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Old 3rd February 2006, 09:10 PM   #21
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Quote:
Originally posted by Dave Cigna


It's not at all clear to me why the screen should be loaded.
Dave,

It is a form of negative feedback. (We are not talking about small anti-parasitic resistors.) When the anode swing goes low, the screen will draw more current, and thus also go down in voltage, and vice versa. It looks like UL performance in a sense but without the direct coupling with the output impedance and thus a form of loss. I have looked at this with a scope and the voltage appearing on G2 is more or less sinusoidal (with sine input), but I have never tried to measure effect on distortion (linearity). I would not do this but rather go to UL with its known characteristics. I have never seen graphs but they would be easily traceable with tube plotters. I still cannot imagine why with EL34 UL operation a 1K screen resistor is recommended specifically - but I believe the above is what is taking place.

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Old 3rd February 2006, 11:30 PM   #22
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In a tetrode, a pentode or a beam tube, this is the g2 that attract the electrons, because the cathode see the g2, not the anode. The gain of the tube depend therefore much more on the voltage on the g2 as from the anode voltage.

The g2 is a very loss grid, so at the majority of the electrons will just pass thru it. After at they have passed thru the g2, they just continues to the anode.

The g2 cannot dissipate a lot of heat. Just a few watts for a power tube as an EL34 or 6L6. The problem is at, when the anode voltage is becoming lower as the g2 voltage, many electrons will be attracted by the g2, and you have to limit this current if you want to not destroy the tube.

To see how much current the g2 can take, look at page 8 and 9 at http://www.tubezone.net/pdf/807curv.pdf
Those curves show the variation of Ig2 for different Ua. It is a curve for different Ug1 and a serie for Ug2=250 et Ug2=300V
The 807 have almost the same electrical charachteristic as the 6L6, but with higher Ua.

On the curve of page 9, you have a point at Ug2=300V and Ig2=300mA. It implies Pg2=90watts. An hifi amp will never go to this point. Even a class B guitar amp will never reach to this point because to get at this point, you need to drive the tube with a power that no one driver will put on the command grid. (page 10 and 11 for Ig1=f(Ua)

But it is still points that we will get without screen grid resistor that can damage the tube.
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Old 3rd February 2006, 11:56 PM   #23
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Take a look at the fat bulb 6CA7 made by General Electric, this is a beam power tetrode and has very different internal construction to both other brands of 6CA7 which were pentodes and European made EL34 which I believe were/are all pentodes. I used to have some of these and if I find one I will try to post the picture, they are different - I used to see them in a lot of old dynaco st-70 amps which had been retubed in the 1960's or later. Seen also in guitar amps where they are considered desirable..

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Old 4th February 2006, 01:06 AM   #24
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Quote:
It is a form of negative feedback. ...
I understand that. In fact I said it here yesterday. But, Dennis Grimwood, the author of the web page from which your quote comes, claims that we want to keep the screen at a high enough voltage to accelerate electrons, but avoid having the electrons go through the screen circuit (instead of the plate circuit) because that wastes power. Specifically, he says

"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."

Well, it's not clear to me that there is an optimum balance (what exactly is being optimized?) or even that the screen should be loaded at all. I understand that applying negative feedback will reduce distortion, blah blah blah, but I don't think it 'clearly' is desirable to do so. More to the point, it's not clear what it even means that a screen be "suitably loaded."

It seems to me that it's just a design priority of his (minimize screen current in order to maximize plate current) and inserting series resistance is his method of choice.

Perhaps I'm a little predisposed: I've read the page in the past and what I've found impossible to forget is Grimwood's claim (at least it seems like he's claiming) that as the screen voltage is raised, when it reaches and exceeds the plate voltage it will rapidly start conducting more current. I just don't believe that it's true. Of course the screen conducts more as its supply voltage is increased, but nothing interesting happens at or around the plate voltage.

We can verify this from published plate curves of any tetrode/pentode/beam tube. If anything interesting happens when we hold Va constant and vary Vg2, then it should also be apparent when we hold Vg2 constant and vary Va. It would appear as a knee in the plate curves at Vg2. (At any given condition there's a certain number of electrons coming of the cathode. If more of them go to the screen then fewer of them will get to the plate.) But that knee isn't there in any of the pentode curves I've seen.

Anyway, I'm not entirely sure that Grimwood understands things as well as he appears to at first. I'm sure that he and I don't agree on our design priorities. Since minimizing screen current seems to be a design priority of his I'm not sure I place much value in his conclusions about svreen resistors.
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Old 4th February 2006, 12:16 PM   #25
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As I said previously, the gain of a pentode or beam tube depend much more on Ug2 as on Ua. That implies at the best solution is to have a stablisation for Ug2 and no srceen resistor, that in order to have a stable gain on the whole range of output levels.

A screen resistor will limit the gain of the circuit, and this limitation will be proportional to Ig2. It will introduce distrortion too. It is a form of negative feedback, but its effect is not constant with the variations of output level.

I agree with Dennis Grimwood when he said at the grid screen is a protection device that limit the current in the g2, but I prefer to use a design where I can remove this resistor and wire the screen directly to the power supply, and that even if that implie at I must decrease Ug2.

If I look at class B amplifier as used in guitar and bass amps, I see at many (all?) manufacturers use fixed Ug2 and don't have such screen grid resistor, and that even if we get much more screen current with a such amp as with a hifi amp. A guitar amp is made to work in clipping, and it is in clipping at Ig2 become very important.

If you look at the Williamson or the Quad II, they doesn't have any grid screen resistor.
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Old 4th February 2006, 08:59 PM   #26
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Quote:
Originally posted by Dave Cigna


I understand that. In fact I said it here yesterday. But, Dennis Grimwood, the author of the web page from which your quote comes.....
If you are referring to my post #21, I did not quote from anybody (cannot recall that I ever read Grimwood's article), just answered your question from basics. Also apologies that I did not think of your earlier post at the time; things get somewhat hazy here late at night... (it was 2:00 in the a.m.)

As said, I agree with you in that I cannot see the advantage of this practice. Has everybody else overlooked it - I believe not. I would also disagree with a sudden change in affairs when the G2 voltage goes higher than the anode if I understand correctly (not having read the article). Then one should have severe effects every time the anode goes on a negative swing. (But there still remains the EL34 findings with a 1K screen resistor.)

D_f, if you are talking of the original Williamson, that is a triode output topology. If you are talking of the later universally used UL, the G2 does of course get a signal on it which changes things (i.e. it does go down partly with the anode), as is the case with the Quad II (also UL).

Kevinkr, I look forward to your pictures, I have not seen that one. I am still asking: Did the characteristics remain the same from pentode to beam tube, and do we know whether the change included grids alignment?
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Old 4th February 2006, 10:08 PM   #27
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Quote:
Originally posted by Dave Cigna
"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."

Well, it's not clear to me that there is an optimum balance (what exactly is being optimized?) or even that the screen should be loaded at all.
"Clearly" is what I call a "fluff-word" - it means that if you say it fast enough and in the right manner, nobody asks awkward questions. Thus, it means the exact opposite of what it says...

By Child's law (3/2 power law) what goes on at the screen grid is non-linear. If we apply voltage from a low impedance source (the 20% or 43% taps on an output transformer) we can impose whatever voltage we want. If we apply that voltage via a resistor, then the non-linear current drawn will change the voltage at the screen grid. Perhaps that's what the resistors are tinkering with? Perhaps there is a null where two non-linear effects cancel?
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Old 5th February 2006, 03:19 PM   #28
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I took a look at http://www.mif.pg.gda.pl/homepages/t...s/blues_jr.gif
It is a fender schema using EL34 and 100 ohms screen grid resistor. If you look at the power supply. it is a 2k2 resistor between the +B for the anodes and the +X for the g2.

It is other schema on the same website, as exemple a marshall with 4 EL34, 1k g2 resistors and a serial coil to do the voltage for the g2.

The problem with such topology is at the DC voltage on the g2 is almost the same as the DC voltage on the anodes. That implies at the screens will conduct during a bigger part of the alternance where the anode is conducting. More, when the tube goes in clipping, the big amount of current in the power supply and the main transformer will do at not only the anode DC voltage will go down, but the DC voltage on the g2 will go down. As the gain of this pp stage is mainly determined by the dc voltage on the g2, the gain will decrease.

Another problem is at all those variation of the working point of the pp stage can cause high frequency oscillations. The output transformer is highly non-linear with the frequency, especialy at high frequency, and it can cause the circuit to oscillate.

A much better approch would be to use a higher DC anode voltage and a lower DC screen voltage. It have at least 2 advantages.

First, we have a bigger difference between the DC anode and screen voltage, so the part of the alternance where the g2 will take a bigger amount of current will be smaller.

Second, it will be easier to obtain a good regulation of the DC screen voltage in the power supply.

With a such topology, it become possible to use a high voltage cap between the 2 anodes of the pp (in parallel with the transformer) to avoid high frequency oscillations.

A valve as the EL34 is much better suited for a such approch as the 6L6.
In the siemens EL34 data sheet, we can read at with Ug2=375V and Ua=375V in class AB, we can reach 35 watts at the output with 5% distortion, and with Ug2=400V and Ua=800V in class B, we can reach 100 watts at the output with the same 5% distortion.

A good compromise would be to have 800V <= Ua <= 750V with 350V <= Ug2 <= 300V. Both philips and siemens recommand the use of a common screen resistor and they give the values. Wath we can see in those data sheet is at it is high variations of Va and Vg2 with the variations of the power output. So I still believe at it would be safe to have no grid screen resistor if Vg2 is stable.

Special car must be on the g2 in a guitar amp, because all the data sheets have the following remarq for the EL34 (it is also true with other tubes): "When using a sinusoidal input signal care should be takent not to exced the maximum admissble Wg2" http://www.mif.pg.gda.pl/homepages/f...030/e/EL34.pdf
A guitar have not a sinusoidal signal, but it IS SINUSOIDAL when we get in larsen. So in fact a sinusoidal generator is a very good instrument to test the limit of a guitar amp. For you safety and the safety of the parts of the output stage, it is good to have a fuse in the cathode circuit.

I have done a few guitar amps with 807s, the first one was very simple, the last one have a regulated supply for Vg2, and no one have screen grid resistor. The first one have a Va of 530V at full output for 300V Vg2, and the last one have Va=600V and Vg2=250V for the same full output. The difference is at the sound of the last one have a much higher dynamic as the sound of the first one, and that even in clipping.

The 807 is a 6L6 with military quality and higher absolute rating. It can work with 600V on the anode at full output, and with intermittent service with 750V at full output.
It is no indication about a screen grid resistor in the 807 data sheets. But the RCA data sheet say at Vg2 must be made from an independant power supply or a voltage divider from the anode voltage (in order to have a good regulation).

I know at the EL34 is not the same tube, but I know at, if I was buying an amp with a such tube, I would try to use a good power supply for the g2 and no screen grid resistor. Only the practice can determine if it will work or not, and the gain in dynamic we get in all cases with a good power supply for Vg2 is worth the try.
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Old 5th February 2006, 10:07 PM   #29
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Default EL34 Screen Resistors

From experience:
EL34 Triode Mode - use 150R minimum
EL34 Ultralinear - Use 1K minimum
EL34 Pentode Mode - use 150R minimum

In Ultralinear Mode the grid stopper action of the resistor is more important than the peak power limiting function - so get that resistor body hard up against the screen pin of the tube socket.

I have a parallel push pull EL34 amp using VDV2100 Toroidal Output Trannies. With 150R screen resistors and Ultralinear Mode it had HF bursts. Driving a sine wave in and looking on an oscilloscope, bursts of RF were clearly (sorry EC8010) seen just after the peak. These bursts were NOT evident in Triode Mode or Pentode Mode.

In Ultralinear Mode 1K was used - this cured the RF bursts.

Cheers,
Ian
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Old 5th February 2006, 10:37 PM   #30
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