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In Reply to: RE: Changing the Gain posted by TomWh on March 16, 2017 at 07:08:49
Tom, depending on the circuit it may be possible to reduce gain by applying some local feedback, eg unbypassed cathode. This may not be what you are looking for as there are consequences to consider.
Your goal and a schematic would help.
Naz
Follow Ups:
For those who believe that no feedback is better than any feedback, you can reduce the gain by reducing the plate resistor value. Gain directly depends on it.
" Gain directly depends on it."No it isn't.
The mu of the tube changes very little no matter what you do.
Lowering the plate load resistor will limit the max output swing but will not lower the mu (and therefore the gain of the stage) at all.Edit, I should have said "will not lower the mu enough to make any real difference"
Edit again, Yes you can lower the mu enough to make a real difference but by the time you've done that the plate resistor is so small that the load line is very vertical and the harmonic distortion is unacceptably high.
Bottom line, for all practical purposes the gain of a tube is what it is. If it's to high, get a different tube.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 03/18/17 03/18/17 03/18/17
Wow...
I think you should try a couple more edits. :)
I think you should show us how to lower the gain of a tube by lowering the value of the plate resistor without increasing harmonic distortion.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I give up. I guess it is too much to expect people to read before answering.
Take a 6sn7
250 volt plate
22k plate resistor
8ma.
About 1k bypassed cathode resistor
Gain 14, DB gain 22.9
Now take a 6sn7
250 volts plate
11k plate resistor
8ma.
about a 1K bypassed cathode resistor
Gain of 11.25, DB gain of 21
That's a gain difference of 1.9db.
That's not enough to make any real difference if he's fighting a situation of "too much gain" but the harmonic distortion will be much higher with a 11k plate resistor vs. a 22k plate resistor.
Am I missing something?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Gain of a 6SN7 using a CCS is pretty much the mu of the tube (assuming it's not driving much load of course).
OTOH Gain of 6Sn7 with unbypassed cathode R of 1K and a low 11K plate resistor is only around 13db according to Spice compared to approx 20db bypassed. And, distortion is approx 10db less than using a 22K despite the lower plate R.
I'm not saying this is the way to go but pointing out that gain can be significantly reduced using tried and tested methods.
Personally I don't like using Rs or Cs in the cathode and prefer to lightly load the plate but many report they love the sound of an unbypassed cathode R. It is a valid (possible) option that just might meet the poster's needs and therefore worthy of putting forward.
From a technical standpoint the combination of a lower plate R, and unbypassed cathode R can significantly reduce gain without increasing distortion. Whether one likes the sound is another thing.
Naz
I agree with your points but Victor said,
"For those who believe that no feedback is better than any feedback, you can reduce the gain by reducing the plate resistor value. Gain directly depends on it."
I was just pointing, without resorting to feedback, lowering the gain of a stage by reducing the value of the plate resistor is not a practical solution. And simply reducing the value of the plate resistor does not lower the gain of the stage to any appreciable amount.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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