# Here's a good one, what kind of typical signal voltages would you expect on a screen?

Discussion in 'Amp Tech Center' started by peteb, Apr 15, 2018.

1. ### petebFriend of Leo's

Apr 25, 2003
use your circuit analysis skills, we know the VDC, what's a max VAC expectation on the screen and what's its source?

Large signal level:

Plate is 200 VAC
Screen is
Grid is 20 VAC
Cathode is 0 VAC

Try it with a dedicated 470 ohm screen resistor and also without.

What do you expect?

High voltages?

Low voltages?

How high?

How low?

2. ### petebFriend of Leo's

Apr 25, 2003
Or try this one

Large signal level:

Plate is 200 VAC
470 R Screen is ?
Grid is 20 VAC
470R Cathode is ? VAC

3. ### BendyhaTele-AfflictedAd Free Member

Mar 26, 2014
Northern Germany
What sort of tube would you be using? Is it pentode or Beam power? What voltage DC are you appling to the screen? Is this by any chance an ultra linear output transformer? If so what percentage tap are you using? Large signal level is a very vague level, can you specify? How well regulated is the screen supply? How is the screen decoupled, and to where?

Why do you want to know?

What would the benifit of knowing be?

Whats are your calculations?

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4. ### RLee77Friend of Leo's

May 15, 2016
Silicon Valley
I’ve got some current flowing through a resistor on my bench. It’s a 4.7k, 1/2w, 5% tolerance, carbon film part. What is the voltage across it?

Last edited: Apr 15, 2018
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5. ### petebFriend of Leo's

Apr 25, 2003
Thanks Bendyha,

Why? It shows something very useful to know about tube operation.

6v6 tube for both cases.

Screen and plate are at 400 VDC in both cases

Screen voltage supply is standard fender, right off the B+

Decoupled?

Here is the schematic for the first case, class AB output, 6G2:

6v6-screen-B2+

Here is the schematic for the second case, class A, AA764:

6v6-screen-470R-B2+

6. ### petebFriend of Leo's

Apr 25, 2003

Thanks RLee,

You have it right.

Vs = Is * Rs

7. ### BendyhaTele-AfflictedAd Free Member

Mar 26, 2014
Northern Germany
Decoupled !

T' what ?

8. ### petebFriend of Leo's

Apr 25, 2003
Here's my calcs:

Compared with the AC plate voltage calculation

The screen carries about 1/10 of the current as the plate.

The screen works into about 1/20 of the load of the plate, 470/8k

Small times small is smaller

I expect the screens to have 1/10 times 1/20 of the signal voltage of the plate: 1/200

The AC on the plate for a large signal is 200 VAC.

A large AC on the screen should be about 1/200th of 200 VAC, so about one VAC max on the screen.

This has been tested and is true.

Now the sticker

what would the max signal voltage on the screen be in the case of no screen resistor, like the 6G2? The answer is really surprising

9. ### petebFriend of Leo's

Apr 25, 2003
Here is the surprising result

Circui:

6v6 - screen - B2+

Here is the effective AC circuit:

6v6 - screen - AC ground

The screen with no screen resistor will theoretically have maximum VAC of zero volts.

The screen is grounded for AC. Just as the tube side of the cathode bypass cap, it is ground for AC.

10. ### petebFriend of Leo's

Apr 25, 2003
This example shows where the AC plate, cathode and screen voltage came from, the voltages don't come from inside the tube, the current comes from inside the tube and makes these voltages outside the tube.

AC:

Vp = Ip * Rp
Vs = Is * Rs
Vc = Ic * Rc

Play on

11. ### ranjamTele-Holic

Dec 29, 2015
Call me a cynic, and I am too lazy to confirm my suspicions, but that big fat filter capacitor would take most (if not all) the AC signal to ground. There will be DC fluctuations, but I wouldn't expect much AC on the Screen Grid. OK, flame suit 'on'.

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12. ### BendyhaTele-AfflictedAd Free Member

Mar 26, 2014
Northern Germany
Volt seconds = Current seconds X Resistance seconds...............Wow ! ....new to me................proof ? (I see Sweeden calling)

But surely the frequency of the current would have to be known. If it was a wire wound resistor, we would need to know its reactance at given frequency.

Can your theory cover all these bases?

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13. ### BendyhaTele-AfflictedAd Free Member

Mar 26, 2014
Northern Germany
The data sheets beg to differ.........

View attachment 506709

........then we have the fact that screen-grid current is not linear...nor is plate current...there is a vague invert relation ship.

No, it is much the sort of answer I expected you to give

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14. ### BendyhaTele-AfflictedAd Free Member

Mar 26, 2014
Northern Germany
Here is what you might expect for a 400V screen

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15. ### petebFriend of Leo's

Apr 25, 2003
Vs is volts on screen.

There is no debate

Measure the VAC on the screen

Do the circuit analysis

Same result either way

16. ### BendyhaTele-AfflictedAd Free Member

Mar 26, 2014
Northern Germany
What ?

You make a proclamation, and nobody is allowed to dissagree with you ?

Since when has this been the standard for TDPRI ?

Do you hereby declare that you have no intrest here in learning of the way that screen-grids really work.....because your theory is the factual truth?

17. ### petebFriend of Leo's

Apr 25, 2003
I posed the question, let people answer, then I gave results.

At the time of my first post I didn't realize I already had data from two different types of amps. The measured results are fairly obvious.

with a screen resistor you might expect the VAC to max out on the screen at about one volt. No screen resistor, the AC will max out at about zero volts.

it makes sense with the circuit too, but it is not as obvious. Put the two together, and there is a result.

Are you surprised how low the AC on the screen is? I was.

When I did the measuring, I could not explain it, but now thru new understanding of where the AC plate voltage and screen voltage come from, it is now easy to understand.

I think most people would not expect the screen VAC to max out at zero or one.

18. ### BendyhaTele-AfflictedAd Free Member

Mar 26, 2014
Northern Germany
I asked you a series of questions....and get no answers.

You rant further, and pose another question......yet it is clear you have no interest in the answers.

Hmmm.....

Last edited: Apr 16, 2018

19. ### petebFriend of Leo's

Apr 25, 2003
i posted answers to your questions in post 5, I was unsure to put for the coupling, but that's OK.

The point is I asked the question and waited.

If anyone is real familiar with screens, they could have answered right away that the VAC on the screen is none to very little. its true across the board.

Nobody offered up that little kernel of knowledge. So now I offer this knowledge, and I noted that I measured and it checks out.

take it or leave it.

20. ### petebFriend of Leo's

Apr 25, 2003

You say that you are not surprised.

I am putting you down as surprised, because..

None of your answers indicated that you knew what kind of VAC is expected on the screen

but its no big deal, we all learn new things every day

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