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Old 15th June 2013, 02:07 AM   #1
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Default Technical help with screen grid voltages

I have recently completed two amplifiers, one a Fender AA764 clone and the other a AX84 6V6 push pull. In both schematics the screen voltage is given as being a few volts less than the anode voltage yet when the screens are measured their voltage is, under static conditions considerably higher than the anode. So why print this screen voltage on the schematic as being lower than the anode? Is it being guessed at this is what it will be under dynamic conditions? I always thought that the anode was a higher voltage than the screen otherwise the screen would act as the anode. I have checked back and this question has been asked by others but I have never seen a good explanation for an answer. Can you help me understand this please?
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Old 15th June 2013, 02:14 AM   #2
AJT is offline AJT  Philippines
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screen grids in a pentode is like a plate in a triode, in a triode, the plate voltage determines plate current, in a pentode, the screen grid voltage controls plate current...
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Old 15th June 2013, 02:16 AM   #3
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I am currently working on an el34 push pull amplifier in pentode mode and g2 goes to B+ through a 1k resistor. Yet the anode goes to the output transformer which suggests it will be at a lower voltage.
I would guess that the g2 voltage will drop down to similar to the anode due to the voltage drop across the resistor as g2 will pick up some current flow.

I don't think g2 is the same as a anode in shape.

Sorry I cant help any more I am fairly new to valves/tubes.
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Old 15th June 2013, 02:48 AM   #4
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Quote:
I don't think g2 is the same as a anode in shape.
that's correct, g2 is a mesh wire in a pure pentode, beam formersnin others like the 6L6, a plate is well, a plate, but did you know that the g2 can be used as a plate?
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Old 15th June 2013, 03:33 AM   #5
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Yes, I did know that the screen can be used as a plate but your answers are not helping me understand the questions I asked.
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Old 15th June 2013, 03:43 AM   #6
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have you seen the datasheets for the tubes in question?

6V6 screens are rated for 285 volts, so that if that is not the case with your actual circuit,
then you need to make adjustments....
change some voltage dropping resistors that supply the screen....
anyway, with signal the plate voltage swings will be large, it will swing below screen voltage
so that enough impedance on the screens are needed to limit power input to the screen...

btw, guitar amps does not strictly adhere to tube specs, those guitar circuits tend to push
the tubes hard so that the tubes live a short life than when used for hi-fi...

so it is up to you how you will push your tubes....
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Old 15th June 2013, 04:15 AM   #7
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Please read my post again, the circuit is a Fender AA764, check out the schematic. The other circuit is the building block 6v6 push pull on the AX84 site.
The components used are as per the schematic and they work very well indeed. However that is not the point, I want to know why the screen voltages are higher than the anode voltages.
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Old 15th June 2013, 06:18 AM   #8
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The only AA764 I can see is the Champ and the VibroChamp. I'll assume you built a Champ, but it would be good to tell us.

The simple answer is that it is close enough.

I am referring to this drawing, if yours is different please say so.
http://www.webphix.com/schematic%20h...a764_schem.pdf

First read the first note, upper right. All voltages are approximate and may vary 20%. They are ballpark figures intended to be helpful, not precise readings. With 360v B+, a change in mains voltage from 115 to 125v would cause a 30v rise in B+.

I see 360v B+. 350v on the screens, that means 10ma through the 1k resistor. 20 more volts dropped across the 10k to the preamp tube. That is 2ma. so that means we have about 8ma screen current.

360v B+ drops to 350v at the plate. On the cathode, I see 19v across 470 ohms, or 40ma, less the 8ma for screen and I get 32ma through the OT. That gives me about 312 ohms for the primary resistance. If I screwed it up, let me know.

The schematic says the screen and plate sit the same. If yours is a lot lower at the plate, what is your OT primary resistance? I assume you did not use a stock vintage Fender transformer. Perhaps your winding has a higher resistance.

EVen though these are approximations, you cannot assume your amp would turn out identical. Transformers have different resistances, and the tubes you are using may have different characteristics from the ones Fender used 50 years ago. which may throw off the current relationship between screen and plate in this circuit.

But the bottom line answer to your question is this: it is all about currents. The voltages at the screens and plates is determined by the currents through the transformer resistance and the screen node resistor. Would we prefer the screen to be lower? Probably so. The screen grid wires are aligned with the control grid wires, so that shadow protects them some from acting out and out like a plate. SO if the voltage is a little higher than the plate, it is not like the screen will take over.

Does any of that help?
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Old 15th June 2013, 06:22 AM   #9
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ok, is this the schematic you built upon? Davidson Amplifier Repair, Nashville, TN : Schematics: Fender: Champ Amp AA764 Schematic

it shows 350v for both plate and screen, are you saying that the screen is actually higher than your plate?

one explanation i can think of is that the plate draws more current than the screen, now depending on the dc resistance of your OPT, the plate voltage can be lower....

from the drawing, at cathode bias of 19 volts with a 470 ohm cathode resistor, that means that the cathode current is about 40mA, so that if 35mA is flowing at the plate, then about 5mA should be flowing on the screen....
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Old 15th June 2013, 06:26 AM   #10
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with tubes, variations of +-10 to 20 percent are nothing to be concerned about most of the time...
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