analyze a circuit...quiescence current and R-Load
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 27th May 2008, 09:05 AM #1 krusty75   diyAudio Member   Join Date: Feb 2008 analyze a circuit...quiescence current and R-Load Hi there, this is a newbie question. I'm looking for a method to calculate quiescence current on a vacuum tube circuit. Let me try to explain better: if I have a circuit: say a marshall JCM800 preamp and I know anode voltages..how can I determine quiescent current on the tubes? Usually the procedure is...choose a quiescence current and the anode voltage and then draw the load line on the tubes curves.. Now I want to do the opposite...I have the Voltage, I have the cathode resistor and the plate resistor..and I want to determine the quiescence voltage... I suppose I'm missing the Load resistor..but how can I calculate on a complex circuit... My example : JCM800 first triode...suppose Anode Voltage:350V, Rk suppose 820R, Load = ??? (how can I calculate it if I have many stages) quiescent current ?? (on the first triode) I would like to know a practical mode (whit an excample if possible on that schematic ) to do this calculation..if any. Thanks a lot for your help Krusty
Merlinb
diyAudio Member

Join Date: May 2006
Location: Lancashire
Re: analyze a circuit...quiescence current and R-Load

Quote:
 Originally posted by krusty75 [B]Hi there, if I have a circuit: say a marshall JCM800 preamp and I know anode voltages..how can I determine quiescent current on the tubes?
Do you mean you know the voltage AT THE ANODE, or the SUPPLY VOLTAGE (HT), which is not the same thing, of course?

Quote:
 Usually the procedure is...choose a quiescence current and the anode voltage and then draw the load line on the tubes curves.. Now I want to do the opposite...I have the Voltage, I have the cathode resistor and the plate resistor..and I want to determine the quiescence voltage... I suppose I'm missing the Load resistor..but how can I calculate on a complex circuit...
If you know the voltage AT THE ANODE, and you know the anode resistor (Ra), then the quiescent current current is simply:
Ia = HT - Va / Ra

Similarly, if you know the cathode voltage (Vk), and the cathode resistor (Rk), then the quiescent current must be:
Ia = Vk / Rk

However, it sounds more like you know the HT, and you have a particular cathode resistor in mind, and you want to choose a load resistor? There is no particular load than corresponds to any particular cathode resistor, unfortunately. A rule of thumb used by lazy designers in the old days was:
Rk = Ra / mu,
or,
Ra = mu x Rk
which would yield 82k in your case, but this tells you nothing about how the stage might operate. 100k is the usual value for an ECC83 in a guitar amp. Perhaps you could explain your problem in more detail?

krusty75
diyAudio Member

Join Date: Feb 2008
Quote:
 Do you mean you know the voltage AT THE ANODE, or the SUPPLY VOLTAGE (HT), which is not the same thing, of course?
Yes you are right...I know Supply voltage not the voltage at the anode.so this is why I want to know a way to calculate quiescent current.

What I want to do is this:
To have a practical example to work with I choose Marshall JCM 800 (first triode) so we can have the same schematic under our eyes.
So...I just want to analyze the preamp (because using the power supply I can "extract" current draw from the tube").
So I have this preamp..with plate resistor fixed (say 100k) and cathode resistor fixed (say 2.7K (but any other value it's ok..it's just an exercise for me)).
What I would like to know (learn how to calculate is)
1) How much is the quiescent current drown by the tube given those parameters. 2) How can I calculate the load seen by this triode? (this is just the first stage of the amp)
Hope my explanation is clearer now.
Krusty

 27th May 2008, 07:17 PM #5 krusty75   diyAudio Member   Join Date: Feb 2008 Thanks a lot!! This post really helps me!! I really appreciated! Krusty
 28th May 2008, 12:54 PM #6 krusty75   diyAudio Member   Join Date: Feb 2008 And how to do the same job if I have a cathode follower stage with no plate resistor? (usually cathode res is smaller than anode) Thanks again.
Merlinb
diyAudio Member

Join Date: May 2006
Location: Lancashire
Quote:
 Originally posted by krusty75 And how to do the same job if I have a cathode follower stage with no plate resistor? (usually cathode res is smaller than anode) Thanks again.
For basic analysis you can use thenormal curves, but add a new voltage scale, indicating thje cathode-to-ground voltage, see:
http://www.freewebs.com/valvewizard1/dccf.htm

However, the curves themselves are altered by the feedback extant in a cathode follower, so you need to take care how you interpret the load line, see page 3: http://www.thermionic.info/cathode_r...eFollowers.pdf

cerrem
diyAudio Member

Join Date: Jun 2005
Location: San Diego, CA
Quote:
 Originally posted by krusty75 And how to do the same job if I have a cathode follower stage with no plate resistor? (usually cathode res is smaller than anode) Thanks again.

The follower is a different beast all together.... It's all feedback...
Here are simple first-order quick and dirty methods that provide close approximations...
The most important voltage to note is the GRID voltage, since the CATHODE will FOLLOW it..... Lets say you have 350V on the plate and the GRID has 190V on it.... The your CATHODE will have roughly 192V minus the bias voltage... which is around 2 volts, give or take, in the applications you have.....since yo have the direct coupled version..... The DC voltage drop, plate voltage, of the gain stage before provides the grid voltage for the follower...which is a 100K ....and the follower uses a 100K in the cathode....which is fine....
You have a 100K in the plate load....this programs the current in the follower....so 190V/100K = 1.9mA ........ You will always get roughly 190V on the CATHODE regardless what value cathode resisitor you use.....so if you use a 56K you would get 3.39mA ....
The proper way to do cathode follower design is to start out with the IV "PLATE" curves..... then there is a way to redraw over them and come up with the "CATHODE" curves.... This way you can see the cathode curves and how to load them and you can see the large voltage swing at the input grid....
If you use too small of a cathode resistor you start to loose gain as it drops below unity....

Chris

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