Another grid resistor question

[Carbsandcaps]

Messing around online and looking at some different biasing arrangements. I see that fixed bias needs let’s say a 100k leak resistor and a cathode his needs a 470k leak resistor. However the grid leak resistor on cathode bias essentially goes to ground and the fixed bias goes essentially to the negative voltage. Does fixed bias not require a ground? Or is it using the negative voltage as a reference to ground? Why do the values change?

Also it seems like fixed bias almost always use a grid stopper resistor and cathode doesn’t. What are the conditions for requiring a grid stopper?

 

[zackthedog]

With fixed bias the grid resistance is grounded through the bias supply. A grid stopper with pentodes or beam power tubes is always a good idea.

 

[love pa amps]

With the fixed bias system, the cathode is usually connected directly to B- with no resistor and the control grid (G1) is connected thru a resistor to the negative bias supply, which can either be a separate dc voltage source whose positive lead is connected to the device's B- common (usually chassis common) and who's negative lead then becomes the negative bias supply in relation to chassis common negative (B-), or created by essentially "floating" B- above chassis negative common by connecting the center tap (B-) of the power transformer B supply to chassis common negative thru a resistor creating 2 separate B- buses, one being negative polarity in reference to the other, sometimes the chassis will be true B- and a B- bias supply will be created by connecting a resistor from true B- to a terminal point where negative (in relation to true B-) voltage can be obtained. Self bias is much simpler, the tube cathode is connected to B- thru a resistor bringing the cathode above B- potential and creating a voltage drop, the control grid is then also connected to B- thru a "leak" resistor causing the tube to create it's own negative bias which will adjust itself depending on the quality of the tube and how much it's drawing. hence "self bias"
Note: only grid resistors in self bias systems are referred to as "grid leak resistors", grid resistors in a fixed bias system are referred to as 'grid bias supply resistors' and cathode resistors are never referred to as "leak" resistors. It's all a lot simpler than it sounds, I hope this helps.

 

[love pa amps]

"grid stopper resistors" are connected between the control grid (G1) signal source (usually a coupling capacitor from the phase inverter stage (in push-pull) circuits or the plate coupling capacitor from the previous stage (usually called the voltage amplifier or driver stage) in single ended (one output tube) devices and connected to the control grid (G1) of the output tube, grid stoppers are never connected as a "shunt" to B- or a bias supply.

 

[love pa amps]

With the fixed bias system, the cathode is usually connected directly to B- with no resistor and the control grid (G1) is connected thru a resistor to the negative bias supply, which can either be a separate dc voltage source whose positive lead is connected to the device's B- common (usually chassis common) and who's negative lead then becomes the negative bias supply in relation to chassis common negative (B-), or created by essentially "floating" B- above chassis negative common by connecting the center tap (B-) of the power transformer B supply to chassis common negative thru a resistor creating 2 separate B- buses, one being negative polarity in reference to the other, sometimes the chassis will be true B- and a B- bias supply will be created by connecting a resistor from true B- to a terminal point where negative (in relation to true B-) voltage can be obtained. Self bias is much simpler, the tube cathode is connected to B- thru a resistor bringing the cathode above B- potential and creating a voltage drop, the control grid is then also connected to B- thru a "leak" resistor causing the tube to create it's own negative bias which will adjust itself depending on the quality of the tube and how much it's drawing. hence "self bias"

With the fixed bias system, the cathode is usually connected directly to B- with no resistor and the control grid (G1) is connected thru a resistor to the negative bias supply, which can either be a separate dc voltage source whose positive lead is connected to the device's B- common (usually chassis common) and who's negative lead then becomes the negative bias supply in relation to chassis common negative (B-), or created by essentially "floating" B- above chassis negative common by connecting the center tap (B-) of the power transformer B supply to chassis common negative thru a resistor creating 2 separate B- buses, one being negative polarity in reference to the other, sometimes the chassis will be true B- and a B- bias supply will be created by connecting a resistor from true B- to a terminal point where negative (in relation to true B-) voltage can be obtained. Self bias is much simpler, the tube cathode is connected to B- thru a resistor bringing the cathode above B- potential and creating a voltage drop, the control grid is then also connected to B- thru a "leak" resistor causing the tube to create it's own negative bias which will adjust itself depending on the quality of the tube and how much it's drawing. hence "self bias"
Note: only grid resistors in self bias systems are referred to as "grid leak resistors", grid resistors in a fixed bias system are referred to as 'grid bias supply resistors' and cathode resistors are never referred to as "leak" resistors. It's all a lot simpler than it sounds, I hope this helps.

additional information: the negative bias voltage will always be picked up at the power transformer "B" supply secondary winding center tap, this being the most negative circuit point in the device.

slight correction

 

[gadget73]

The resistors are smaller with fixed bias to control any grid leakage. If there is gas or whatever in the tube that allows the grid to drift less negative, current flow increases which tends to make the grid even less negative, etc. The smaller value resistor bleeds that off.

With carthode bias, if the grid drifts slightly positive and bumps current flow, the cathode simply becomes more positive and restores the voltage difference so the resistor can be larger without any concern of the tube running away and melting down.

 

[TriodeLuvr]

Note: only grid resistors in self bias systems are referred to as "grid leak resistors"

There was a time when this terminology was only applied to a stage that actually employed grid leak bias. When that's not the case, I refer to it simply a as grid resistor regardless of whether fixed or self bias is in use.

Jack

 

[gadget73]

or created by essentially "floating" B- above chassis negative common by connecting the center tap (B-) of the power transformer B supply to chassis common negative thru a resistor creating 2 separate B- buses, one being negative polarity in reference to the other, sometimes the chassis will be true B- and a B- bias supply will be created by connecting a resistor from true B- to a terminal point where negative (in relation to true B-) voltage can be obtained.

This isn't really fixed bias. Its called "back bias", which is really just cathode bias wired differently. The only functional difference is that it creates bias from the current flow of all tubes, not just the output tubes. In most amps the other tubes contribute a fairly small percentage of the total current flow, so this really doesn't matter very much. In say a receiver where you have a pile of tubes it can be a different story.

What is Back-Biasing?

Guitar amplifiers

www.aikenamps.com

I know there is some famous amplifier that uses it, but I can't remember what off the top of my head. I was thinking it was either a Brook 300b or a Lincoln Walsh 2A3 design but the schematics don't agree with my apparently incorrect memory.

 

[Carbsandcaps]

Ok instead this in the wrong thread but here it is.
So how about this circuit that goes to the negative supply AND ground though a 10k resistor.

 

[Palustris]

I know there is some famous amplifier that uses it, but I can't remember what off the top of my head. I was thinking it was either a Brook 300b or a Lincoln Walsh 2A3 design but the schematics don't agree with my apparently incorrect memory.

The Peerless A100A used back bias. It was considered a kind of "sliding" bias. As current increased in the output tubes, the bias would go more negative allowing a larger signal to be able to drive the tubes increasing power output.

 

[kyle]

If you're talking about R41 and R44, they're there so that if the balance pot if fully at one end or the other, there is still a resistance for the bias that will stop a runaway situation.

 

[gadget73]

^^ they won't actually do that. The resistors are there to provide a voltage divider so the pot does something. If you cut those resistors out, the nearly-zero current draw through the grid would mean moving the pot doesn't change the grid voltage any and therefore can't adjust the tubes to idle at the same current. If the wiper of the pot lifts the tubes nuke either way from lack of grid supply.

 

[love pa amps]

Ok instead this in the wrong thread but here it is.
So how about this circuit that goes to the negative supply AND ground though a 10k resistor. View attachment 3274496

the cathode resistor goes directly to ground, the circuit is drawn just showing a common ground point for several circuits, because the cathode resistor is solidly grounded it is not influenced by negative bias or any other voltages in the circuits grounded at the same point, it's just drawn that way for simplicity and convenience to avoid confusing overlap of drawn lines

 

[Carbsandcaps]

So is there a reason the negative grid voltage goes before the stopper or is it just easier that way?

 

[gadget73]

The stopper is supposed to be as close to the tube as possible in order for it to work. If they moved it somewhere else it wouldn't do anything.

 

[Carbsandcaps]

So what’s the issues with using a grid leak resistor that’s larger than the data sheet shows?

 

[nerdorama]

Maybe nothing depending on how much larger or maybe a redplate from not controlling the grid voltage well enough particularly with fixed bias where there is no cathode resistor to help control the current in the tube. Depends on the power tubes in question and also the condition of them. Usually it is best to trust the data sheets but there are modern instances with new versions of tubes that have required using an even smaller max resistance than the original data sheets specify. Some of the modern 7591's come to mind.

 

[Carbsandcaps]

Ok because I’m working on this amplifier that in order to get the bias correct I would have to use a 250k ohm resistor on a 6l6 that calls for 100k for fixed bias

 

[Mr Boochie]

Messing around online and looking at some different biasing arrangements. I see that fixed bias needs let’s say a 100k leak resistor and a cathode his needs a 470k leak resistor. However the grid leak resistor on cathode bias essentially goes to ground and the fixed bias goes essentially to the negative voltage. Does fixed bias not require a ground? Or is it using the negative voltage as a reference to ground? Why do the values change?

Also it seems like fixed bias almost always use a grid stopper resistor and cathode doesn’t. What are the conditions for requiring a grid stopper?

The lower the grid resistor's resistance, the more voltage to the ground. Most tubes have a range based on their electric characteristics.

I've seen grid leak (grid-to-ground) resistors in both fixed and cathode bias amps.

 

[egellings]

Messing around online and looking at some different biasing arrangements. I see that fixed bias needs let’s say a 100k leak resistor and a cathode his needs a 470k leak resistor. However the grid leak resistor on cathode bias essentially goes to ground and the fixed bias goes essentially to the negative voltage. Does fixed bias not require a ground? Or is it using the negative voltage as a reference to ground? Why do the values change?

Also it seems like fixed bias almost always use a grid stopper resistor and cathode doesn’t. What are the conditions for requiring a grid stopper?

Whether at physical ground or at a DC level, both are at AC ground, so long as any DC level is sufficiently stiff at all AC frequencies involved. An ideal DC source is an AC short circuit.