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Old 29th September 2010, 02:59 AM   #1
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Default Emulating remote cut off

Since (I think I am correct here) the gain of a pentode is affected by the screen voltage. If one took a portion of the input signal to the control grid and amplified or attenuated it and applied the resulting signal to the screen (in addition to a fixed screen supply voltage) could one emulate a remote cut off characteristic using a sharp cut off pentode?
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Old 29th September 2010, 10:22 AM   #2
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Yes, to a certain extent. You could just leave the screen unbypassed. But why not use a genuine remote cutoff valve? There was even one intended for audio - EF83.
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Old 29th September 2010, 07:44 PM   #3
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Your bets bet for emulating a remote cutoff characteristic is to use a single series dropping resistor for the screen. If you don't regulate the DC, the screen voltage will come up as the screen current goes down. That will tend to keep the pentode out of cutoff. It's one of the reasons why you don't want to do that if your goal is linearity.

What are you trying to accomplish here? If it's for a receiver, well, there are just esssssss-loads of remote cutoff pents designed expressly for this purpose.
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Old 29th September 2010, 09:08 PM   #4
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Quote:
there are just esssssss-loads of remote cutoff pents designed expressly for this purpose.
Many of the sharp cutoff pentodes that are used in audio amps and receivers have a remote cutoff version. The internal difference is the way the control grid is wound. A sharp cutoff pentode has a grid wound with a constant winding pitch. The remote cutoff grid does not have a constant pitch, so part of the grid cuts off before the rest.

6AU6 --> 6BA6
6EJ7 --> 6EH7
6SJ7 --> 6SK7

If this is for a variable gain amplifier look into a "dual control pentode". G1 and G3 are both control grids.
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Old 30th September 2010, 01:08 AM   #5
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Actually it is not an rf or hi-fi application. The idea is a signal processor. What I am trying to do is amplify one side of the wave form more than the other and have it adjustable. Also would like to do it with current production tubes if possible.
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Old 30th September 2010, 01:23 AM   #6
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Is this for a guitar processor? Is sand allowed?
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Old 30th September 2010, 01:30 AM   #7
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1)Yes.
2)If really necessary.
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Old 30th September 2010, 02:00 AM   #8
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I don't know if it is necessary, but it makes things easier if a few opamps are wired up to the tubes. Your design calls for some type of rectification to alter the gain on one half cycle. It could be a tube grid, or a diode. An opamp with a diode - resistor circuit in the feedback loop seems like a place to start.


I have been working on a guitar processor on and off for a while. I plan to stuff the whole thing inside a solid body guitar too. There will be tubes, and the primary signal path will be pure tube, but since there will be a microprocessor to store and recall settings, the opamps seem like a given.

I plan to enter the guitar in another design contest. My last chips and tubes entry won a prize that lead to writing a magazine article. Both paid quite well.

As I have said before, If you plan to sell any of your creations to working musicians, it must use tubes that can be obtained at a music store in the middle of nowhere. That severely limits the choices.
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Old 30th September 2010, 02:13 AM   #9
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Yes, tube availability is an issue. I assume that small signal pentodes of any type are uncommon in guitar shops. I don't mind some op amps in the peripheral circuitry in this case.

My thought was that putting a large enough in phase signal on the screen would make the gain continuously variable without any need for rectification as you would need for something like a compressor. Am I missing something fundamental?
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Old 30th September 2010, 02:36 AM   #10
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My thought was that putting a large enough in phase signal on the screen would make the gain continuously variable without any need for rectification as you would need for something like a compressor. Am I missing something fundamental?
Probably not. I haven't tried it myself. The original Fender Champ used a 6SJ7 driving a 6V6. I built a few clones about 40 years ago. I could get a cool distortion sound out of it by swapping in a 6SK7 and driving the Champ with s simple one transistor preamp. THe extra gain allowed me to control the distortion with the guitars volume control and the playing style.

I discovered this by playing around. With guitar effects, that is mandantory. Another thing, what sounds great with one guitar, may sound bad on another. In my case the guitar build is far enough along that I can play it. The circuit experiments are just getting started.
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