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Old 25th December 2005, 10:20 AM   #1
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Default is this true????

hello,

i was reading on the internet just lately and came upon the following article:

http://www.webace.com.au/~electron/tubes/oes.html

first thought.......yeah right.....but then again, it might work....

i will try this on my learning amp (very simple 5687->el84 SE)
can you also implement this in ultralinear mode???
i have a 10k-5k-2,5k opt....so i could connect the el84 ultralineair (5k-2,5k taps) and put the diode on g2????

merry xmas,

kathodyne
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Old 25th December 2005, 10:18 PM   #2
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Kathodyne,

I seem to be the first to react to this .... and I must admit it is difficult to decide where to start. The simplest (and wise) answer would be to just reject the lot. But a little explanation is perhaps deserved.

Basically a screen grid draws d.c. current, which can go up or down during operation, but never reverses. And only when it reverses, will a diode in the line have effect. Otherwise it simply stays in conduction with a voltage drop of some 0.6-1V. It does not know that there is a.c., signal or whatever in question. As long as it is forward biased, it will conduct. (The varying voltage drop across it as a result of G2 current fluctuations can be neglected for this very basic discussion.)

I must frankly admit that I do not know what the author is about by claiming that the screen will suddenly not expend power because a diode in its circuit meraculously "isolates" it from signal. Also, proceeding to G3, there has been informative post on that on another thread about where to take G3 in an EL34. Again just to summerise, the amplification factor to anode from G3 is so low as to be negligible, compared to others.

Furthermore it is safe to say that while the respective grids' position and effect on space charge could be an interesting topic, the manufacturers' initial decision on geometry inside a tube would not purposely leave a lot of room for spectacular improvement in performance. And that discovery suddenly now, 50 years after the hey-day of vacuum tubes!

Lastly, the suggestions re electrolytic capacitors as that they mainly pass signal in one direction only or some such concept reveals lack in basic understanding and strengthens the impression that there is nothing of real value here.

So, I would say this is simply a no-go - sorry. Unless some well-enlightened member can point out where we are all wrong, and that I should claim my university class fees back. (I need the money, but I doubt whether I can depend on that as a source of income.)

Regards.
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Old 26th December 2005, 09:55 AM   #3
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thank you johan for your understandable reply!

i already thought this was too good to be true, with my little understanding i already found it very strange that current would go back into the psu or transformer (via g2)


hmmm...first snake-oil experience, almost bought it!!!!
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Old 1st January 2006, 10:37 PM   #4
marconi is offline marconi  Europe
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There is such a thing as screen inversion in power tubes, it is a well known effect in VHF power tetrodes. The screen current reverses under certain conditions, and this is usually prevented by a bleeder resistor -or a gaz tube or sometimes more fancy stuff- to the cathode.
Not to be mistaken for "secondary emission" which is a different animal..
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Old 1st January 2006, 11:01 PM   #5
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Marconi,

There is a saying: He who asks is a fool for 5 minutes; he who does not ask is a fool for the rest of his life.

I remember something about screen inversion, but it was lo-o-o-ng ago. To avoid falling foul of the latter part of the saying, please refresh my memory (and perhaps that of others) and briefly explain that again.

Thanks!
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Old 1st January 2006, 11:11 PM   #6
marconi is offline marconi  Europe
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A quick search on google keywords: tetrode-screen-reverse- will enlighten you.
Try this one:
http://www.cpii.com/eimac/sec2-2.htm
Rgds
Marcel
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Old 1st January 2006, 11:15 PM   #7
kevinkr is offline kevinkr  United States
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Presumably adding diodes in series with the screens of classic tetrodes might eliminate the tetrode kink when the plate voltage falls below the screen voltage. (By preventing it from hogging the plate current.) With modern "kinkless tetrodes" and all ultralinear configurations this ought not to be an issue at all?

I am just starting to experiment with the C3m which has separate connections for all grids and the very comprehensive data sheet shows the effects of connecting the suppressor grid (g3) to the plate, and to the cathode - one is clearly tetrode, kink and all, and the other is classic pentode. Take a look at those curves - it's quite informative. This would be a good tube type to determine if there is any merit to these claims.

I don't think there is a lot original here, I have seen some of this stuff elsewhere.

I have even seen amplifiers with series diodes in the screen built by people I am sure never encountered this website if for no other reason than the web didn't exist at the time. I never verified whether or not there was any merit to the approach, the amplifiers in question worked. (I was already into triodes by then )

His claims in regards to cathode bias circuits, driver transformers and bypassing are standard good engineering practice, not at all original, and should NOT be marked copyrighted in conjunction with his claims or anything else. And I don't like electrolytics either, but not for the reasons cited.

There is a lot of high sounding prose and absolutely no measurements backing up any of these claims. Until someone actually verifies or refutes these claims they need to be taken with a grain of salt. (At least)
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Old 1st January 2006, 11:28 PM   #8
kevinkr is offline kevinkr  United States
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Reading the eimac application information it is quite clear that adding a diode to prevent reverse screen current from flowing would destroy these particular tetrodes.
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Old 2nd January 2006, 12:07 AM   #9
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Marcel,

Yah me man! There it comes back from I dare not say how long ago!

We did these experiments at the time (varsity) with the good old 807. But since then I have not done anything in r.f., at least not with tubes. And I think this effect will not be evident in audio, which is why one never reads about it. ... might have to think about it though on anode peaks of non-UL operation - but then the screen supply in audio is mostly low impedance. (I am pondering though, the prescribed use of 1K screen resistors for the EL34. Might be interesting to measure there...) Also figure 9 in your reference juggled the old grey matter again.

Much obliged to you!
Johan
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Old 25th January 2006, 06:53 PM   #10
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Johan ,

I entirely agree with you .

Carlos
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