the role of screen grid voltage

[tubby]

could someone explain how tone is affected when the screen grid voltage is changed,
e.g. from 40V below anode voltage to equal anode voltage (say 400V anode voltage).

i've seen some power reduction cicuits with an adjustable screen voltage regulation, but i can imagine adverse effects on tone... (?)
thanks.

[tubeswell]

Higher screen grid voltage (up to about = to plate voltage) results in higher grid voltage lines spaced further apart on the tube characteristics chart (the 'knee' of the Vg0 line is higher - enabling a higher load line), and higher power output from the tube. This allows for more headroom and more bite. The 'role' of the screen grid is to provide an 'electron accelerator' between the cathode and the plate - reducing inter-electrode capacitance and impedance. Take away the screen voltage and the tube loses power output

[Chuck H]

Take away the screen voltage and the tube loses power output

I'm guessing the above also comes with increased effect from the inter electrode capacitances?

I'm asking because I haven't built a "power scaling" circuit yet. But I may.

IME and as it applies to typical builds, reducing screen volts also results in increased power supply resistance, which results in softer attack and more sag. Along with reduced output power. Or correct output power if your reducing screen volts to keep the tubes from sef destruction

If you were to use a parallel supply for the screens along with a pi filter or a sizable choke you can reduce screen volts with less "sag" effect. But this is not typical in most amps due the parts expense. Not to mention that there are other easier options to achieve the desired output wattage.

Chuck

[Robert M. Martinelli]

IME and as it applies to typical builds, reducing screen volts also results in increased power supply resistance, which results in softer attack and more sag. Along with reduced output power.

Hi Chuck,
I knew about the screen grid purpose in a pentode, so did I about the power reduction issue; what I don't get is how "sag" is supposed to increase reducing screen voltage. ( rectifier ) "Sag" is the result of the voltage drop across the rectifier imposed by the current flowing through it; if we lower the screen grid voltage we reduce the current flow through the power tubes as well, so, with all the other factors being the same, with a lower screen voltage the rectifier should "sag" less, not more ( less voltage should drop across it ).

Can you elaborate on this?

Thanks and best regards

Bob

[Chuck H]

Sure...

The screens draw current too. Substantial current. And behave very dynamic under operating conditions. More resistance in the screen supply increases that effect. If you have a scope you can increase screen supply resistance and see for yourself.

Yes, if you lower screen volts the lower plate watts would result in less sag IF your using a rectifier tube or some other resistance before the plates. But the amount of effect from screen sag seems disproportional. Again, you should see it on a scope. I was actually dissapointed because I only wanted lower screen volts and not so much more sag.

One of the first things to consider when building or modding an amp for bass guitar or just solid clean tone is to tighten up the screen supply.

Chuck

[ETR]

Why does everyone think of plate and screen voltages as fixed just because they are x volts at a certain bias point. Your plate voltage will sag regardless of whether you have a tube rectifier or not. That's what happens when you pull curent thru your power supply. The same thing happens with your screen voltage except it will sag a lot more if you have some impedance in line with it. The screen grid actually has more effect on your conductance than your control grid does, so when it drops you get the good ol compression effect that us guitarists love and bass players don't as much.

[Chuck H]

Why does everyone think of plate and screen voltages as fixed just because they are x volts at a certain bias point.

Who said that?

Your plate voltage will sag regardless of whether you have a tube rectifier or not. That's what happens when you pull curent thru your power supply.

True. But to make the statement in that way, and in this context implies that the difference is negligable. It's not. With a diode rectifier and an adequate power supply you will only see a few volts of sag on the plates between idle and full tilt boogie. But with a rectifier tube or an equivalent resistance in the power supply you will see 15 to 20 volts of sag between idle and dime. This is a very audible difference and also my point in stipulating resistance in the power supply. The amount of sag on the plates without a resistance in the power supply IS negligable.

The same thing happens with your screen voltage except it will sag a lot more if you have some impedance in line with it. The screen grid actually has more effect on your conductance than your control grid does, so when it drops you get the good ol compression effect that us guitarists love and bass players don't as much.

Finally... We agree

Chuck

[ETR]

Who said that?



True. But to make the statement in that way, and in this context implies that the difference is negligable. It's not. With a diode rectifier and an adequate power supply you will only see a few volts of sag on the plates between idle and full tilt boogie. But with a rectifier tube or an equivalent resistance in the power supply you will see 15 to 20 volts of sag between idle and dime. This is a very audible difference and also my point in stipulating resistance in the power supply. The amount of sag on the plates without a resistance in the power supply IS negligable.



Finally... We agree

Chuck

A few volts difference with a SS recto? You've got to be kidding. 10% or more is typical. With 460 volts at idle its more like 50 volts. Check it out sometime. A 5AR4 in a Deluxe Reverb will give you less sag. If you want to start worrying about 10 volts or so on your plates you better not plug your amp into line voltage that varies more than a few volts. Better unplug the fridge.

[Chuck H]

A few volts difference with a SS recto? You've got to be kidding. 10% or more is typical. With 460 volts at idle its more like 50 volts. Check it out sometime.

Maybe with a 100mA PT in a 50 watt amp

A 5AR4 in a Deluxe Reverb will give you less sag

Ok. I just want to "frame" this as the time when someone implied that a tube rectifier sags less than a diode rectifier.

Understand that the conditions include 'all other things being equal'. ie: same mA rating on the PT and the same voltage and bias.

If you want to start worrying about 10 volts or so on your plates you better not plug your amp into line voltage that varies more than a few volts. Better unplug the fridge.

I've seen line volts fluctuate even more than that. And unplugging the fridge isn't a bad idea (if only because the noise from my fridge is horrid ) But the discussion isn't about the standing voltage. It's about the effect of sag. Even if the my awful antique "icebox" were to come on right as I hit a power chord the amps response wouldn't be the same as the effect of sag due to plate supply resistance. (with the exception that things get colder )... Get it...Bias cools with plate sag, fridge gets colder at the same time, yuk yuk...

Amp builder, not comedian.

Chuck

[Robert M. Martinelli]

It's always true that all power supplies "sag" somewhat when you pull current, but, as already stated, all other factors being the same a tube rectifier will always sag more than a SS rectifier - period. The inherent series resistance of a tube rectifier is always higher.

A 10% or more drop ( 50V ) under working conditions makes me think about poor/improper transformer/PS sizing, as Chuck pointed out. A 10 to 20 V drop is to be considered normal with a tube rectifier, which becomes less than 4V with 4x1N4007 as rectifiers ( a bigger drop with 1N4007s indicates an undersized transformer IMHO, but that would be "transformer sag", not "rectifier sag" ). This is why it is said that SS rectifiers "don't sag" - they actually sag, but at so small a degree for this to be negligible.

This said, I agree that the screen grids have a big effect on mutual conductance, because they act as accelerators and reduce the inter-electrode capacitance ( this is why pentodes were created in the first place ), only, I didn't realize at first that, using Chuck's words, the effect of a voltage variation on the screens was "disproportional".

Regards

Bob

[Enzo]

One thing I thought when Chuck was discussing reducing screen voltage increased sag, was the nature of that sag. If you simply add resistance between the screen node and the plate node on the B+, well then sure it will increase sag along with lowering the screen volts. But in the spirit of the original question, what if the screens had their own variable supply separate from the plate? Now you can reduce the screens without adding resistance. I suspect a lot of that sag would go away. We'd be left with the original question.

[Bruce / Mission Amps]

Hold on... and think about this too

Hmmm... I think you fellows ought stop talking or typing and get back to some basic bench testing.
Go get your DC volt meters out and connect the positive lead to the power tube's plate and the negative lead to it's cathode....
then put a steady signal in the amp and set the volume high enough which will get the amp at full power output (dummy loads please) ... then look at your DC volt meter....
Why? Because any decent power tube (driven well into full output), can pull it's cathode and plate together so close together that there is a VERY big DC drop from anode to cathode and I think its caused by the variable impedance of the tube, the DC resistance of the OT and yes, the power supply's "Zed" (impedance) as a black box.
But it is not unusual to see 50 to 100v drop or more at the plates with many guitar amps regardless of the PT's DC ma rating.
Actually, the huge variation in the DC is how the damn thing works with respect to driving the OT and subsequently the speaker.
Now if you are only talking about the & of VDC drop right at the first main filter cap.... then that is a different matter, but all that AC and DC current has to come from somewhere.

[ETR]

Hmmm... I think you fellows ought stop talking or typing and get back to some basic bench testing.
Go get your DC volt meters out and connect the positive lead to the power tube's plate and the negative lead to it's cathode....
then put a steady signal in the amp and set the volume high enough which will get the amp at full power output (dummy loads please) ... then look at your DC volt meter....
Why? Because any decent power tube (driven well into full output), can pull it's cathode and plate together so close together that there is a VERY big DC drop from anode to cathode and I think its caused by the variable impedance of the tube, the DC resistance of the OT and yes, the power supply's "Zed" (impedance) as a black box.
But it is not unusual to see 50 to 100v drop or more at the plates with many guitar amps regardless of the PT's DC ma rating.
Actually, the huge variation in the DC is how the damn thing works with respect to driving the OT and subsequently the speaker.
Now if you are only talking about the & of VDC drop right at the first main filter cap.... then that is a different matter, but all that AC and DC current has to come from somewhere.

Yes check the DC volts at the first filter cap at idle and then crank your sig gen and amp up with a dummy load. I do this test about 10x a week typically on SS recto BF Fenders, which are hopefully a decent enough design. Generally the plate voltage will drop at that point around 10%. Please try this before you say it ain't so. This is part of my routine test when I'm working on an amp. I also check the AC ripple at idle and when cranking.

[Chuck H]

ETR, I think you misread Bruce's post. He didn't say "look here, there is fluctuation at the first filter node." He said that the tubes do sag, give and take, moving alot of current even though it DOESN'T always show at the first filter node. And I must digress to his observation. He's right that this discussion has become pedantic because the current moving through the amp does have to come from somewhere if it's going to make watts. The way I see it there have been many accurate statements made that do not accurately represent the subject. We are all right. And we are all wrong. Except for Bruce, who is only right and was good enough to point out the neglected elements. And I hope that I am coming away from this with better understanding.

Chuck

[Robert M. Martinelli]

Actually, the huge variation in the DC is how the damn thing works with respect to driving the OT and subsequently the speaker.

"The huge variation in the DC".....OK, but that's not DC anymore, because it's been "modulated" by the incoming signal, so here we're talking about an AC amplified signal going to the OT ( it has to be AC for the OT to work ) and subsequently to the speakers. This huge variation is not "directly" related to the rectifier's differential ( forward ) resistance, but, as Bruce pointed out, it's directly related to how the d@mn thing works ; the large signal ( or modulated DC, if you like ) swing you observe is "sound" related, not "PS related".

When we're talking about "rectifier sag" This means we're thinking 'bout and try to analyze the DC variation ( drop ) due to the rectifier's forward resistance alone, and there's no denying that a SS forward resistance is way lower when compared to a tube rectifier.

This is why, to use Chuck's words, "we're all right and we're all wrong", it's because the above ones are two completely different issues IMHO.

Peace

Bob