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Old 1st June 2004, 09:58 PM   #11
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Default Plate Voltage Definition and Conditions

Greetings All,

My searches return this thread as the most relevant to my current conundrum. In an attempt to make my questions slightly less idiotic, I am flogging the reluctant mule of my brain through the excellent Audio Classroom articles on the AudioXpress website.

It will come as no surprise that I am confused, this time with the concept of Plate Voltage in the Voltage Amplifier article. The perplexing passage occurs at the end of page 4, and reads:
Quote:
"...the plate voltage here does not have the same meaning as the B+ voltage we have been working with. Here it means voltage on the plate ; it does not allow for the drop in the plate-load resistor... If the tube was operated from a very high B+ voltage, and fed through a large load resistor so as to give 250V on the plate, an amplification of 100 would be approached.
So here's what I'm wondering:

1. What is plate voltage measured with respect to? The emitter?

2. Under what condition is it measured? This is what is puzzling me, for it seems to me that, under the conditions in the quoted passage, when I = 0 there will be no voltage drop across the plate-load resistor, and the plate voltage will not be 250V, but rather the "very high B+".

So what am I missing?
Barring active cathode-loads, don't all load lines pass through the point (0 mA, B+ V)? So aren't all tubes in such circuits stressed by B+ V on their anodes at 0 current?

I've tried to be very explicit in my question, so that it will be easy to answer.

If I am diligent enough, I hope to someday understand the SYclotron circuit.

Thanks for your help, and happy tweaking!

Best,

George "Enthusiastic Amateur" Ferguson
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Old 1st June 2004, 10:20 PM   #12
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Hi,

Quote:
1. What is plate voltage measured with respect to? The emitter?
The cathode.

Quote:
2. Under what condition is it measured?
Static. IOW whilst idling.

Quote:
when I = 0 there will be no voltage drop across the plate-load resistor, and the plate voltage will not be 250V, but rather the "very high B+".
I = 0 does not occur, there's always current flowing unless you interrupt the current flow. By pulling the tube, for instance.

Just imagine a class B amp and know that tubes never reach total cut-off condition, you'll realise you just can't stop current from flowing.

Hope this helps a little,
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Old 2nd June 2004, 12:06 AM   #13
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Hey Frank,

Thanks for the reply.

Okay, plate voltage is measured between the anode and cathode.

And you said:

Quote:
Static. IOW [In Other Words?] whilst idling.
Forgive my thickheadedness, but this I do not yet understand--for the literature I am reading seems to imply that the plate voltage limit comes into play during dynamic conditions.

Looking at page two of the "The Power Stage" article on the AudioXpress website, I see the following passage:

Quote:
[plate voltage] is specified in one of two ways: the real limitation is the maximum positive excursion, because the danger is that of flashover from a high potential difference between the plate and the grid;
(continuing my question numbers, at three...)

3. My interpretation--since the grid is relatively close to ground in most circuits, and B+ is positive wrt ground, the greatest potential difference (and flashover danger) occurs when the plate reaches its most positive value, for this is when the grid-plate potential is the greatest magnitude. Correct? That makes sense to me.


Quote:
but most circuit designers prefer to have a maximum working-voltage figure, which is the highest supply voltage that can be connected to the plate circuit.
4. Also known as B+, right?

Quote:
In any output stage the plate-supply voltage is connected to the end or the center tap of the output-transformer primary, the plate being connected to the other end. In the quicescent or no-signal condition, the plate voltage is almost equal to the supply voltage, being reduced only by the small DC drop in the winding.
5. This is because X(L) = 2PiFL, right? And because no inductor is is ideal in terms of having 0 DC reactance?

Now comes the fun part:

Quote:
When the plate voltage changes because of an applied audio signal, the fluctuations will go positive as well as negative from this supply voltage.
6.

How can the plate voltage go positive from the supply voltage? Doesn't this mean the plate voltage is swinging above B+? I don't see how this can happen.

Any guidance appreciated. Otherwise I'll just keep pondering, and will figure it out eventually. Thanks for your patience!

Best,

George "Amateur With A Headache" Ferguson
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Old 2nd June 2004, 01:38 AM   #14
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Hi,

Quote:
Forgive my thickheadedness, but this I do not yet understand--for the literature I am reading seems to imply that the plate voltage limit comes into play during dynamic conditions.
It does but when I said measure it so and so I meant it as a way of checking what the actual plate voltage was, to cross-check if the circuit is operating correctly if you like.

What NC is discussing are dynamic conditions that can occur while the amp is actually working.

It's been ages I read that series of articles but I get the feeling he's talking about the plate and screengrid potential flashover here.

4. Is not correct, that would actually be the highest possible plate voltage as given in the datasheet of that particular tube.
At least that's how I read it.
B+ can be as high as you like for as long as the tubes see a correct plate voltage it won't care what the actual B+ is.

5. That's just because OPTs are chokes and chokes inevitably have DC resistance, hence a voltage drop across them.

Quote:
Doesn't this mean the plate voltage is swinging above B+? I don't see how this can happen.
I think what NC means is that the output voltage at the plate can swing above quiescent plate voltage....

Not too sure what he means here so:

I'll see if I can find the articles, I think they were once published in SP....
It sure gets confusing especially for a poor non-american like myself...

Cheers,
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Old 2nd June 2004, 04:36 AM   #15
DougL is offline DougL  United States
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Quote:
How can the plate voltage go positive from the supply voltage? Doesn't this mean the plate voltage is swinging above B+? I don't see how this can happen.
My understanding is a Choke loaded tube can swing voltages in eccess of battery plus. (B+) The "extra" voltage comes from the energy stored in the inductence. Same principal applys for a transformer. Energy is stored in the primary inductence.

On the intuitive level, a transformer loaded tube has its plate sitting at b+ minus the dc resistance of the winding. if the transformer couldn't swing above b+, we would be left with about class b operation.

Hope this helps;

Doug
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Old 2nd June 2004, 06:04 AM   #16
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Hi Doug,

Yes, your message is very helpful, and addresses exactly the questions I am pondering. Stored energy in the transformer's E-M field--that could be just crazy enough to work!

For example, take a look at this schematic of the J.E. Labs 300B--It shows exactly the sort of situation Crowhurst, Degrove and you describe:

I'm assuming the schematic's voltages refer to the quiescent conditions you all describe. They show the output tube's plate (quiescent voltage?) at 425V, out of a B+ of 435V. Knowing that SETs operate in class A, I was at a loss to explain this situation.

But capacitors store energy in voltage, and inductors in current--yeah! So my guess is that, when the amp warms up to idling conditions, the increasing current stores energy in the inductor (and while the current changes there is probably a relatively low frequency voltage across the secondary/speakers?

Let's see, it takes an infinite current through a capacitor to create an instantaneous change in voltage across it, and an infinite voltage across an inductor to create an instantaneous change in current through it, right?

So when our idling amp receives an AC signal, it responds by changing it's anode current, and the OT responds by changing its voltage drop?

And the sign of that voltage change reverses for current increases and decreases?

Hmmm... doesn't that imply that (ignoring the DC drop across the cathode bias resistor) the plate voltage can swing from (almost) 0V to (almost) twice B+? (This part I am even less clear on )

Wow! For "simple" devices, there is a lot going on in even a basic SET! This only increases my respect for the folks that created them.

Thanks Frank and Doug for your help.

Best,

George "Headache Slightly Better" Ferguson
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