What are remote and sharp cutoff pentodes??????

6/16/2009 1:58:26 PMCHRIS

Greetings.I read something about cutoff pentodes having their grids more open near the edges so when there are strong radio signals less gain is produced or something.Is this true and what is the difference between a sharp and remotecutoff etc. pentode?But the real question here is why do some (AUDIO} amplifiers use these in their preamps.Do they "limit" strong audio signals or does this have absolutely nothing to do with their purpose?Any info appreciated.Thanks

6/16/2009 6:09:58 PMBill G.

Hi Chris,
The difference between sharp cut off and remote cut off is created by an engineering compromise. In a sharp cut off pentode like a 12AU6 or 12SJ7 the spec for cut off is well controlled in manufacture. The cost is dynamic range. A remote cut off such as 12BA6 or 12SK7 has its ability to amplify optimised, at the cost that its cut off and saturation parameters are more variable than for the sharp cut off.
Remote cut off are used where amplification is important and the likelyhood of saturation or cut off is low, such as RF amplifiers or first IF amplifiers. Sharp cut off are used as limiters in FM always because a limiter is commonly saturated and cut off in its operation. It is also commonly used in subsequint IF stages beyond the first.

Usually tube substitution books show them as not replacing each other, however, in a pinch you can do it for a temprorary replacement.

Best Regards,

Bill Grimm

6/16/2009 7:04:17 PMCHRIS

:Hi Chris,
: The difference between sharp cut off and remote cut off is created by an engineering compromise. In a sharp cut off pentode like a 12AU6 or 12SJ7 the spec for cut off is well controlled in manufacture. The cost is dynamic range. A remote cut off such as 12BA6 or 12SK7 has its ability to amplify optimised, at the cost that its cut off and saturation parameters are more variable than for the sharp cut off.
: Remote cut off are used where amplification is important and the likelyhood of saturation or cut off is low, such as RF amplifiers or first IF amplifiers. Sharp cut off are used as limiters in FM always because a limiter is commonly saturated and cut off in its operation. It is also commonly used in subsequint IF stages beyond the first.
:
: Usually tube substitution books show them as not replacing each other, however, in a pinch you can do it for a temprorary replacement.
:
:Best Regards,
:
:Bill Grimm

THANKS BILL

6/16/2009 8:18:24 PMWalt

Would the remote cutoff tube be similar to the variable-mu (gain) tube? Would these gain characteristic classifications also extend to triodes?
Is there a similar classification for solid state devices, like a variable-beta device?

6/17/2009 10:12:05 AMBill G.

: Would the remote cutoff tube be similar to the variable-mu (gain) tube? Would these gain characteristic classifications also extend to triodes?
: Is there a similar classification for solid state devices, like a variable-beta device?

Hi Walt,
The gain is set by the load resistances and the biasing of the grid and the tube characteristics.
The remote cut off is made for gain so it is high gain. The sharp cut off is optimised for predictable cut off and saturaiton characterisitcs. To achieve this it has to be lower gain.

Best Regards,

Bill Grimm

6/16/2009 8:23:59 PMin laymans terms please?

6/16/2009 10:23:36 PMit is! are you stupid?

::Hi Chris,
:: The difference between sharp cut off and remote cut off is created by an engineering compromise. In a sharp cut off pentode like a 12AU6 or 12SJ7 the spec for cut off is well controlled in manufacture. The cost is dynamic range. A remote cut off such as 12BA6 or 12SK7 has its ability to amplify optimised, at the cost that its cut off and saturation parameters are more variable than for the sharp cut off.
:: Remote cut off are used where amplification is important and the likelyhood of saturation or cut off is low, such as RF amplifiers or first IF amplifiers. Sharp cut off are used as limiters in FM always because a limiter is commonly saturated and cut off in its operation. It is also commonly used in subsequint IF stages beyond the first.
::
:: Usually tube substitution books show them as not replacing each other, however, in a pinch you can do it for a temprorary replacement.
::
::Best Regards,
::
::Bill Grimm

6/17/2009 10:09:16 AMBill G.

In the following text I refer to these specs:
http://www.mif.pg.gda.pl/homepages/frank/sheets/049/6/6SJ7.pdf
http://www.mif.pg.gda.pl/homepages/frank/sheets/049/6/6SK7.pdf

Tubes in the radios we restore typically operate in Class A operation. This means that they are constantly drawing current and the signal is presented to the grid in a manner to cause this current to vary in the linear regeon of the tube's performance characteristics.
According to the RCA specs noted above the 6SK7 (remote cut off) will not cut off until you get a grid voltage of -15. The 6SJ7 will cut off at -5, one can expect more gain or dynamic range out of the 6SK7 (remote cut off).
One may wonder why bother with a 6SJ7 (sharp cut off) ever.
Once out of the linear range a tube will start to get into non-linear regeons and start to distort the signal. If this can happen in a circuit, such as an FM limiter, and needs to be managed to minimize distortion. A remote cut off may cut off or saturate at widely varied points. This can be seen on the second graph on the 6SK7 spec. The three load lines approach hte zero plate current (cut off) at a very gradula angle. As a result 6SK7 which perform in spec will cut off at a rather wide range of grid voltages. This low slope tells us this since it is an average.
On the 6SJ7 things are different. The five lines shown show achievement of cut off (zero plate voltage) at a relatively sharrp angle. So if your supressor grid is 100 volts, line 2, you will achieve cut off between -5.6 volts and -6.1 volts. With the 6SK7 you will get to zero somewhere between -25 and -50. RCA doesn't tell you much on the 6SK7 graph because you shouldn't be counting on this anyway.
Over all -6 volts is not a remote as -35 volts, and a load line that intercepts zero ad a sharp andle is a sharp cut off.

Best Regards,

Bill Grimm

6/18/2009 3:08:25 PMLewis L

:In the following text I refer to these specs:
:http://www.mif.pg.gda.pl/homepages/frank/sheets/049/6/6SJ7.pdf
:http://www.mif.pg.gda.pl/homepages/frank/sheets/049/6/6SK7.pdf
:
:Tubes in the radios we restore typically operate in Class A operation. This means that they are constantly drawing current and the signal is presented to the grid in a manner to cause this current to vary in the linear regeon of the tube's performance characteristics.
:According to the RCA specs noted above the 6SK7 (remote cut off) will not cut off until you get a grid voltage of -15. The 6SJ7 will cut off at -5, one can expect more gain or dynamic range out of the 6SK7 (remote cut off).
: One may wonder why bother with a 6SJ7 (sharp cut off) ever.
: Once out of the linear range a tube will start to get into non-linear regeons and start to distort the signal. If this can happen in a circuit, such as an FM limiter, and needs to be managed to minimize distortion. A remote cut off may cut off or saturate at widely varied points. This can be seen on the second graph on the 6SK7 spec. The three load lines approach hte zero plate current (cut off) at a very gradula angle. As a result 6SK7 which perform in spec will cut off at a rather wide range of grid voltages. This low slope tells us this since it is an average.
: On the 6SJ7 things are different. The five lines shown show achievement of cut off (zero plate voltage) at a relatively sharrp angle. So if your supressor grid is 100 volts, line 2, you will achieve cut off between -5.6 volts and -6.1 volts. With the 6SK7 you will get to zero somewhere between -25 and -50. RCA doesn't tell you much on the 6SK7 graph because you shouldn't be counting on this anyway.
: Over all -6 volts is not a remote as -35 volts, and a load line that intercepts zero ad a sharp andle is a sharp cut off.
:
:Best Regards,
:
:Bill Grimm

What it has to do with things is using the tube in an Automatic Volume Control. The remote cut off is wound with the grid closely wound at the edges, but far apart at the center. Therefore, as you place a negative Voltage on the grid, the tube will amplify less as the closely wound part of the tube will be cut off, and the widely spaced windings will be doing the amplifing. So, you have an Automatic Volume Control. (AVC) If you were a radio engineer forty years ago, You would be using the same tubes in an audio limiter circuit, where a negative DC Voltage was passed back into the input of a limiting amplifier to keep the modulation of your transmitter at 100%.
Today, they use diodes as variable resistors (At least, the public adress amplifiers in airplanes do it this way)

Lewis

6/18/2009 8:08:43 PMCHRIS

::In the following text I refer to these specs:
::http://www.mif.pg.gda.pl/homepages/frank/sheets/049/6/6SJ7.pdf
::http://www.mif.pg.gda.pl/homepages/frank/sheets/049/6/6SK7.pdf
::
::Tubes in the radios we restore typically operate in Class A operation. This means that they are constantly drawing current and the signal is presented to the grid in a manner to cause this current to vary in the linear regeon of the tube's performance characteristics.
::According to the RCA specs noted above the 6SK7 (remote cut off) will not cut off until you get a grid voltage of -15. The 6SJ7 will cut off at -5, one can expect more gain or dynamic range out of the 6SK7 (remote cut off).
:: One may wonder why bother with a 6SJ7 (sharp cut off) ever.
:: Once out of the linear range a tube will start to get into non-linear regeons and start to distort the signal. If this can happen in a circuit, such as an FM limiter, and needs to be managed to minimize distortion. A remote cut off may cut off or saturate at widely varied points. This can be seen on the second graph on the 6SK7 spec. The three load lines approach hte zero plate current (cut off) at a very gradula angle. As a result 6SK7 which perform in spec will cut off at a rather wide range of grid voltages. This low slope tells us this since it is an average.
:: On the 6SJ7 things are different. The five lines shown show achievement of cut off (zero plate voltage) at a relatively sharrp angle. So if your supressor grid is 100 volts, line 2, you will achieve cut off between -5.6 volts and -6.1 volts. With the 6SK7 you will get to zero somewhere between -25 and -50. RCA doesn't tell you much on the 6SK7 graph because you shouldn't be counting on this anyway.
:: Over all -6 volts is not a remote as -35 volts, and a load line that intercepts zero ad a sharp andle is a sharp cut off.
::
::Best Regards,
::
::Bill Grimm
:
:
:
:
:What it has to do with things is using the tube in an Automatic Volume Control. The remote cut off is wound with the grid closely wound at the edges, but far apart at the center. Therefore, as you place a negative Voltage on the grid, the tube will amplify less as the closely wound part of the tube will be cut off, and the widely spaced windings will be doing the amplifing. So, you have an Automatic Volume Control. (AVC) If you were a radio engineer forty years ago, You would be using the same tubes in an audio limiter circuit, where a negative DC Voltage was passed back into the input of a limiting amplifier to keep the modulation of your transmitter at 100%.
:Today, they use diodes as variable resistors (At least, the public adress amplifiers in airplanes do it this way)
:
:Lewis Thanks Bill and Lewis!