficult question and will depend somewhat on the brand of tubes and of course the speakers - however, does anyone have an opinion?

Cheers,

Rob
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Old 28th October 2007, 10:22 PM   #2
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Define "best", if you please. IMO, there is no best, in the absolute sense. After all, we're talking: whose room, whose ears, which speakers, what style of music, etc. ad infinitum.

There is a definite difference in the general voicing of true pentodes and beam tetrodes. The EL34 and the 6CA7 are electrically equivalent, but they don't sound the same.
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Old 29th October 2007, 12:28 AM   #3
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Just to help keep things clear for me, can you verify the following?

6V6 - beam tetrode
6L6 - beam tetrode

EL84 - pentode
6BQ5 - beam tetrode?
6pi14pi - ?

EL34 - pentode
6CA7 - beam tetrode

6550 - I've seen this described as a "beam power pentode" - what the heck is that? I believe the 6550 is the evolution of the 6L6, which would suggest it is a beam tetrode.

KT88 - "kinkless tetrode" - so is this really a tetrode as the name implies, or is it a true pentode like the EL84 and EL34?

Can any generalizations be made, such as American tubes tend to be beam tetrodes and the European tubes are usually pentodes?

Which do you tend to prefer, beam tetrodes or pentodes?

Do you find any truth in this quote?
"Pentodes have a poorer damping factor compared to Tetrodes in similar circuits, which means we can get more output tube distortion. Of course, with a bit of feedback in the amplifier, we'd be hard pressed to differentiate between the two sonically, but the fact remains that the pentode output stage is run without any negative feedback, making clean headroom take a back seat. The only 'drawback' to a real Pentode versus a Beam Power Tetrode is the sad fact that Pentode tubes are more susceptible to a potentially damaging Screen current rise at maximum signal than Power Tetrodes. This is because the beam forming plates in a Beam Power Tetrode drastically cut down on secondary emission, which will help keep the Screen Grid current increase at full throttle to a minimum. "
http://www.tone-lizard.com/Vox_Myths.htm
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Old 29th October 2007, 12:46 AM   #4
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I would have thought that this question would have brought forth many more "expert opinions" by now. As previously stated there is really no "best tube". The "best tube" for one particular amp, may sound so - so in another amp, or in the same amp under different conditions, or even in the same amp under similar conditions with different music.

This brings me to my opinion. I have a SimpleSE set up so that I can run EL34's, KT88's or 6L6GC's without changing anything. I find that EL34's reveal more of the details in the music than the other tubes, and the KT88's have more bass and flat out rock better than the others. So if you want to play Metallica turned up to 11, use the KT88's, if you want to hear the details in Norah's voice, use the EL34's. 6L6GC's fall somewhere in between.

I have not tested any of the high dollar tubes (not in my budget). My favorite EL34's are JJ's and the KT88's are Electro Harmonix. I tend to swap between these tube sets depending on my musical preferences at the time, and how loud I am going to crank the volume.
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Old 29th October 2007, 01:35 AM   #5
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6V6, 6L6, are indeed beam tetrodes. As are the original KT88 and 6550. I have never seen any of these tubes that were not beam tetrodes.

The original 6BQ5's and EL84's were true pentodes. It is possible that some newer tubes could have beam tetrodes, but all of the ones that I have seen are pentodes.

The original EL34's and 6CA7's were true pentodes, although I have seen both type numbers on some beam tetrodes.

As for generalizations, as the vacuum tube era waned and production ramped down, many manufacturers all over the world started stuffing all sorts of unusual combinations into the glass.

The statement that pentodes may be more succeptable to screen current overload may have some basis. This should not make much difference in a HiFi application since the amp should not spend too much time in the overload region. The story is different is a guitar amp cranked to eleven. Some tubes got poor reputations from the guitar amp industry. Some manufacturers built tubes specifically for the guitar amp industry. The Sylvania 6L6GC STR was one of these. The EL34 was heavilly used in guitar amps, and the beam tetrode versions were probably made for survival in guitar amps. Most of the ones that I have seen were Japanese.
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Old 29th October 2007, 02:23 AM   #6
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I like KT77's also, they're worth a try. Think the EL34's slightly older best friend.
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Old 29th October 2007, 02:26 AM   #7
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A few more differences I'm aware of, between pentodes and beam tetrodes:

* BTs have a sharper 'knee' in the pate characteristic curve.

* BTs often do well in Class AB2 (grid current), e.g. 807 and 6146, but I'm not aware of any pentodes that do.

* BTs need much lower grid 'leak' resistors than pentodes for bias stability.

* BTs are reputedly more prone to parasitic oscillations.

* BTs, in general, take lower screen current than pentodes, as a percentage of plate current.

With all the differences between the two, it seems very likely that they should sound different. As to which sounds better, well, that's a matter of personal opinion I guess. I don't think there's anything fundamentally wrong with either design that would cause it to sound inferior, provided it's used properly.
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Old 29th October 2007, 02:38 AM   #8
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Putting a KT88 into a circuit fit for EL34's can often sound dull at the top end and the square wave will show it and also the otherway round.. It depends on what circuit and output config you are using. Although many IO tubes are interchangable, there will be bias and load Z differences which (the latter) will effect the sound.
The most influencial I find is between a 6550 and EL34. The 6550 is a lower running Z tube and with a well matched o/p tranny can provide stunning performance but look at the bias requirements ?. So said for others.
I like EL34's, all makes behave well. Personally I would abandon the KT77 and use KT88's instead.

This reminds me about the 6L6. Data sheets quote B+ max at 360V. Who's using them higher than this and in what mode. I came across one design at 450V+ true p-p ?? That's pushing it.

rjich
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Old 29th October 2007, 03:37 AM   #9
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Quote:
Originally posted by Ty_Bower
6550 - I've seen this described as a "beam power pentode" - what the heck is that? I believe the 6550 is the evolution of the 6L6, which would suggest it is a beam tetrode.

KT88 - "kinkless tetrode" - so is this really a tetrode as the name implies, or is it a true pentode like the EL84 and EL34?
It's a difference in terminology. Real tetrodes, those with a cathode, control grid, screen grid, and plate disappeared early on. Real tetrodes have a nasty negative resistance "kink" in the plate characteristic that must be avoided (unless you're making UHF oscillators). Pentodes were originated to eliminate that negative resistance characteristic.

The "beam" tubes were developed to beat a patent, and so the developers avoided the word "pentode" deliberately, referring to it as a "kinkless tetrode" to avoid legal problems. However, "pentode" simply means "five electrodes", and that's how many beam tubes have: 1) cathode 2) control grid 3) screen grid 4) beam formers 5) plate. Therefore, it is a pentode. Once the pentode patent lapsed, they became to be called "beam pentodes".

Quote:

Do you find any truth in this quote?
"Pentodes have a poorer damping factor compared to Tetrodes in similar circuits, which means we can get more output tube distortion. Of course, with a bit of feedback in the amplifier, we'd be hard pressed to differentiate between the two sonically, but the fact remains that the pentode output stage is run without any negative feedback, making clean headroom take a back seat. The only 'drawback' to a real Pentode versus a Beam Power Tetrode is the sad fact that Pentode tubes are more susceptible to a potentially damaging Screen current rise at maximum signal than Power Tetrodes. This is because the beam forming plates in a Beam Power Tetrode drastically cut down on secondary emission, which will help keep the Screen Grid current increase at full throttle to a minimum. "
http://www.tone-lizard.com/Vox_Myths.htm
Don't know; don't care. I'm not one of those audiophools who thinks that gNFB is the spawn of the devil. Using local and/or global NFB can improve the damping factor of any amp that uses pentode finals. So far, I've done designs with the 807 and the 6BQ6 (both BPPs) run as pentodes, and the included gNFB keeps the speeks under control as well as eliminating that pentode nastiness. Both sound just great.
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Old 29th October 2007, 04:48 AM   #10
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Quote:
Originally posted by Miles Prower


Once the pentode patent lapsed, they became to be called "beam pentodes".

I always thought that true beam pentodes were Telefunken LS-50 and Soviet version GU-50. They both have precisionally aligned grids. (By the way, speaking of best sounding output toobs)
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