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?
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.
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.
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....
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....
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.
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.
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 heaven/www.schematicheaven.com/fenderamps/champ_aa764_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?
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 heaven/www.schematicheaven.com/fenderamps/champ_aa764_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?
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....
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....
Guitar amps routinely run tubes outside the published specs. We expect to have to change tubes a lot more often than we did for dad's old table radio or TV. Think of it like race cars. They wear out a lot faster than if we drove politely down the street. But if you do that, you are no longer racing.
If you look at plate and screen voltages with a scope you will see nice steady DC on the screen, but probably 10 Volts peak to peak or more on the plate. Many times the screen voltage will be between the positive and negative peaks of the plate voltage. Now if you look at the published plate curves of a power tube you will see that the plate voltage can move atleast 100V up or down and the plate current won't change by more than about 5%. So it just doesn't matter if the screen is 5V or 10V higher than the plate when the tube is at idle.
You're talking about g3, which is designed to prevent secondary emission of electrons from the plate from flowing to the screen grid. G2 (the screen) is always a mesh wire grid that is aligned with the control grid. Sorry to nitpick
yes, sorry my bad, beam formers are indeed g3 in beam tetrodes...
Hi Guys
Poor Phil has not received an answer to his original question. So, here it is:
The voltages listed on the schematic are at idle. With a standard DC voltmeter of any type you will see voltages approximately what are listed, or at least in similar proportion.
Measuring the plate voltage under load with anything other than an oscilloscope is pointless. The plate can pull the OT end almost to ground - usually within 40-80V, anyway. At the opposite end of the signal swing, the plate voltage soars above B+ by the same signal peak voltage due to the flyback action of the OT. This is all normal. One might expect that the DC meter reading here would simply give an average - which it tries to do but it will be inaccurate, and again rather pointless.
Keep in mind that every power supply sags a bit under load. Guitar amps are built economically, so a 10% sag is expected and 20% is rather typical. If you measure the plate voltage at the point with your DC meter, you will note that it is much lower than the schematic lists, but so too will be screen voltage and every other voltage in the amp.
Screen voltage in an of itself is not important except that it is what grid control voltage relates to - not plate voltage. Screen resistors drop a tiny voltage that is unimportant overall in the operation of the tube, particulalrly with respect to protecting the tube while clipping. That protection comes from the value of this resistor. The 100R value commonly used with EL-84 is a joke - it should be 1k-1W minimum and better as 2k2-1W. The 470-2Ws Fender uses for 6L6s - which everyone copies - should be 1k-5W. All screen Rs should be flame-proof types - not carbons that burst into flames. 'The Ultimate Tone' (TUT) series books explain all of this in detail, along with tonnes of other guitar amp circuitry.
With respect to guitar amp designs "violating" tube ratings - this does not happen. Tube specs changed in the 1950s or so even though the tubes did not. A more important facet is that the _applications_ do not imply limitations. And still another facet is the blissful ignorance that those apps and the RDH bible follow tube hifi rules, not tube safety rules. Again, our FAQ and books explain this.
Have fun
Kevin O'Connor
Poor Phil has not received an answer to his original question. So, here it is:
The voltages listed on the schematic are at idle. With a standard DC voltmeter of any type you will see voltages approximately what are listed, or at least in similar proportion.
Measuring the plate voltage under load with anything other than an oscilloscope is pointless. The plate can pull the OT end almost to ground - usually within 40-80V, anyway. At the opposite end of the signal swing, the plate voltage soars above B+ by the same signal peak voltage due to the flyback action of the OT. This is all normal. One might expect that the DC meter reading here would simply give an average - which it tries to do but it will be inaccurate, and again rather pointless.
Keep in mind that every power supply sags a bit under load. Guitar amps are built economically, so a 10% sag is expected and 20% is rather typical. If you measure the plate voltage at the point with your DC meter, you will note that it is much lower than the schematic lists, but so too will be screen voltage and every other voltage in the amp.
Screen voltage in an of itself is not important except that it is what grid control voltage relates to - not plate voltage. Screen resistors drop a tiny voltage that is unimportant overall in the operation of the tube, particulalrly with respect to protecting the tube while clipping. That protection comes from the value of this resistor. The 100R value commonly used with EL-84 is a joke - it should be 1k-1W minimum and better as 2k2-1W. The 470-2Ws Fender uses for 6L6s - which everyone copies - should be 1k-5W. All screen Rs should be flame-proof types - not carbons that burst into flames. 'The Ultimate Tone' (TUT) series books explain all of this in detail, along with tonnes of other guitar amp circuitry.
With respect to guitar amp designs "violating" tube ratings - this does not happen. Tube specs changed in the 1950s or so even though the tubes did not. A more important facet is that the _applications_ do not imply limitations. And still another facet is the blissful ignorance that those apps and the RDH bible follow tube hifi rules, not tube safety rules. Again, our FAQ and books explain this.
Have fun
Kevin O'Connor
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