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Old 17th November 2005, 09:26 PM   #1
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Default 6l6gc Tube Drive

1. Does anyone know the input impedance of a 6L6GC?

2. What voltage peak to peak will drive it to it's maximum output?
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Old 17th November 2005, 11:03 PM   #2
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Hi Chris,

I have a copy of the GE 6l6gc data sheet, got it from the www?

It shows a number of conditions and @ 55w ab1 has the following

g1 -37v
g2 400v
drive 70v pp
a 450V
peak g2 22ma
peak Ai 210ma
A load PP 5K6R
Dist 1.8%

I'a try to email the spec sheet
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Old 17th November 2005, 11:52 PM   #3
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"1. Does anyone know the input impedance of a 6L6GC?"

Yup. It's 100M (nominal) for class A(1) or AB(1). For other operations, it varies.

As for the rest: Frank's has what you're looking for.
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Old 18th November 2005, 12:14 AM   #4
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Hi Miles, Chris,

Now that's an interresting site, one of the better ones i've seen.

The 6l6GC by GE is the data sheet I have been working off.

saves me battling our very slow www access today ;o0


Robert
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Old 18th November 2005, 01:57 AM   #5
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OK If I got this straight, with fixed bias (adjustable) the grid input impedance of either a 6V6 or 6L6 is 0.1megaohms (100K). So if a 12AX7 was used to drive a 6V6, it should drive a 6L6 as well. Where did you get 100M for grid resistance? BTW thanx for that great link!
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Old 18th November 2005, 02:17 AM   #6
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With cathode bias, 470K grid resistor can be used, which the 12AX7 will be happier driving... the 12AX7 has about a 60K plate resistance, so with 100K typical 'AX7 plate load, about a third of its drive capability is lost driving 100K grid resistor, vs. about 10% loss with 470K.
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Old 18th November 2005, 05:36 AM   #7
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"Where did you get 100M for grid resistance?"

For most tubes operated conventionally, the nominal impedance into the grid is 100M, give or take. I wasn't aware you were actually asking for the grid resistor, not the nominal impedance before connecting anything to it.
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Old 18th November 2005, 01:22 PM   #8
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As pointed out in the thread:

Driving a single 6L6GC with 12AX7

A 12AX7 is commonly used in guitar amp applications to drive a 6L6. This was probably done originally for cost reasons and blindly copied for decades. If you are designing a HiFi amplifier, and flat frequency response without serious phase shift is important a tube with a lower plate resistance may be a better choice (12AU7, 12BH7, 12AT7, 6CG7, etc). Most guitar amps have little response above 10KHz.

The input resistance of a 6L6 (or most tubes) is very high until it is overdriven, then it drops like a rock. The overload characteristics of the driver in this case is one of the attributes that determine the sound character of a guitar amp when it is "cranked to eleven." The input of any tube is also capacitive, and the drivers ability to drive this capacitance is one of the factors that determine the frequency response of the amp.
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Old 18th November 2005, 03:48 PM   #9
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Hi mrhotmark,
It's more a question of capacitance instead of resistance. In class AB2 you drive the grids positive and the impedance drops like a stone.

So you need to qualify the circuit, single ended, push pull AB1 or AB2 (or A even). In class AB1 you can go near the grid bias voltage as a peak value. So for a bias of -40V, the drive might be +/- 40V peak.

A tube manual is a help here. It'll give an approximate capacitance as well.

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