what does "remote cutoff" mean?

[ELS122]

I am constantly seeing these "remote cutoff" pentode's in rf stages. what does "remote cutoff" mean and what does it do?

[ELS122]

and can I use it in an audio amplifier (guitar amp) and what effects will it add if it's usable?

[TimFox]

A remote-cutoff tube is one where a relatively large negative grid voltage is required to reduce the cathode current to near zero (cutoff).  In practice, this is done with a grid winding where the spacing is not constant along its length.  It is roughly equivalent to two tubes in parallel, where the tube with a tight spacing cuts off at a lower grid bias, but the other continues to conduct up to a higher negative bias voltage.  This grid structure is commonly used in pentodes, triodes, and mixer tubes in RF applications for better control of gain with small input signals.
They are not used in ordinary audio circuits, since they are inherently more non-linear than the “sharp cutoff” variety.  Typical examples:  6AU6 (sharp) / 6BA6 (remote), or the older 6SJ7 (sharp) / 6SK7 (remote).  If you compare the plate characteristics graphed in the data sheets for these pairs, you will see the difference.
In a guitar amplifier, you might exploit the distortion of the remote-cutoff tube, which would have large even-order harmonics at high levels.

[ELS122]

Ok that cleared things up a lot, I might try to experiment whit remote cutoff pen tides in guitar amplifiers now. I will post if any interesting stuff happens when I try do use that kind of pentode

[ELS122]

Wait so it would add a dc bias to the signal basically if it needs a lower voltage to “turn off” or will it just like respond linearly to the audio signal?

[TimFox]

If the audio signal applied to the grid is small, then varying the DC voltage on the grid will vary the gain from grid to plate.  When building a theremin using a 6L7 pentagrid mixer tube, which has a remote-cutoff grid 1 for signal input, I found that the gain for the signal on grid 1 was a very accurate exponential function, with an almost constant slope in dB/volt,over a large range of DC bias.  A sharp-cutoff tube will also vary the gain with DC bias, but over a narrower range of bias.

[T3sl4co1l]

Ah, for guitar amps where distortion is desirable to some extent -- you may need a fair bit of overdrive (10s of volts) but you can expect a remote-cutoff to generate more gradual distortion than a more linear type.

Conversely, a more linear type tends to cut off or saturate more sharply, which means that, when overdriven, they tend towards a clipping distortion waveform rather than a saggy rounding/peaking shape.

For small signals, the gain does not vary much during a cycle and the distortion is small; you only get significant distortion on larger signals.

If the audio signal applied to the grid is small, then varying the DC voltage on the grid will vary the gain from grid to plate.  When building a theremin using a 6L7 pentagrid mixer tube, which has a remote-cutoff grid 1 for signal input, I found that the gain for the signal on grid 1 was a very accurate exponential function, with an almost constant slope in dB/volt,over a large range of DC bias.  A sharp-cutoff tube will also vary the gain with DC bias, but over a narrower range of bias.

Hm, that's cool, and very handy.  Which further implies a response similar to a transistor, although with much less gain (i.e., requiring many volts to cut off, rather than many 10s of mV).  In other words, you could use a transistor to much the same end, given some consideration of transistor properties like temperature sensitivity.

Tim