the role of screen grid voltage

[ETR]

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

I think you should reread my post, do the test for youself. I understood Bruce's post completely. The only thing he left out was doing the test I suggested.

[ETR]

"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

Sorry I'm not wrong on this one. I think what maybe you and Chuck are probably missing is the drop across the power transformer. When you talk about power supply sag that's usually the biggest contributer. The DC sag in voltage is quite easily measured as I suggested everyone try that doubts this at the first filter cap. It's quite simple really.

[Chuck H]

No, I didn't miss the drop across the PT. As I mentioned earlier, an underated PT will drop volts. This would be evedent regardless of the rectifier type. But using a proper PT would minimize this. Not that all Fender amps used such a PT.

You know what. This is just getting stupid. The same argument keeps coming up and you keep turning it over 50 times because you don't want to be the knuckle head that said a diode rectifier sags just like a tube rectifier. Knock it off.

Chuck

[Robert M. Martinelli]

I think what maybe you and Chuck are probably missing is the drop across the power transformer.

I am with Chuck, as I don't miss the drop across the PT either.

When I said " all other factors being equal" I meant it - than meant "changing the rectifier type only", keeping all other components ( including the PT ) the same.

A properly sized PT only drops a few Volts - period.

The "DC swing" due to the signal being amplified is something different from rectifier sag - period. ( I think this has been pointed out clearly enough in my last post, if in doubt, re-read it thoroughly )

And this is, and will be always true: A tube rectifier drops more voltage than a SS rectifier because its forward resistance is higher - period.

Regards

Bob

[Steve Conner]

Hello all,

Just a brain dump on this issue:

A PT can sag quite a bit. I have an old one that's 520V off load, and 400V on full load, with a solid-state rectifier. It was a FWCT type designed for use with a tube rectifier: a PT designed from the outset to work with a solid-state bridge rectifier will sag far less. Tube rectifiers needed resistance to limit the peak current, so the transformer designers probably used to build it in by using thinner wire. And the bridge rectifier needs only half the turns for the same output voltage, so the designer can use thicker wire still, without using up any more room on the bobbin.

Tubes do not sag by themselves, except maybe the rectifier tube: the word "sag" refers to droop in the B+ voltage under load.

Reducing the screen voltage will normally increase sag in the screen node, because the usual way of doing it is with a series resistor. This isn't necessarily so, though: you can do it with a separate transformer tap, or a regulator. It can be an old-fashioned gas tube with a funky purple glow, like some amps used to have, or in this day and age, you can use zeners and MOSFETs.

The screen voltage functions like the boost on a turbocharged car: it directly multiplies the tube's maximum current delivery. Hence, lowering the screen voltage leads to reduced power output and earlier clipping. (and probably a longer, happier life for your "engine" too.)

And, since the screen is like another plate whose current varies in sympathy with the actual plate current, unbypassed screen resistors also lower the tube's gm, which impacts the NFB loop gain, and hence speaker damping.

If you bypass your screen dropper to get the gm back, you have an R-C filter whose time constant can get your amp pumping like a compressor (which most P-P amps do to some extent anyway)

Any effect on the tone is likely to be explained by the above three factors. Broadly speaking, it probably sounds like you'd pulled some tubes or swapped them for smaller ones.

[ETR]

No, I didn't miss the drop across the PT. As I mentioned earlier, an underated PT will drop volts. This would be evedent regardless of the rectifier type. But using a proper PT would minimize this. Not that all Fender amps used such a PT.

You know what. This is just getting stupid. The same argument keeps coming up and you keep turning it over 50 times because you don't want to be the knuckle head that said a diode rectifier sags just like a tube rectifier. Knock it off.

Chuck

It would be benificial for you to not misquote me or call me names. I never said a tube rectifier dropped more voltage than a SS recto. Reread my post and since you started it get a clue asswipe.

[Enzo]

Alright, knock it off everyone. We don;t need namecalling in any direction. Doesn;t matter who started what, no one has to "finish." This place is for amps, not attitudes.

[Chuck H]

Ok...Sorry. I lost my cool on this and it's my fault it got kicked over. I am indeed an a$$w!pe. Sorry ETR, sorry Enzo, I'm back in my "happy place".

Chuck

[mooreamps]

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.

If you have it set up right, there is no effect on tone.

-g

[Merlinb]

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.

For a given load impedance, lowering the screen voltage will push you further towards triode-like operation, and a triode-ish sound, which is tonally less interesting, usually. Raising the screen voltage pushes you towards typical pentode operation and beyond, into high odd-harmonic and screen-sag territory (and into the danger zone for screen burn out, of course). Of course, you have to know what your load impedance is before you know which 'tonal region' you're operating in!

[mooreamps]

No. If you have it set up right, there is no effect on tone.

-g

[Merlinb]

No. If you have it set up right, there is no effect on tone.

-g

Hmm, I cannot see how you come to that conclusion. Suppose I have it set up right, then I lower the screen voltage to zero (for example), you think that won't have an effect on tone? I think you need to qualify your statement.
Everything about the pentode is determined by the screen voltage; suggesting it has no tonal effect seems a little naive to me...

[mooreamps]

Hmm, I cannot see how you come to that conclusion. Suppose I have it set up right, then I lower the screen voltage to zero (for example), you think that won't have an effect on tone? I think you need to qualify your statement.
Everything about the pentode is determined by the screen voltage; suggesting it has no tonal effect seems a little naive to me...

Well, if using combination bias on an self-bias SE power amp, if you lower the screens all the way to zero, then of course that will put the power tubes into stand-by. But what is does do is allow the player to get the saturated tones at lower volume levels, which is what I was led to believe the purpose of electronic power scaling in the first place. Besides, most of my guys who play my equipment on stage run the power brake anywhere from 50% up to 90% anyways. . . . .

Secondly, WADR, I'm not required to qualify anything.

-g