# Plate resistor/cathode res question



## JDW3 (Sep 23, 2009)

In reading Aikens page, he said that if you raise your cathode resistor, you should alter your plate resistor. He didn't say how. So, what is the formula for this? 

I lowered my cathode resistor on V1 to 820 from 1500 ohms. The plate resistor is stock, at 100K. Should the plate resistor be lowered or raised?


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## Shiny_Beast (Apr 16, 2009)

I'd leave it the same, you don't see many plartes less than 100k, probably for a reason. I think the higher you raise one, the higher you have to set the other to keep the same bias. However, a different bias may be desirable.


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## keeperofthegood (Apr 30, 2008)

I am going to sound stupid here but I am a n00b and still trying to fathom this myself.


It helps me to look at the cathode, the grid, and the plate as three things on a string (using a triode just for the sake of ease). If I put a black mark on the string for the cathode, for the grid, and for the plate, no matter where the ends of the string go, the relation stays the same between those three dots. So, if you raise up the plate end of the string you also have to push up the cathode end.

That should be as clear as mud, however I can recomend an evenings reading with working examples of how this all works that I have read and still need to re-read (going from 0v up makes sense to me, going from -150 up boggles my mind still, even though I know the c-g-p relationships don't change).

*http://www.angelfire.com/electronic/funwithtubes/Basics_03_Diodes.html

http://www.angelfire.com/electronic/funwithtubes/Basics_04_Triodes.html

http://www.angelfire.com/electronic/funwithtubes/Basics_05_Multigrid.html


*


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## parkhead (Aug 14, 2009)

JDW3 said:


> In reading Aikens page, he said that if you raise your cathode resistor, you should alter your plate resistor. He didn't say how. So, what is the formula for this?
> 
> I lowered my cathode resistor on V1 to 820 from 1500 ohms. The plate resistor is stock, at 100K. Should the plate resistor be lowered or raised?


can you quote or link to the page ? I can't find this 



p


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## JDW3 (Sep 23, 2009)

parkhead said:


> can you quote or link to the page ? I can't find this
> 
> 
> 
> p



This is in the Q&A on Aikens site:

Q: Can I change the preamp cathode or plate resistors to change the gain or tone of my amp? 
A: Yes, but you should change the plate resistor value when you change the cathode resistor value, in order to compensate the quiescent plate voltage shift, to rebias the tube near the center of the plate voltage swing, or to the place it was originally biased. If you don't change the plate resistor, the change in the static DC bias point may cause the tube to clip very asymmetrically, and the headroom will be lower, which may or may not be a good thing, depending on your needs. The two go hand-in-hand. If you change the cathode resistor, the plate resistor should (ideally) be changed, and if you change the plate resistor, the cathode resistor should be changed, unless you are designing for an asymmetric bias point. At any rate, it is always a good idea to check the operating conditions of the tube circuit on a scope to make sure things aren't getting out of hand. People tend to view preamp tubes differently from power tubes, but in reality, they are the same animal, just on a different scale. Preamp tubes need to be biased, just as power amp tubes do, and there are ranges that are more optimum than others. This is why it is not a fair test to just swap different tube types and compare tones. A 12AX7 will require different plate and cathode resistors than a 12AT7, for instance, for optimum bias point and tone.


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## keeperofthegood (Apr 30, 2008)

AH! I found the other two pages pertinent to understanding this:

http://www.angelfire.com/planet/funwithtransistors/Book_CHAP-4A.html

http://www.angelfire.com/planet/funwithtransistors/Book_CHAP-4B.html

He takes you through the math, which is admittedly a bit of a trip in molasses the first time through >_>

How Rp is established in his walk through is by using values from the data sheets I think. However, yes, you are maintaining the electrical distance between the cathode and plate.


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## parkhead (Aug 14, 2009)

For our purposes simply lowering the cathode resistor is OK unless you get some distortion you don't like in the process.

It's ok to trust your ear & keep in mind dropping the cathode from 1500 to 820 is not a massive shift. Also keep in mind we are not looking for the maximum linear gain from the stage... we are simply looking for a little more gain

We are looking to make the amp "squeeze like tube of toothpaste"
If you look at 100's of schematics you will see 1500, 820, 2k7, 5k6 and 10k cathode resistors all paired up with the generic 100k plate load. 

We are taking an already well engineered amp an nudging some values to get the tone we want... if you were sitting down with a blank sheet of paper, or tweaking an HI FI system you'd want to do more math, and less trail and listen ...

p


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## JDW3 (Sep 23, 2009)

parkhead said:


> For our purposes simply lowering the cathode resistor is OK unless you get some distortion you don't like in the process.
> 
> It's ok to trust your ear & keep in mind dropping the cathode from 1500 to 820 is not a massive shift. Also keep in mind we are not looking for the maximum linear gain from the stage... we are simply looking for a little more gain
> 
> ...




Well guys, I got the amp sounding good and put her back in her cab before any more foolishness ensues!

I raised the plate resistor from 100K to 150K. The caps in tone stack are factory stock. The cathode r/c is 820/25. But in changing the slope resistor and mid resistor on the bass pot, it's all a bit different.

I have other amps that need work. Luckily, as I would probably keep tweaking! 

With the amp back in it's 1x15 cab, I am really happy with the results. I need a get a camera and get something posted on youtube. 

I slammed a P-90 SG into it, as well as a really good sounding telecaster. The amp sounds great at all volumes. The 15" is a Weber Cali. I'm thinking I want a Weber Neo, just for clean tones (so I've heard) and light weight.

I also ran the amp into a 2x12 Marshall cab with Vintage 30s. Best tones I've gotten from the amp. Doing this, I know that the low end looseness is from the 15 and not the amp.


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