6L6GC / 6BG6 x4 Ultralinear Guitar Amp

[Printer2]

Originally Posted by nazaroo

This is interesting, because its the opposite of what I have thought:

I supposed that lower resistance meant a higher driving voltage at a given preamp/driver level (and smaller accumulated distortion from previous stages) and less strain on the first half of the amp.

I assumed that high resistances would cause not only voltage loss requiring higher voltages in previous stages for a given loudness (and more distortion), but also a much less linear behaviour by the output tubes, because of the 'voltage-divider' effect of the resistor in the circuit with the grid current.

Of course I have never seen any proper detailed explanation of the interaction of the components as the output tubes are driven into grid-current conditions.

The little I could gather comes from the strange curves (when makers bother to provide them) as grids are driven positive.

High impedance source (the driving stage) into a high impedance input, life is good. Get to 0V on the input and the high impedance turns into a low impedance diode drawing current. In comes the capacitor, blocking distortion. Go to Class AB if you do not like the rules of the game. Transformer coupled sounds ideal. Mind you, probably should run with lower voltages and higher current then.

[nazaroo]

Originally Posted by Printer2

And a Fender, Marshall, Vox, or any of their offshoots are not musical?

Fender: Bright - metallic, harsh at high volumes. Lots of 'clean' power next to a Marshall, but meh,...not musical enough.

Marshall: lots of lovely messy breakup overdrive, that driven sound. Sure, musical if you can play the right stuff, like Howlin Wolf. Would I own one? No. Why buy an amp so badly designed that I have to buy output tubes every 2 weeks?
Idiots.

Vox: I guess if I was playing the blues solo in a coffee house, this or some old tweed would be my choice. Lots of character, but the cliche' is now played to death, like a sitar on a Beatles track.

Not talking about overloading output transformers, its the tubes where the action is. Don't like sag? Guess you fall in line with the high gain metal guys.

You're right: the tubes are where the action is.
Thats why you shouldn't like a typical guitar amp,
with underpowered cheap output transformer,
with its flux mushing out like a fart from a tuba.

You mean power supply sag?
Sure that can be tasty.
In fact load it up with tube rectifiers,
the loss of power will be more than compensated by
the clean solid regulated voltage, with its smooth slide downward under increasing load.

High metal guys?
I loved Lee's explanation of how Metal was shipwrecked by record company formulas. Awesome Youtube fun.

Well if you like thin preamp distortion.

Its only 'thin' if you leave it in the hands of drugged up design-clowns.

I would prefer to roll my own thanks.

So it is a clean amp you want. Why not go SS?

If you want a clean amp, don't go solid state.
All solid state gear is shiite.
My tube amps blew Brystons out of the water in a straight A/B listen-off.

As said above, maybe tubes are not for you.

Guess I've been in the wrong business for 30 years...

Most designs now days take overloading the grids inputs in consideration. What may look like RF mitigation is more tone shaping and nonlinear operation control.

This is an interesting observation/claim.

Can you give examples, and document it a little bit?
I'd love to read about how some amp designer chose a grid stopper based on tested class AB/B performance.

[DF96]

Under normal circumstances the grid input impedance is high, so the circuit behaviour is determined by the output impedance of the previous stage and the grid bias resistor. When overdriving causes grid current to flow the only resistance is the previous stage o/p Z in series with the grid stopper. Unless the grid stopper is large this circuit can charge the coupling cap quite quickly but it then has to discharge slowly through the grid resistor. If you want to avoid blocking you need a small grid resistor and a large grid stopper, so the time constant hardly changes as grid current flows. This would cause the voltage attenuation you describe, so is not usually done.

If NFB is present the situation can get more complicated because the previous stage might cutoff, so its output impedance shoots up to just the anode resistor value. This might help, because it augments the grid stopper during clipping.

If you want to avoid blocking, then either avoid overdriving or use a proper AB2 driving arrangement.

[Printer2]

Originally Posted by nazaroo

Fender: Bright - metallic, harsh at high volumes. Lots of 'clean' power next to a Marshall, but meh,...not musical enough.

As I asked, which is musical enough? Heck of a lot of good music out there made with Fender amps. Which sounds metalic and harsh, the Tweeds or the Blackfaces?

Fender Harvard+Silvertone 1448 - YouTube
G & L Telecaster ASAT Classic Part 2 clean - YouTube

Marshall: lots of lovely messy breakup overdrive, that driven sound. Sure, musical if you can play the right stuff, like Howlin Wolf. Would I own one? No. Why buy an amp so badly designed that I have to buy output tubes every 2 weeks?
Idiots.

I know of musicians that actually make them last as long as a month. Imagine that?

Vox: I guess if I was playing the blues solo in a coffee house, this or some old tweed would be my choice. Lots of character, but the cliche' is now played to death, like a sitar on a Beatles track.

Don't care for the Beatles much myself.
18watt EF86 custom combo amp - YouTube

You're right: the tubes are where the action is.
Thats why you shouldn't like a typical guitar amp,
with underpowered cheap output transformer,
with its flux mushing out like a fart from a tuba.

You are playing the wrong amps.

You mean power supply sag?
Sure that can be tasty.
In fact load it up with tube rectifiers,
the loss of power will be more than compensated by
the clean solid regulated voltage, with its smooth slide downward under increasing load.

Well there is hope for you.

High metal guys?
I loved Lee's explanation of how Metal was shipwrecked by record company formulas. Awesome Youtube fun.

?

Its only 'thin' if you leave it in the hands of drugged up design-clowns.

I would prefer to roll my own thanks.


If you want a clean amp, don't go solid state.
All solid state gear is shiite.
My tube amps blew Brystons out of the water in a straight A/B listen

Well since you were saying you have not hear a tube amp you liked...

Guess I've been in the wrong business for 30 years...

I've wasted a lot of my youth also.

This is an interesting observation/claim.

Can you give examples, and document it a little bit?
I'd love to read about how some amp designer chose a grid stopper based on tested class AB/B performance.

Not sure you understood me. Grid stoppers are used to control the frequency response and the harshness that can happen when you overdrive the grid. Not sure what you mean by tested Class AB/A performance.


I think I will enjoy seeing what you will come up with.

[Printer2]

Originally Posted by DF96

Under normal circumstances the grid input impedance is high, so the circuit behaviour is determined by the output impedance of the previous stage and the grid bias resistor. When overdriving causes grid current to flow the only resistance is the previous stage o/p Z in series with the grid stopper. Unless the grid stopper is large this circuit can charge the coupling cap quite quickly but it then has to discharge slowly through the grid resistor. If you want to avoid blocking you need a small grid resistor and a large grid stopper, so the time constant hardly changes as grid current flows. This would cause the voltage attenuation you describe, so is not usually done.

If NFB is present the situation can get more complicated because the previous stage might cutoff, so its output impedance shoots up to just the anode resistor value. This might help, because it augments the grid stopper during clipping.

If you want to avoid blocking, then either avoid overdriving or use a proper AB2 driving arrangement.

Some work done with zeners clamping the voltage on cathode biased amps also.

[nazaroo]

Originally Posted by DF96

Under normal circumstances the grid input impedance is high, so the circuit behaviour is determined by the output impedance of the previous stage and the grid bias resistor. When overdriving causes grid current to flow the only resistance is the previous stage o/p Z in series with the grid stopper. Unless the grid stopper is large this circuit can charge the coupling cap quite quickly but it then has to discharge slowly through the grid resistor. If you want to avoid blocking you need a small grid resistor and a large grid stopper, so the time constant hardly changes as grid current flows. This would cause the voltage attenuation you describe, so is not usually done.

If NFB is present the situation can get more complicated because the previous stage might cutoff, so its output impedance shoots up to just the anode resistor value. This might help, because it augments the grid stopper during clipping.

If you want to avoid blocking, then either avoid overdriving or use a proper AB2 driving arrangement.

This is the most clear and succinct explanation so far.

But I have some issues with the analysis:

(1) The current is literally flowing out of the tube when there is grid current. It can go in TWO directions: into the input Cap (at least momentarily, or for a time), and into ground (or into the Negative BIAS supply circuit.

(2) Thus the previous stage Zout is not the 'only resistance'.

(3) Apparently the ability to charge the input Cap without the complimentary ability to discharge it is what you and others have been referring to as "blocking" or "blocking effect/distortion". Like a stuffed up nose, current is stifled, and resistance rises rapidly.

(4) This is only relevant if one has a blocking cap (normally to keep out DC from the previous stage). It may be a better argument for direct or resistive coupling than against class AB/B operation.

(5) the "solution" of increasing the grid-stopper resistance as a 'cure' does appear ridiculous, and frequency-dependent too.

(6) As grid current is diverted into ground instead of back into the previous stage (now blocked by a full cap), the stability of the time constant seems irrelevant, because current flowing through the grid-leak resistor to ground drastically alters the BIAS, causing a potential runaway condition.

(7) When such loss of control over the current flow occurs, even loss of driver voltage becomes irrelevant as well.

(8) I think its "not usually done" for more serious reasons than mere drive-signal attenuation.

(9) The advice to avoid overdriving (i.e., crossing the 0-bias line) or redesign the circuit is great advice, but the steps need explicit expression.

(10) I would guess that both control-loss, undesirable current and voltage changes, and runaway tubes would make proper driver/output stage interfaces mandatory.

[Simon B]

Originally Posted by Printer2

Guess you fall in line with the high gain metal guys.

I don't think he's going to like that much....

[doozerdave]

Originally Posted by nazaroo

Well, yep I am coming from a 'hifi perspective'. And I love Jazz.
The goal for me is a really musical guitar amp.
If that means it ends up "outside the box" as far as previous guitar amps and experience goes, oh well, I won't be heartbroken.

This is a very entertaining thread, and I'm going to admit right off the bat that I haven't read every single post in this thread yet, but this one caught my eye. Right from the thread title my first thought was "Ultra-linear guitar amp? I can't imagine that will sound very nice.", and then I saw the quoted post. Isn't all of what makes tube amps "musical" and "pleasing to the ear" the fact that tubes are inherently non-linear? 2nd order harmonics are the key to great tube sound, especially in guitar amps. If you make a hifi style, ultra-linear guitar amp aren't you going to defeat a lot of that character?

[nazaroo]

Originally Posted by Printer2

As I asked, which is musical enough? Heck of a lot of good music out there made with Fender amps. Which sounds metalic and harsh, the Tweeds or the Blackfaces?

I guess that would be the Twin.

Fender Harvard+Silvertone 1448 - YouTube

Well, the bass on this sounded pretty good, but the whole treble/upper mid was just noisey and distorted in a non-musical way.

Here's where the guitar belongs, far below the harmonica,
and with the treble properly turned off, when you're using
a crappy amp like that:

[yt="How Many More Years"]4Ou-6A3MKow[/yt]

http://www.youtube.com/watch?v=A1FK620bS7A

I know of musicians that actually make them last as long as a month. Imagine that?

I know a lot of repairmen who've had to modify hundredss of dangerously crappy Marshalls, to stop them eating EL34s like cheeseys.

Don't care for the Beatles much myself.
18watt EF86 custom combo amp - YouTube

Well,

You are playing the wrong amps.

Well you got a great tone finally on that last amp/link recording.
Rare for a Youtube vid.

Well there is hope for you.

Well since you were saying you have not hear a tube amp you liked...

except my own, it goes without saying...
But then, I haven't built any guitar amps.

I've wasted a lot of my youth also.

it goes without saying...

Not sure you understood me. Grid stoppers are used to control the frequency response and the harshness that can happen when you overdrive the grid. Not sure what you mean by tested Class AB/A performance.

Okay, so you envision grid-stoppers being selected and used
as treble-controls.
That's quite interesting too in its own way.
Again, I'd like to get to see some examples of commercial units designed that way.
I don't doubt that private experimentors have tried every design philosophy possible.

I think I will enjoy seeing what you will come up with.

Thanks for the advance vote of confidence!

[Alvis]

Originally Posted by nazaroo

Okay, so you envision grid-stoppers being selected and used
as treble-controls.
That's quite interesting too in its own way.
Again, I'd like to get to see some examples of commercial units designed that way.
I don't doubt that private experimentors have tried every design philosophy possible.