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Changing the Gain

174.238.4.61
Posted on March 16, 2017 at 07:08:49
TomWh
Audiophile

Posts: 764
Location: Tucson Az
Joined: August 7, 2003
 I have some Russian tube that say mu is like 35-55. I always thought the mu was pretty much fixed. If you can adjust it what is the procedure.

Thanks
Tom

 

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RE: Changing the Gain, posted on March 19, 2017 at 07:42:08
Tre'
Industry Professional

Posts: 17297
Location: So. Cal.
Joined: February 9, 2002
 "I always thought the mu was pretty much fixed."

It is. If you need less gain use a different tube.


Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Changing the Gain, posted on March 18, 2017 at 05:03:42
Naz
Audiophile

Posts: 2184
Location: Sydney
Joined: September 2, 2005
 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

 

RE: Changing the Gain, posted on March 18, 2017 at 09:55:53
Victor Khomenko
Manufacturer

Posts: 55317
Joined: April 5, 2000
 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.


 

RE: Changing the Gain, posted on March 18, 2017 at 19:40:52
Tre'
Industry Professional

Posts: 17297
Location: So. Cal.
Joined: February 9, 2002
 " 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"

 

RE: Changing the Gain, posted on March 19, 2017 at 06:02:43
Victor Khomenko
Manufacturer

Posts: 55317
Joined: April 5, 2000
 Wow...

I think you should try a couple more edits. :)


 

RE: Changing the Gain, posted on March 19, 2017 at 07:51:14
Tre'
Industry Professional

Posts: 17297
Location: So. Cal.
Joined: February 9, 2002



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"

 

RE: Changing the Gain, posted on March 19, 2017 at 08:35:31
Victor Khomenko
Manufacturer

Posts: 55317
Joined: April 5, 2000
 I give up. I guess it is too much to expect people to read before answering.


 

Read what?, posted on March 19, 2017 at 09:36:30
Tre'
Industry Professional

Posts: 17297
Location: So. Cal.
Joined: February 9, 2002
 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"

 

RE: Read what?, posted on March 21, 2017 at 04:56:54
Naz
Audiophile

Posts: 2184
Location: Sydney
Joined: September 2, 2005
 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

 

Agreed, posted on March 21, 2017 at 07:46:03
Tre'
Industry Professional

Posts: 17297
Location: So. Cal.
Joined: February 9, 2002
 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"

 

RE: Changing the Gain, posted on March 17, 2017 at 23:11:27
twystd
Audiophile

Posts: 2723
Location: Austin,Texas
Joined: December 9, 2000
 Tom, the gain of the tube is tied to several factors, manufacturing tolerances, loads, a higher load will result in more mu (to a point), also operating points effect the mu as well.

twystd

 

RE: Changing the Gain, posted on March 16, 2017 at 09:24:49
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Some tubes are made with a control grid helix that varies in pitch. Remote cutoff pentodes associated with AGC are pretty common. In triodes, the 6ES8 comes to mind.

The mu varies with the amount of bias voltage applied. The association with AGC should be obvious.


Eli D.

 

RE: Changing the Gain, posted on March 16, 2017 at 10:26:53
TomWh
Audiophile

Posts: 764
Location: Tucson Az
Joined: August 7, 2003
 Hi Eli

These are Russian 6s3p-ev a buddy send me. I am running them a little hot around 3.2 watts but they sound very good. Will be going down on b+ and current to see what happens.

Is it linear how the bias and mu react. Also is it always in the same direction?

Good luck with the 46 amp have them in the 1st stage of my power amps.

Thanks Tom

 

RE: Changing the Gain, posted on March 16, 2017 at 13:27:55
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 In the case of the 6s3p-ev, the +/- 15 for mu seems to be manufacturing tolerance, as opposed to intentional variable mu.

The acceleration/impact ratings suggest aerospace and/or military service as the intended use.

The 1T4 is a fair example of a super-control/remote cutoff type.
http://www.mif.pg.gda.pl/homepages/frank/sheets/127/1/1T4.pdf


Eli D.

 

RE: Changing the Gain, posted on March 16, 2017 at 16:31:12
Victor Khomenko
Manufacturer

Posts: 55317
Joined: April 5, 2000
 ***The acceleration/impact ratings suggest aerospace and/or military service as the intended use.***

Well, not really, those are pretty mild specs. Some military tubes are designed to handle 150g repetitive shocks and 500g single shocks, plus 20g vibration.


 

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