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 Post subject: A Minor Voltage Detail
PostPosted: Dec Mon 02, 2019 5:23 pm 
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This is a very minor point. The pic I took today of a very simple C.W. transmitter. Point of interest is the power supply. Post filter, the voltage is supposed to be 400 DC and that will apply to the anode.
What do you suppose if the reference point here for the screen voltage?
The schematic simply says expect to find 150 volts on the screen.
Just not sure why they designed it this way, as above. However, it is only a very minor detail. Note the 1200 volts capacitor between the anode circuit and transmission circuit. At max output the amp draw should be about 100 milliamps.


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 Post subject: Re: A Minor Voltage Detail
PostPosted: Dec Mon 02, 2019 5:28 pm 
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A resistive divider to get a lower voltage---why is that unusual?
What exactly is your question???

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 Post subject: Re: A Minor Voltage Detail
PostPosted: Dec Mon 02, 2019 5:36 pm 
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That is a very standard circuit. As Mark noted, a resistive voltage divider (in conjunction with the normal screen current draw) provides the required screen voltage. In addition to the economy of using the plate supply to also furnish screen voltage, it has the additional tube safety benefit of avoiding the issue of a "hot" screen with no plate voltage which can easily happen when separate supplies are used.

The 1,200 volt capacitor rating is there because:

1. It has to withstand the DC plus peak RF voltage developed in the amplifier stage.
2. It is a safety related component that blocks DC from appearing on the antenna output and using a conservatively rated capacitor reduces the odds of that happening. However you don't show the full schematic and a proper design will have a RF safety choke near the antenna connector which will short the DC supply to ground and blow the fuse rather than allow a dangerous level of voltage to appear on an ungrounded antenna element if the blocking/coupling capacitor develops significant leakage or shorts.

Rodger WQ9E


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 Post subject: Re: A Minor Voltage Detail
PostPosted: Dec Mon 02, 2019 6:22 pm 
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The exact screen voltage will also be greatly affected by the stage loading and tuning
since the screen current will vary accordingly!

Usually most designs just use a screen dropping resistor and let the screen current determine
the screen current and voltage.... but something taken from the internet needs a grain of
salt, since anyone can publish any circuit, vetted or not.

BTW, it is also standard practice to have a 2.5 mH RF choke from the output of the pi
network to ground--that further assures no DC voltage will appear on the antenna
should the plate blocking capacitor fail.

Pete W1BR

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 Post subject: Re: A Minor Voltage Detail
PostPosted: Dec Mon 02, 2019 6:31 pm 
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What the divider does is give the same voltage with a lower source impedance (compared to the simple series resistor). This means that the screen voltage is less sensitive to changes in screen current.
How this benefits the performance of the circuit is above my pay grade......:)

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 Post subject: Re: A Minor Voltage Detail
PostPosted: Dec Mon 02, 2019 6:40 pm 
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Peter Bertini wrote:
The exact screen voltage will also be greatly affected by the stage loading and tuning
since the screen current will vary accordingly!

Usually most designs just use a screen dropping resistor and let the screen current determine
the screen current and voltage.... but something taken from the internet needs a grain of
salt, since anyone can publish any circuit, vetted or not.

BTW, it is also standard practice to have a 2.5 mH RF choke from the output of the pi
network to ground--that further assures no DC voltage will appear on the antenna
should the plate blocking capacitor fail.

Pete W1BR

"TW, it is also standard practice to have a 2.5 mH RF choke from the output of the pi
network to ground--that further assures no DC voltage will appear on the antenna
should the plate blocking capacitor fail."

This is mentioned in the schematic, yes. Maybe someone had gotten a significant buzz at one time.

I drew a diagram of this divider and referenced the voltage at two points. I would like to see some figures if anyone has a spare moment. The voltage struck me as high at face value. Given the screen is being used as an oscillation plate. The divider itself is 400 supply then 11 K plus the 50 K. Just any thoughts would be interesting.
It's not a trick question. The design just interests me. Thanks.


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 Post subject: Re: A Minor Voltage Detail
PostPosted: Dec Mon 02, 2019 6:43 pm 
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pixellany wrote:
What the divider does is give the same voltage with a lower source impedance (compared to the simple series resistor). This means that the screen voltage is less sensitive to changes in screen current.
How this benefits the performance of the circuit is above my pay grade......:)


it's a very simple but high power transmitter operating on 40 and 80 meters. Interesting though. Must have been late 50s design. Needs to be dipped with an output lamp that should glow.


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 Post subject: Re: A Minor Voltage Detail
PostPosted: Dec Mon 02, 2019 6:45 pm 
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Since it is a single stage transmitter design, the reason for the stiffer screen regulation is explained here:

http://bliley.net/XTAL/docs/eng_bulls/E6/E-6_11.html

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 Post subject: Re: A Minor Voltage Detail
PostPosted: Dec Mon 02, 2019 6:59 pm 
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10.7Megahertz wrote:

I drew a diagram of this divider and referenced the voltage at two points. I would like to see some figures if anyone has a spare moment. The voltage struck me as high at face value. Given the screen is being used as an oscillation plate. The divider itself is 400 supply then 11 K plus the 50 K. Just any thoughts would be interesting.
It's not a trick question. The design just interests me. Thanks.
I'm still not clear on what your question is.

Assuming that the 400volt buss has a low impedance**, we start by finding the equivalent circuit of the divider. The voltage is (50 / 61) * 400 = 328. The equivalent series resistance is the parallel combo of the 50K and 11K, which is 9Kohms.

If the screen is at 150 volts, then the drop on the equivalent resistance is 178, meaning that the current is roughly 20mA


**In circuit design and analysis, "low" means low enough that we don't have to think about it

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: A Minor Voltage Detail
PostPosted: Dec Mon 02, 2019 7:28 pm 
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pixellany wrote:
10.7Megahertz wrote:

I drew a diagram of this divider and referenced the voltage at two points. I would like to see some figures if anyone has a spare moment. The voltage struck me as high at face value. Given the screen is being used as an oscillation plate. The divider itself is 400 supply then 11 K plus the 50 K. Just any thoughts would be interesting.
It's not a trick question. The design just interests me. Thanks.
I'm still not clear on what your question is.

Assuming that the 400volt buss has a low impedance**, we start by finding the equivalent circuit of the divider. The voltage is (50 / 61) * 400 = 328. The equivalent series resistance is the parallel combo of the 50K and 11K, which is 9Kohms.

If the screen is at 150 volts, then the drop on the equivalent resistance is 178, meaning that the current is roughly 20mA


**In circuit design and analysis, "low" means low enough that we don't have to think about it

As a rough estimate the 328 volts is referenced from chassis. I often reference the quoted screen current value for the tube but this often may not correspond to reality. I just thought I'd upload the pic and invite comments or observations.


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 Post subject: Re: A Minor Voltage Detail
PostPosted: Dec Mon 02, 2019 11:33 pm 
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The previous set uses an OD3 voltage regulator on the screen of the oscillator. Yet that oscillator will get up to 21 megacycles. These are very "heavy duty" transmitters but feel free to comment. Would you build the transmitter this way or what might you modify?


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 Post subject: Re: A Minor Voltage Detail
PostPosted: Dec Mon 02, 2019 11:50 pm 
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I really do not understand what you are trying to do.....and it's not obvious that you understand my explanations.

For some of the last statements/questions, we'd need to see the whole schematic

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: A Minor Voltage Detail
PostPosted: Dec Tue 03, 2019 3:44 am 
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Greetings to Mark and the Forum:

I got the same answer that you did, but I don't understand at all how you got there. Too esoteric for me, I suppose.

I began by assuming zero screen current and calculated the ohms per volt for the divider as a whole. That got me only 78 volts drop across the 11K, so I had to assume significant screen current. Since the high side of the 11K is at 400 volts and the low side is at 150 volts (nice of them to give us both of those numbers), the current through the 11K is about 23 mA. Since the 50K resistor will draw 3 mA at 150 volts, the screen current is therefore 20 mA. That sounds like an awful lot of screen current to me, but since we don't know the tube type, maybe not.

Regards,

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 Post subject: Re: A Minor Voltage Detail
PostPosted: Dec Tue 03, 2019 2:02 pm 
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You hit the nail on the head, Jim. For me, personally, something here isn't right. To put it another way, If I wanted 150 volts on the screen of said tube, I wouldn't have predicted those values of resistance. As in the schematic. What we have is simply supply volts to the screen with no voltage drop as in anode cathode drop, as in a rectifier tube. My maths too is kind of dated in the past so I just do a voltage divider the normal way and then calculate any parallel currents. I actually wondered whether the reference point was the drop just from the positive line. I looked up screen current for this tube and about 16 milliamps is listed. Of course the usual way when a single dropper is used is to reference the screen current, subtract desired voltage from supply and use ohms law to drop the necessary voltage. Normally that resistance would be high. Here we have a divider plus a bleeder. 11k doesn't drop a great deal from the 400 supply.


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 Post subject: Re: A Minor Voltage Detail
PostPosted: Dec Tue 03, 2019 2:19 pm 
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pixellany wrote:
I really do not understand what you are trying to do.....and it's not obvious that you understand my explanations.

For some of the last statements/questions, we'd need to see the whole schematic

I had a good look at your calculation and I can see your angle. Although I wasn't sure how you got into parallel resistance in this case. Not that I'm not aware there are various methods in maths. We all concluded therefore that the screen current is a factor. What I'm seeing is bleeder at the supply and then a simple divider of 11k and 50 k. The screen is at the junction so we assume chassis as reference point. As Jim stated we now assume another current value. I did try a few calculations to tie all of this in but got odd screen current values. All in all something is niggling me still.


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 Post subject: Re: A Minor Voltage Detail
PostPosted: Dec Tue 03, 2019 2:58 pm 
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The equivalent circuit is only one way to do these things.

Suppose you want 150 volts on the screen, and you know the current will be 30mA. Then you might decide to add a parallel resistor to add some extra current through the dropping resistor. If you want to add another 30mA, that's a 5kohm parallel resistor.
Now the series resistor is carrying a total of 60mA. The drop is 400-150 = 250 volts, so the series resistor will be 250/60 = 4.17Kohms.
As already stated, the purpose of a shunt resistor here is simply to make the screen voltage less sensitive to the screen current. I do not know why the designer thought that was needed.

Lookup equivalent circuits here:
https://en.wikipedia.org/wiki/Equivalent_circuit

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 Post subject: Re: A Minor Voltage Detail
PostPosted: Dec Tue 03, 2019 3:03 pm 
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10.7Megahertz wrote:
The previous set uses an OD3 voltage regulator on the screen of the oscillator. Yet that oscillator will get up to 21 megacycles. These are very "heavy duty" transmitters but feel free to comment. Would you build the transmitter this way or what might you modify?



What does the screen have to do with oscillator maximum frequency?? The screen is
bypassed for RF by a .01 uF cap. There is no RF on screen.

Peter

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 Post subject: Re: A Minor Voltage Detail
PostPosted: Dec Tue 03, 2019 3:20 pm 
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Quote:
What I'm seeing is bleeder at the supply and then a simple divider of 11k and 50 k. The screen is at the junction so we assume chassis as reference point.

I do not understand this at all.

We specify and measure voltages to a wide variety of "reference points". In a tube circuit, we are concerned about voltages relative to the cathode. Sometimes that is the chassis ground---sometimes not. What counts in any of these problems is that the reference point is specified when discussing voltage measurements. If it is established that the reference point is going to be the chassis, then the wiring of resistors to get the right screen voltage does not change that.

If you really want to learn to do problems like this, I recommend you do some reading on circuit analysis.

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 Post subject: Re: A Minor Voltage Detail
PostPosted: Dec Tue 03, 2019 3:52 pm 
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For a transmitter, the voltages are usually referenced to chassis. I suspect that is being cathode keyed--cathode would be at chassis potential when keyed.

As an aside, it is possible for screen grid current to go positive or negative depending on tuning and
loading for some tubes--this is very common in tubes like the 4CX250A, etc. And there is a lot of
assumption that the data on the drawing was ever correct to begin with. Been there,
as an editor for twenty odd years.

Pete

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 Post subject: Re: A Minor Voltage Detail
PostPosted: Dec Tue 03, 2019 5:21 pm 
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Peter Bertini wrote:
10.7Megahertz wrote:
The previous set uses an OD3 voltage regulator on the screen of the oscillator. Yet that oscillator will get up to 21 megacycles. These are very "heavy duty" transmitters but feel free to comment. Would you build the transmitter this way or what might you modify?



What does the screen have to do with oscillator maximum frequency?? The screen is
bypassed for RF by a .01 uF cap. There is no RF on screen.

Peter


Lots of oscillators such as the Tritet will use the screen as a "plate" in the way the plate of a triode is used in a tickler oscillator. However, now you mention it....... The reference book describes this oscillator as practically a Colpitts oscillator although I'm not sure this makes things easier. Regardless a Hartley or Colpitts will have the cathode tapped at an intermediate point of the RF coil (or between two capacitances. These are positive feedback oscillators.
Here as above the screen is at RF ground and the signal is fed to that (screen) and the grid. The screen is therefore the plate of the oscillator and the high voltage anode will amplify the signal.
By my reckoning here the DC is amplifying the RF. Although, I never expect to be taken purely on my word and urge you to explore some of these points and form your own view. Or disagree, of course.
A lot of the more complex oscillators will use a crystal and the screen will act as the oscillator plate.
This is why I got curious about the voltage on the screen here. The current could be stabilised more by adding an OD3 gaseus regulator with an operating voltage of 150 volts. When a designer does something different, I get interested. Tonight, I will go into some more detail if time allows.


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