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efnikbug
08-04-2010, 06:00 PM
On a typical Fender using the shorting tip and 68K resistor in series with the guitar signal:

1) What happens in the absence of this 1Meg resistor?

2) What happens if you change the value +/-?

3) Can this resistor be located somewhere other than the input jack? Can it be located on the board? Or must it be on the jack because it's also connected to the shorting switch portion of the input jack.

Cyg_1963
08-04-2010, 06:24 PM
It is there to match the impedance of the pickup(s) to the desired impedance of the input stage of the amp.
I have never played with this resistor.

I usually alter the next (usually 68K) to alter the signal of the input.

Putting it on the jack probably helps with potential RF issues.

phsyconoodler
08-04-2010, 06:48 PM
No.It's to prevent any guitar signal from being shunted to ground.The low input doesn't have one so less of the guitars signal gets to the grid of the tube.
It is like a voltage divider.
The 68k resistor in series with the input signal is to prevent RF interference.

jay42
08-04-2010, 06:51 PM
It provides a ground path for the input tube grid(s) no matter what happens...for instance: a guitar cord is attached, but no guitar...buzzzt! Some Ampeg V series amps have a 4.7M or 5.6M resistor there. Some amps have a small cap in parallel for RF problems. The 68K networks are there for the RF rejection, however, not all amp designs use them.

Cyg_1963
08-04-2010, 07:31 PM
So it would not affect the tone or gain in any way.

Some things I have never questioned, others I have just taken on assumption.
This is one of them.

Ronsonic
08-04-2010, 08:50 PM
You're looking at it wrong. While it's often located at the jack, the purpose is to provide a ground reference for the grid of the tube that follows. That tube is biased by having its grid at ground potential and a positive voltage produced at the cathode by the cathode resistor - this has the effect of keeping the grid at a negative 1 - 1.5 VDC relative to the cathode. Without that resistor the grid would gradually accumulate stray electrons, go positive, and then really start pulling electrons and go way positive until it turns into a diode and melts down.

You'll notice on tube data sheets there is a spec for the largest grid resistor that a given tube can use. It's that important.

In normal use, either the jack is shorted because nothing is plugged in, or there's the volume control of a guitar at the other end of the cord. A good designer won't count on that. What if someone unplugs the guitar while the amp is still on and the cord plugged in. Or, maybe someone plugs in a gadget that has an output cap but no pull down resistor after it.

Short version: It's the grid resistor for the first tube stage and it is very, very necessary.

Ronsonic
08-04-2010, 09:43 PM
with the cable plugged in, but no guitar, what does the amp 'see'? The cable acts like a capacitor or something? (As far as the amp is concerned)?

With that resistor? It sees the resistor in parallel with the cable capacitance.

Without, just an open circuit shunted by the small capacitance of the cable.

phsyconoodler
08-05-2010, 10:01 AM
Quote:Ronsonic said:"You're looking at it wrong. While it's often located at the jack, the purpose is to provide a ground reference for the grid of the tube that follows. That tube is biased by having its grid at ground potential and a positive voltage produced at the cathode by the cathode resistor - this has the effect of keeping the grid at a negative 1 - 1.5 VDC relative to the cathode. Without that resistor the grid would gradually accumulate stray electrons, go positive, and then really start pulling electrons and go way positive until it turns into a diode and melts down."

That statement does not apply to a cathode biased preamp tube,only to a grid-leak biased system.
You can choose to use a grid resistor or not,it will not make the tube 'burn up' unless it's grid-leak biased.
You don't have to have the 1meg resistor either;it's just to prevent part of the guitar's signal from going to ground.It's no different than turning the guitar down when you plug into the low input on most amps.
Most guitar amps use a shorting jack that grounds the grid when no cable is plugged in.

PKBlues
08-05-2010, 10:34 AM
Then .... would changing (upping )the value of this resistor help control feedback potential when a ( higher impendence) Harp mic is plugged in to the jack.. ?

Ronsonic
08-05-2010, 11:52 AM
Quote:Ronsonic said:"You're looking at it wrong. While it's often located at the jack, the purpose is to provide a ground reference for the grid of the tube that follows. That tube is biased by having its grid at ground potential and a positive voltage produced at the cathode by the cathode resistor - this has the effect of keeping the grid at a negative 1 - 1.5 VDC relative to the cathode. Without that resistor the grid would gradually accumulate stray electrons, go positive, and then really start pulling electrons and go way positive until it turns into a diode and melts down."

That statement does not apply to a cathode biased preamp tube,only to a grid-leak biased system.
You can choose to use a grid resistor or not,it will not make the tube 'burn up' unless it's grid-leak biased.
You don't have to have the 1meg resistor either;it's just to prevent part of the guitar's signal from going to ground.It's no different than turning the guitar down when you plug into the low input on most amps.
Most guitar amps use a shorting jack that grounds the grid when no cable is plugged in.

Without that (typically) 1 Meg grid resistor how does the grid reference ground? Without a ground reference, what is your bias voltage? Without a bias voltage what happens?

The 68K grid stoppers are optional. Mesa has been leaving those out for decades. Others just use a smallish 10-22K to limit HF interference but you need that grid resistor.

As I mentioned, no competent designer trusts that nobody will ever unplug the guitar while leaving the cable plugged in. Or trust that someones foot-toy or other gadget, LPB-1 for example will have a pull down resistor on the output.

phsyconoodler
08-05-2010, 12:27 PM
The shorting jack provides a ground reference when no cable is plugged in and the guitar provides the completed circuit when plugged in.
And a cathode biased amp does not respond unless a grid voltage is applied unless the grid is grounded.The guitar is the voltage source for the grid.
Plug in a guitar cable without a guitar and it starts to make noise and in some cases it may cause the tube to run away but I'm sure you'll hear it and do something long before it causes the tube to melt down.
Take a look at a Fender amp.The high jack uses the 1 meg resistor so that no signal voltage bleeds to ground and the low input doesn't so that SOME does leak to ground so there is less at the grid.
No voltage applied to the grid simply makes the tube not do anything.It makes noise.Nothing else.
Same with a cathode biased output tube.No voltage applied to the grid and the tube idles.It doesn't run away' and burn up.
Lots of Fender and other amps have had sprung shorting bars on switchcraft jacks over the years.They don't make the tube 'run away'.They simply make noise because the grid is ungrounded.The tube does nothing unless a voltage or RF signal is applied.

phsyconoodler
08-05-2010, 12:47 PM
pkblues said:
"Then .... would changing (upping )the value of this resistor help control feedback potential when a ( higher impendence) Harp mic is plugged in to the jack.. ?"

No,feedback is caused by an impedance mismatch.You can control it by removing the cathode bypass capacitor on the preamp tube.You can also use a preamp between the harp mic and the guitar amp that matches the impedance.Hot mic signal in and guitar level signal out.
You could also try simply adding a diode in the guitar cable that allows the mic signal to only go in the direction of the amp.Remember that the instrument causes the mic to produce an AC signal that enters the amplifier.If that signal is too hot,it cause feedback.What's feeding back?The harp mic in this case.So preventing that AC signal from returning to the harp mic with a diode,it prevents the mic from feeding back.

jawjatek
08-05-2010, 01:13 PM
Sheesh. LMAO at this thread, sorry. Carry on. :D

Cyg_1963
08-05-2010, 07:40 PM
Like I said, some thing I just do religiously and don't alter.
That is one.

But it is nice to know why.
I'll just continue...

hasserl
08-05-2010, 09:02 PM
The shorting jack provides a ground reference when no cable is plugged in and the guitar provides the completed circuit when plugged in.
And a cathode biased amp does not respond unless a grid voltage is applied unless the grid is grounded.The guitar is the voltage source for the grid.
Plug in a guitar cable without a guitar and it starts to make noise and in some cases it may cause the tube to run away but I'm sure you'll hear it and do something long before it causes the tube to melt down.
Take a look at a Fender amp.The high jack uses the 1 meg resistor so that no signal voltage bleeds to ground and the low input doesn't so that SOME does leak to ground so there is less at the grid.
No voltage applied to the grid simply makes the tube not do anything.It makes noise.Nothing else.
Same with a cathode biased output tube.No voltage applied to the grid and the tube idles.It doesn't run away' and burn up.
Lots of Fender and other amps have had sprung shorting bars on switchcraft jacks over the years.They don't make the tube 'run away'.They simply make noise because the grid is ungrounded.The tube does nothing unless a voltage or RF signal is applied.

In a typical Fender #2 input the 1M ground reference resistor is bypassed and the 68k grid resistor for the #1 input becomes the ground reference. This provides a greatly reduced impedance, the pickups are loaded down much more with the result of reduced gain at the input stage.

Ronsonic
08-05-2010, 09:05 PM
Same with a cathode biased output tube.No voltage applied to the grid and the tube idles.It doesn't run away' and burn up.
Lots of Fender and other amps have had sprung shorting bars on switchcraft jacks over the years.They don't make the tube 'run away'.They simply make noise because the grid is ungrounded.The tube does nothing unless a voltage or RF signal is applied.

Those amps remain stable because they have a resistor referencing the grid to ground. If you remove all DC connection between the grid and ground you will have a major malfunction.

The grid lives in a cloud of electrons off the cathode, it will pull positive if left without a ground reference. Really. With a 100K plate resistor in series even a 12AX7 won't red plate, but it will run as a diode instead of a triode. You need to reference grid to ground for the tube to function as an amplifier.

Do you actually attempt to build amps without grid resistors?

Ronsonic
08-05-2010, 09:08 PM
Like I said, some thing I just do religiously and don't alter.
That is one.

But it is nice to know why.
I'll just continue...

Continue.

Bias is set by the voltage relationship between the grid and cathode. Without that ground reference you don't have a stable voltage at the grid. That simple.

You'll notice that the guy who says it isn't necessary uses them too.

Shiny_Beast
08-05-2010, 09:25 PM
Not knowing if it's just going to be a guutar plugged straight in or not I'd say it's required, although most pedals have an output pot that has less than 100k to ground.

The grid needs some kind of drain for excess electrons or something like that. Try running a stage without any grid leak to ground, it sounds gated like a fuzz biased wrong. That's what I thought was wrong at the time anyhow.

schmidlin
08-05-2010, 09:59 PM
I knew this would heat up, but here is my 2 cents:
What is a Strat pickup? 8K to ground? Throw in the 250K pot to ground and it reduces further, so there goes your need for the 1M. Seems like a straw man argument assuming that some *mystery* instrument has infinite impedance and will screw up the V1 bias.

That said, I throw it in there in my builds; maybe I'm superstitious, or maybe I have bigger fish to fry in the tinkering department. Also there may be some weird input combo problem that this 1M prevents from causing. Maybe if the shorting input jack doesn't short? Maybe they did it before they used shorting jacks and you never know...

zzmoore
08-05-2010, 10:10 PM
In a typical Fender #2 input the 1M ground reference resistor is bypassed and the 68k grid resistor for the #1 input becomes the ground reference. This provides a greatly reduced impedance, the pickups are loaded down much more with the result of reduced gain at the input stage.
Isn't it 136k from the tip to ground?
Thanks

donnyjaguar
08-06-2010, 09:22 AM
In theory you don't need that 1M resistor. In practice you do. Think instrument cable plugged into amplifier but not instrument. :)

BTW, its not there for impedance matching as the input impedance of the amplifier is normally much higher than the output of the instrument.

phsyconoodler
08-06-2010, 10:23 AM
Ok,I'll reiterate.Anyone have a Fender amp that had a sprung input jack?No grid to ground reference there is there?
Anyone have their tubes runaway?Nope.
Who leaves an amp on and a cable plugged in with no guitar anyway?

donnyjaguar
08-06-2010, 10:34 AM
Ok,I'll reiterate.Anyone have a Fender amp that had a sprung input jack?No grid to ground reference there is there?
Anyone have their tubes runaway?Nope.
Who leaves an amp on and a cable plugged in with no guitar anyway?

All the time brutha, all the time... Even my kids are conditioned to the loud CRACKLE!! BUZZZ!!! CRACKLE!! of changing instruments on the fly. :)

Structo
08-06-2010, 10:43 AM
Hehehehe, this gets better and better.
Carry on!

jay42
08-06-2010, 12:29 PM
Hehehehe, this gets better and better.
Carry on!:facepalm

Ronsonic
08-06-2010, 12:57 PM
Ok,I'll reiterate.Anyone have a Fender amp that had a sprung input jack?No grid to ground reference there is there?


Yes, there is.

Look at the schematic it is the 1M resistor we've been talking about.

Run a cathode biased tube without a resistor from grid to ground. Go ahead. Please do this.

Ronsonic
08-06-2010, 01:02 PM
In theory you don't need that 1M resistor. In practice you do. Think instrument cable plugged into amplifier but not instrument. :)

Or signal source with an output cap, like some EH treble boosters, or piezo pickups or any number of other oddities and science fair projects.

The difference between theory and practice is that in theory, there's no difference.

BTW, its not there for impedance matching as the input impedance of the amplifier is normally much higher than the output of the instrument.

I will say that the 1M is a pretty happy point for most guitars. To my ear most electrics start getting harsh and brittle sounding much over that. Which sorta makes sense, pickup designers are going to work around the amps that they see and that 1M is pretty much universal.

phsyconoodler
08-06-2010, 01:18 PM
What I meant by 'sprung' is when the shorting bar no longer touches.It's a 'damaged' jack.They do not make a ground contact.The grid does not run away on itself,it simply hums.

You guys better take a look at some schematics of some old amps like Kalamazoo's,Kents',Magnatones,etc that use cathode biased 12AX7's.Not a 1 meg grid to ground reference resistor in sight.

wizard333
08-06-2010, 04:10 PM
The 1M resistor is to set the input impedance for your amp. It works in parallel with your guitar's volume knob when the guitar is plugged in. Remove it, and you have no ground reference and it wont work. 1M is a standard value on tube amps. Higher will be brighter (to a point), lower sounds more dull, just like using a smaller volume pot sounds more dull because you are losing high end.

The 68k is there as a grid stopper for the input stage and is designed to attenuate 20K + audio signals from hitting the grid of your 1st stage, so you dont get RF interference and have a "Spinal tap at the Airforce Base" moment with radio coming through your amp at high volume.

reaiken
08-06-2010, 05:23 PM
Okay, guys, here's the deal:

The 1Meg is there to provide a ground reference for the grid. The tube works by developing a bias voltage across the cathode resistor with respect to the grid, which must be at or near ground, DC-wise, or it won't work.

Since the tube grid is a very high impedance, much higher than the value of the resistor, the resistor value also sets the input impedance of the amp. The input impedance is not extremely critical, as long as it is high enough, because preamp stages are voltage amplifier stages, not power amplifier stages, and you don't match impedances like you do with a power output stage, where you are striving for maximum power transfer. In a voltage amplifier stage, the goal is to make the input impedance high for maximum voltage transfer, and to avoid loading the output of the driving stage, which, in this case, is the guitar pickup. A typical rule of thumb is that the load impedance should be at least ten times the value of the driving source impedance in order to be negligible.

Now, do you need a resistor there with a guitar amp? Not always, because a "standard" (non-piezo) guitar pickup is a coil of wire with one end grounded. The pickup coil will then become the DC ground return for the amplifier's input stage, and it is in parallel with the 1Meg resistor, so a resistor is not always necessary.

However, as correctly pointed out, it is a very good idea to add it, because you don't know what might be plugged into the input. The driving source may be AC-coupled though a capacitor from a stompbox output, which would result in the grid floating.

What will happen with a floating grid? Will it cause the tube to melt down? No. The typical 100K plate load resistor will limit the maximum current to the plate to around 2.5 mA at most, and the resulting voltage drop across the resistor would limit the plate voltage to around 50V at most, so there is not enough power dissipation to cause any damage. The worst case that would happen would be the resistor would burn up if it weren't rated for high enough power. In addition, the tube's grid is such a high impedance, it will sit there and "float" for awhile, so you may not even get much current.

Having said this, the best approach is to add a blocking capacitor in series with the input, with the 1Meg grid reference resistor after it. This will provide a stable DC biasing point for the tube no matter what is plugged into the input, even a floating cable or an AC-coupled stompbox, or an effects unit with a high DC offset at the output.

This capacitor is a necessity on a grid-leak biased amp, like the Magnatone T-32, because the tube wouldn't be able to develop enough grid-leak bias voltage at the grid when connected to a guitar amp pickup's resistance without the DC-blocking capacitor. As mentioned, it is also a good idea on a cathode-biased amp, and is used in some commercial amplifiers, like the Peavey Classic 30, for instance. It will also keep the guitar volume pot from making scratchy noises when turned up and down if there is a bit of grid current in the first preamp tube, which will develop a small DC voltage at the grid that can result in these noises.

Randall Aiken

hasserl
08-06-2010, 06:23 PM
Yep, that sounds about right. :D

Hey Randall, where were you when we were talking about using multiple taps on an output transformer simultaneously? ;)

reaiken
08-06-2010, 06:26 PM
Hey Randall, where were you when we were talking about using multiple taps on an output transformer simultaneously? ;)


Which thread was that?

RA

davemccarthy707
08-06-2010, 06:32 PM
Yep, that sounds about right. :D

Hey Randall, where were you when we were talking about using multiple taps on an output transformer simultaneously? ;)

Running multiple speakers off multiple taps is easy.............:hide2 JK

Dave

Ronsonic
08-06-2010, 06:48 PM
Hi Randall, maybe he'll believe you.


Okay, guys, here's the deal:

The 1Meg is there to provide a ground reference for the grid. The tube works by developing a bias voltage across the cathode resistor with respect to the grid, which must be at or near ground, DC-wise, or it won't work.

reaiken
08-06-2010, 06:52 PM
Hi Randall, maybe he'll believe you.

Or maybe not, since you could argue both sides. If you have a guitar pickup providing the DC return path for the grid reference, the resistor is technically not necessary, unless your source is AC-coupled, and since it won't likely kill anything if the cord is unplugged from the guitar...

:hide2

Then again, maybe I've provided a nice diplomatic way out for all parties involved... :)

RA

Tone_Terrific
08-06-2010, 10:03 PM
What I meant by 'sprung' is when the shorting bar no longer touches.It's a 'damaged' jack.They do not make a ground contact.The grid does not run away on itself,it simply hums.

You guys better take a look at some schematics of some old amps like Kalamazoo's,Kents',Magnatones,etc that use cathode biased 12AX7's.Not a 1 meg grid to ground reference resistor in sight.
:huh
Looking at a Fender amp schematic shows the 1M always available, sprung jack or not.

hasserl
08-06-2010, 11:39 PM
:huh
Looking at a Fender amp schematic shows the 1M always available, sprung jack or not.

When using the #2 input the 1M is bypassed. Look at the #1 input, see how when a plug is not inserted the switch provides a path to ground, bypassing the 1M.

Tone_Terrific
08-06-2010, 11:51 PM
http://www.drtube.com/schematics/fender/bassman-5f6-a-schematic.gif

I see now how it works on the newer ones.
The old ones had it always in there.^

reaiken
08-07-2010, 02:42 PM
When using the #2 input the 1M is bypassed. Look at the #1 input, see how when a plug is not inserted the switch provides a path to ground, bypassing the 1M.

The point is that there is always a resistor to ground to provide a DC ground reference for the grid, either the 1Meg or the attenuator made from the 68k/68k voltage divider when the bottom jack is in shorting mode. In this case, the lower 68k resistor is the grid reference for the tube, in the other case the 1Meg is the grid reference. Note that in the 68k/68k case, the input impedance of the amp drops to 136k ohms.

The grid has to have some sort of ground reference, in the event the amp is driven from an AC-coupled source. As mentioned earlier, if the pickup is DC-coupled, the grid can get its ground reference through the pickup coil itself, so technically, it isn't necessary, but it is still a good idea.

RA

donnyjaguar
08-09-2010, 03:52 PM
I'm sure someone will come up with a modification to remove this 1M resistor in the never ending quest for more mojo. :)

phsyconoodler
08-09-2010, 06:17 PM
Again,my point is that it isn't really necessary but it's a good idea.Lots of older amps didn't have them.There also weren't many stompboxes and effects pedals back then either.And most people plugged in an accordian or a guitar.
The tube still works without the 1meg resistor but it needs it to operate at full potential.

Tone terrific wrote:"Looking at a Fender amp schematic shows the 1M always available, sprung jack or not."
That's only true if the jack is plugged in.If the shorting bar is not making contact and the jack is not plugged in,there is no ground.

Tone_Terrific
08-09-2010, 07:48 PM
Tone terrific wrote:"Looking at a Fender amp schematic shows the 1M always available, sprung jack or not."
That's only true if the jack is plugged in.If the shorting bar is not making contact and the jack is not plugged in,there is no ground.

This is trivial but that's why we're here...
I posted the 5F6A Bassman schematic link.
1 M always to ground, or shorted out to provide a 68k to ground

The new ones (HRD for example) also have the 1M connected directly to ground.
I see some ground reference through the 1M or 68k at all times, regardless of jack switching.
Are we all looking at the same thing?

n8b
08-09-2010, 10:17 PM
That's only true if the jack is plugged in.If the shorting bar is not making contact and the jack is not plugged in,there is no ground.

The input is not grounded, but there is a DC ground reference through the 1M resistor no matter what the jacks are doing.

Edit: Unless you are talking about something other than a Fender. If so, please post context.

donnyjaguar
08-10-2010, 08:18 AM
Okay, guys, here's the deal:

(much excellent text removed)

Having said this, the best approach is to add a blocking capacitor in series with the input, with the 1Meg grid reference resistor after it. This will provide a stable DC biasing point for the tube no matter what is plugged into the input, even a floating cable or an AC-coupled stompbox, or an effects unit with a high DC offset at the output.

Randall Aiken

Now I could be mistaken, but I believe there is a potential pitfall to adding a DC blocking capacitor in front of the first voltage amplifier. I'm thinking this would add a reactive component to the pickup-coil/instrument cable LRC circuit and result in unexpected frequency response. I believe this is the argument in hi-fi circuits, and circles, about adding a capacitor to a moving coil input of a phono stage. Of course, I could be wrong.

hasserl
08-10-2010, 10:02 AM
Note: I just "uncovered" one of those old amps with no 1M grid to ground resistor. IT's an old Kent amp, made in Canada, circa 1960's, it is one of those transformerless amps. No grid referencing resistor at all. The input jack is a switching type with the switched contacts going to ground. I went ahead and added the resistor.

Note that this amp has operated for a very long time like this, indicating the resistor is not a requirement and the tube won't melt without it.

phsyconoodler
08-10-2010, 10:40 AM
There are lots more like that.Take a look on schematic heaven.Quite a few.

The point is,it's not going to harm the amp.
Guys,you had better take a closer look at a Fender switchcraft jack.If the switch is sprung,there is no ground reference.The tip lug needs to come into contact with the switch contact to complete the path to ground WHEN NO JACK IS PLUGGED IN.Thats why it's switched.You obviously don't need it when the jack is plugged in.
They pick up outside noise and the amp hums with a sprung jack.And they sometimes pop when you plug in a cable.
With modern amps you need the resistor because of modern effects and who knows what guys plug in these days?

n8b
08-10-2010, 12:51 PM
Guys,you had better take a closer look at a Fender switchcraft jack.If the switch is sprung,there is no ground reference.

The tip being ungrounded and "no ground reference" for the grid are two different things. Unless I am missing something.

phsyconoodler
08-10-2010, 04:01 PM
Yeah,you're right.I meant no tip to ground reference if the jack is sprung.Grounding it just makes the tube inoperative.

reaiken
08-10-2010, 04:33 PM
Guys,you had better take a closer look at a Fender switchcraft jack.If the switch is sprung,there is no ground reference.The tip lug needs to come into contact with the switch contact to complete the path to ground WHEN NO JACK IS PLUGGED IN.Thats why it's switched.You obviously don't need it when the jack is plugged in.

I'm not seeing what you are saying.

Take, for instance, a 5F6-A bassman schematic here:

http://www.kbapps.com/audio/schematics/tubeamps/fender/bassman5f6a.html

If the tip switch is "sprung" and not making connection from the tip to the switching lever terminal of the jack, and no cable is plugged in, the 1M is still connected to ground from the grid of the tube (through the 68K grid stopper).

You get hum because it is a high impedance (1Meg), but the tube works and amplifies the hum because it is still biased properly with a ground reference. If the switch is not sprung, it shorts out the 1Meg resistor, so there is a 68K resistor from grid to ground (actually 34k since there are two 68k's effectively in parallel). The tube still works and is still biased properly, but the short to ground removes the high-impedance hum pickup source.

RA