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Old 1st February 2006, 11:32 PM   #11
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Default Slight distortion reduction in UL PP

Small value screen resistors in a push-pull ultralinear output stage are purported to slightly reduce distortion. This is probablly hard to analyze; it would be an empirical selection.
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Old 2nd February 2006, 04:54 PM   #12
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I used 100 ohm resistors in series with the screens right at the sockets in all of my EL34 based amplifiers, whether triode or UL connected, this was mainly to prevent vhf parasitic oscillations.. It did also help at clipping in UL to limit the screen current. I never built any pentode connected amplifiers commercially.

Kevin
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Old 2nd February 2006, 09:32 PM   #13
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I have graphs somewhere that show that the distortion for p.p. EL34s in UL with 1K screen resistors is noticably lower at maximum output than for very much lower values. I will have difficulty in finding them though - boxes, boxes, boxes.....This is the only tube for which I have seen this.

D_F,

One is never too old to learn (hopefully!) The EL34 as a beam tube? I admit that I have seen them described as such but always regarded that as a rough generalisation. The construction is significantly different, as are the screen characteristics, so one wonders why they did it under the same designation. You are not talking about some of the firms from the east (and north) that merely save by planting the same construction inside a glass bulb? Equating KT66 and 6L6GC I can understand (with an effort, because replacing 6L6s by original KT66s can land one in trouble with heater current), but a pentode with a beam tube? I am lazy; is there somewhere on the net where I can read about this, maybe see a picture of the innards?

Regarding guitar amps I can agree with you; I had to oblige someone once with a design. They are part and parcel of the sound required and I had to do some pretty strange things electronically speaking but the guy ended up very satisfied. (It was quite an adventure to convert to electronics what he wanted soundwise.) In that sense one cannot qualify them as hi-fi, but I also do not "look down" on them if that is what they are required to do. One can say that the guitarist uses say a Fender to produce his particular flavour of sound, but when he replays a guitar CD at home he will use a hi-fi amplifier.

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Old 2nd February 2006, 09:48 PM   #14
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Me again.

Dominique_free, I read right over the web-site you quoted and only opened it after my previous post. It does not appear to give exact details of construction, but set me a-thinking. I have seen ECL82 or ECL86 (pentode section) as beam tubes, but only by virue of beam-forming plates round the G3s. The suppressor grids were still there; I did not bother to investigate whether G1 and G2 were actually of equal pitch and aligned. This was a long time ago, but as above as I recall. Is this what might be meant by pentodes as beam tubes, or are the grids actually aligned?

It might be interesting to note that sensitivity is not always down with beam tubes. The formidable 8417s need a signal of only 17Vp to drive a pair to 100W. (They have a gm of 23000 umho.)

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Old 2nd February 2006, 11:41 PM   #15
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Besides limiting screen current, these resistors provide a form of negative feedback. They're sorta similar to unbypassed cathode resistors in that the AC signal current through them produces an AC voltage at the screen that is 'negative.' It doesn't surprise me at all that measured distortion is reduced with larger value resistors. In my experience the size does affect the sound.

I also remember seeing PP circuits (I think in old tube manuals) with one resistor shared by both tubes. Just like a shared cathode resistor, I think these tend to improve balance.

-- Dave
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Old 3rd February 2006, 12:56 PM   #16
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The tetrode is an evolution of a triode, the problem is at the g2 take a lot of current at low Ua. The pentode is an evolution of the tetrode that supprim this effect but give a tube with an extremly high output impedance.
The beam tetrode is in fact an evolution of the pentode. It is a pentode where the g2 concentrate the electron beam. As every pentode, it have 5 electrodes. The goal was to archieve a tube with as much of the advantages of the triode (low output impedance) and of the pentode (high sensibility). As it is the g2 that is specialy formed to archieve the beam effect, you have to break the tube to see if it is a pentode or a beam tube. In Europa, we call a such tube both as beam tetrode or beam pentode.

Now, an EL34 have those 5 electrodes on the socket when a 6L6 or 807 have only 4 of them on the socket. But that doesn't mean at a 6L6 or 807 have only 4 electrodes inside the tube. They have 5, the only difference is at the g3, the suppressor, is internly wired to the cathode with a 6L6, when you have to wire it externly with an EL34.

I have found this page: http://www.jogis-roehrenbude.de/EL34...EL34-Story.htm
On the first page, we can see at when philips/mullard was manufacturing this tube as pentode, the siemens version was a beam tube.
On the second page, we see at even philips have made some beam EL34 under the name of ECG EL34, and at siemens manufactured some EL34 as pentode. The zenith EL34 from sylvania (philips) was a beam tube, as the sylvania EL34 made in USA.
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Old 3rd February 2006, 01:18 PM   #17
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Quote:
Originally posted by Dominique_free
As it is the g2 that is specialy formed to archieve the beam effect,
It is not exact. The wires of the g1 and the g2 are aligned and the g3 is specialy formed. See http://www.jogis-roehrenbude.de/EL34...nnenaufbau.htm
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Old 3rd February 2006, 05:02 PM   #18
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I found a very good reading on the screen grid: http://www.webace.com.au/~electron/tubes/screens.htm

On the screen grid resistor, it write:
"What we want is for the Screen Grids to be at a DC potential sufficiently high enough to attract and accelerate electrons towards the Plates but, to maximise power output, not to collect and divert them to earth through the B+ supply.

Clearly there will be a particular value of Screen Grid Stopper Resistor that will provide optimum balance between the conventional "short-circuited" Screen Grid configuration and an arrangement whereby the Screen Grids are suitably loaded.

The optimum value will clearly be variable depending upon the particular circuit configuration and operating voltages.

However, as a rule of thumb, and noting the advice of Philips Miniwatt to instal a value of Grid #2 resistor of between 500 to 1000 ohms in each Grid #2 supply lead, we can assume that a value of 50% of the Plate to Plate primary load impedance is an approximate ideal for the Screen to Screen loading.

This will result in a grid stopper resistor value of:

500 ohms per Screen Grid when the transformer primary load impedance is 2,000 ohms Plate to Plate
1000 ohms per Screen Grid when the primary transformer primary load impedanceis 4,000 ohms Plate to Plate
2000 ohms per Screen Grid when the primary transformer primary load impedanceis 8,000 ohms Plate to Plate

For other values of Plate to Plate load, calculate on the basis that each Screen Grid resistor should be 25% of the transformer Plate to Plate primary load impedance

In all cases, pursuant to Philips Miniwatt advice, the Screen Grid resistor is "non-decoupled" - ie is unbypassed.

This resistor must be installed directly to the Grid #2 pin of the tube socket and be preferably non-inductive.

The Screen resistors must have sufficient heat rating to operate safely and reliably without distress."
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Old 3rd February 2006, 08:38 PM   #19
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Quote:
Originally posted by Dominique_free
Clearly there will be a particular value of Screen Grid Stopper Resistor that will provide optimum balance between the conventional "short-circuited" Screen Grid configuration and an arrangement whereby the Screen Grids are suitably loaded.
It's not at all clear to me why the screen should be loaded.
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Old 3rd February 2006, 08:55 PM   #20
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Dominique_free,

Many thanks for pointing my nose at those websites. I have not seen so many EL34s in my life! I hunted for a cut-open view though, so that one could see whether the grids were in fact aligned, which is the main advantage of the 6L6 type. I will call it that, because as often happens a device is called by its most obvious feature but not the most important one. You had to correct yourself for what I also often do - concentrate on the screen alignment feature. I did seem to notice beam-forming electrodes in some of the many pictures, but in all where it was descernable a G3 was evident. The question remains whether beam confining electrodes were merely added to the existing EL34 topology, in which case the advantage would be secondary.

We used quite a munber of EL34s in my days at the CSIR (see my CV), mostly European manufacture but the construction was always the classic one. Though they were comparatively sensitive I went off EL34s because I had glowing spots on G2 when I did not want them. Cutting the tubes open, this appeared to have been in places where several consecutive screen windings were quite open to the cathode (i.e. not shielded in any place by a G1 winding), but that was difficult to see with any certainty. Using beam tunes - OK then, grid-aligned types - I never had the problem. I only encountered 6CA7s once, but they were the same construction as the classic EL34.

Thanks once again,
Johan
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