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In Reply to: RE: confused I am... posted by vinnie2 on August 17, 2015 at 07:31:43
>> so that one is plate to either of the filament ends correct?
Correct
>> plate current and grid volts I am not sure how to measure.
The voltage across the cathode resistor is your bias voltage.
Divide that voltage by 850 (cathode resistor)
and you get the plate current.
Ohms Law (Versions of same equation)
V = I*R
I = V/R
R = V/I
School's Out 8^D
DanL
Follow Ups:
Basically, if you do as Danl says, you'll be fine, but I always think of "bias voltage" as the difference in voltage (with respect to ground) between the grid and the cathode. The grid V always has to be negative wrt the cathode V. So that family of parallel curves you posted will be at progressively more negative bias voltages (0, -2, -4, etc). The more negative the bias voltage, the more the tube is turned off, and vice-versa. Whereas, the "cathode voltage" is just one parameter needed to calculate bias voltage. For some, this may be a matter of semantics.EDIT: This was meant to appear below Vinnie's response to Danl. Sorry.
Edits: 08/17/15
I checked grid to ground and it was almost zero vdc (0.003), so in this case it is a moot point; but I believe you are correct.
The only time the grid will be negative with regard to ground is when the grid resistor is connected to a negative bias supply. That's called "fixed" bias.
The bias voltage of a tube that uses a cathode resistor for bias is the voltage difference between the grid and the cathode.
Since you know that the grid to ground voltage is (more or less) zero, then the bias voltage for the tube is the cathode to ground voltage.
When you look at the plate curves with the load line, use the cathode to ground voltage as the bias voltage and the plate to cathode voltage as the plate voltage.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Another one for the notebook. Thanks!
Thanks DanL. So grid volts and bias voltage are the same thing?
Nest question is how far away from the line can you be before you are too far away?
I guess i will play with the cathode resistor a bit and see how close I can get to being on the line.
The graph is an estimate or ideal tube.
Tube variation is normal and IMO
as long as the bias is over 2.8V,
I would say it's fine.
Don't worry about the gragh.
DanL
Well that's good to know. I played around with the cathode resistor and 820 ohms brings everything in real close to P's "ideal" specs. Sounds great, so I think we are in good shape. Thanks!
Edits: 08/17/15
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