Grid stop resistor.

[Alan0354]

I have a question, we use grid stop resistor to avoid the awful sound when over drive and the grid current conduction cause the coupling cap to charge up. The result is a very unpleasant distorted. Notably the good old Marshall Plexi that sounds really good when you crank to 6 but it really break up at 10.

The cascade gain amp all use quite large grid stop resistor like 100K for preamp stages. Why the grid stop resistor for the power tube is very low? I saw a few Fender only use 1500 ohm and more amp use lower than 2.4K.

I have problem with my experiment that I can hear the typical cracking sound of the grid being over drive on the 6L6GC. A 20K resistor sure help. My question, why people don't increase the resistor? I understand if you bias a AB2, grid current conduct in part of the cycle. BUT for most differential stage ( phase splitter) have 47K or higher plate resistors, it does not really source a whole lot of current, maybe 3 to 4 mA max.

If I make the amp to stay in AB1, only problem I can think of is the frequency response as the input cap and grid-plate cap is larger in big tubes. So I look up 6L6GC. grid plate cap is 0.6uuF and input cap is 10uuF.

First, I take that 10uuF = 10pF as 10EE-6X10EE-6= 10EE-12!!! Right?

Typical gain for RL=6K give u=gm(RL//rp) = gmRL as rp is very high.

So gain is 6000umhoX6000=36 give and take as I make serious assumption. So miller cap is only (36+1)X0.6pF=22pF. Together with 10pF input cap, the total capacitance shunting the input of the 6L6GC is 32pF. The pole frequency is

1/(2\piRC)= 200KHz for a 25K grid stop resistor, that is not going to be in the picture. Am I missing something?

Thanks

Alan

A side question, AB2 supposed to provide more power for the given stage than AB1. I had an adjustable grid voltage in my Bassman and tested before, I use the attenuator so I cranked up all the way to distortion. I found it is louder at bias a -39V than -54V as the original Bassman. Why? Obviously -54V bring the stage into AB2. at -39V, it is close to AB1. Why do I get louder volume. Remember, this is with attenuator driving to distortion. The power amp was clipping to compare. It was max out in both case.

[Enzo]

Maybe I am unsure of what you describe, but -54v is closer to cutoff than -39v.

[Alan0354]

Maybe I am unsure of what you describe, but -54v is closer to cutoff than -39v.

That's what Fender bias a lot of their silver face amps give and take a little. But they are always -50V or over.

[jpfamps]

So gain is 6000umhoX6000=36 give and take as I make serious assumption. So miller cap is only (36+1)X0.6pF=22pF. Together with 10pF input cap, the total capacitance shunting the input of the 6L6GC is 32pF. The pole frequency is

1/(2\piRC)= 200KHz for a 25K grid stop resistor, that is not going to be in the picture. Am I missing something?

You need to include the output impedance of the PI in your poll frequency calculation.

[jazbo8]

That's what Fender bias a lot of their silver face amps give and take a little. But they are always -50V or over.

I think Enzo means that -39v is closer to AB2 than -54v, you had it the other way around.

From Aiken: "The total resistance is the sum of the series grid resistor and the bias feed or "grid-to-ground" resistances, so if the max spec is 300K, for example, and there is a 220K bias feed resistor, the largest grid resistance that can safely be used is 80K. Of course, in practice, tubes should not be run that close to the edge of their specifications, to insure reliability."

More at Aiken's site: http://www.aikenamps.com/InputRes.htm

Jaz

[Alan0354]

I think Enzo means that -39v is closer to AB2 than -54v, you had it the other way around.

From Aiken: "The total resistance is the sum of the series grid resistor and the bias feed or "grid-to-ground" resistances, so if the max spec is 300K, for example, and there is a 220K bias feed resistor, the largest grid resistance that can safely be used is 80K. Of course, in practice, tubes should not be run that close to the edge of their specifications, to insure reliability."

More at Aiken's site: Grid Resistors

Jaz

I guess I miss spoke, correct me if I am wrong.

At -50V, the idle current is lower, one of the tube is off in bigger part of the cycle than if the amp is bias at -39V. But at -50V, you can drive more signal into the input before the power tube start to draw grid current.

at -50V, a sine wave signal has to have 100V p-p before hitting the zero grid point where grid current starts. For bias at -39V, it only takes 78V p-p to reach the zero grid point. That's the reason -50V is more closer to AB1 than -39?

Am I correct.

[Alan0354]

You need to include the output impedance of the PI in your poll frequency calculation.

Yes, I forgot. The plate resistance of PI is 47K, so the total resistance is 72K, the corner frequency is 67KHz. Still way higher than we care. Beside, more power amp has negative feedback ( closed loop feedback). Some even put a 2000pF from output of the PI to ground on each side to create a dominant pole to stabilize the power amp. Large grid stop might be able to replace that as dominant pole.


I still have the other part of the question, why do I get more output when I biasd at -39V. I thought you get more power biasing more negative. As I said, in both case, I drive the amp all the way to saturation, so the fact that you need more input voltage to drive full power on -50V has been taken care of.

[jazbo8]

Yes, but perhaps it is better to say -50v is more class B than -39v, or conversely -39v is more class A than -50v, and since you are limiting the input drive up to 0v for both, they are both class AB1 by definition. I too, would like to learn more about class AB2 in guitar amps, so this is a good topic, there are some other threads on this forum that I need to go through still, so much to learn...

Jaz

[pdf64]

'why do I get more output when I biasd at -39V'
I can imagine that the amp may sound and respond differently at very different bias levels, but did you measure the relative output levels?
Pete

[Alan0354]

'why do I get more output when I biasd at -39V'
I can imagine that the amp may sound and respond differently at very different bias levels, but did you measure the relative output levels?
Pete

Yes, it sound different. -39V is tighter. I just did not expect -39V get louder as it is moving towards class A ( well not quite but you know my drift). That's why I was expecting it to be not as loud. No I did not put a scope or anything, I just crank the dial to 10, both distorted on the clean channel so I drive the amp full tilt. So the volume represents the power. It is noticeable difference.

[frus]

Well you also changed the frequency response/harmonic distortion a bit, so it might seem louder...?

Regarding the grid stoppers, you can put up to 100k without audible difference. I tried it, amp sounded fine, worked fine.

[Alan0354]

Well you also changed the frequency response/harmonic distortion a bit, so it might seem louder...?

Regarding the grid stoppers, you can put up to 100k without audible difference. I tried it, amp sounded fine, worked fine.

I thought about this too, but I tested, if I have the volume less than max, the two setting has the same volume. Only when I gradually cranked up then one stop gettting louder towards the end and the other still went up a little bit more.

Yes, this is very non scientific, just my observation.