Hi guys,

I tried to set up a small experiment to calculate the actual plate
resistance for a 12AX7 under given conditions, and compare the value
to what I found in the plate resistance curve. I think my
reasoning/understanding is way off though, because my calculation
doesn't even come close to what I see in the curves.

The set up includes the following:

1) A 107VDC B+ voltage through a 100k resistor (Ra) into the
12AX7 plate.

2) A 1.5k resistor (Rk) ties the cathode to ground.

3) A 1Meg resistor ties the grid to ground.

(resistance values are +- 5%)


These are the measurements I took:

1) Voltage drop across Ra = 39V

2) Voltage drop across Rk = .547V

3) current (measured between cathode and Rk) = .00037A



Based on these measurements I calculated what I thought was the plate
resistance as follows:

1) Voltage drop between plate and cathode = (107-39-.547) = 67.453V

2) Plate resistance (Rp) = (67.453V / .00037A) = 182305 ohms



However, when looking at the plate resistance curve
(http://www.mif.pg.gda.pl/homepages/f...93/1/12AX7.pdf)
(PAGE 4), it appears that the resistance should be about 120k when the
plate current is .37mA. I'm off by 60k!!


I've obviously got something wrong either in my set up or my
understanding of what the plate resistance is - or both. Any
suggestions or clarifications are most appreciated.

Thanks,
Joe

On Tue, 01 Feb 2005 04:28:47 GMT, Joe wrote:

I've obviously got something wrong either in my set up or my
understanding of what the plate resistance is - or both. Any
suggestions or clarifications are most appreciated.

Plate resistance is a dynamic condition. It's defined as the
(very small) change in plate voltage divided by the (very
small) change in plate current.

Because the errors in DC measurements tend to eat your lunch,
most folks do this with an AC signal, and factor out all the
load impedances.

At any rate, what you're measuring is not plate resistance.

Chris Hornbeck
"Don't be foolish, like the others..." _Lola Montes_, 1955

"Chris Hornbeck" wrote in message
...
Plate resistance is a dynamic condition. It's defined as the
(very small) change in plate voltage divided by the (very
small) change in plate current.

Yep.

At any rate, what you're measuring is not plate resistance.

In fact, grab the graph again and mark the point of plate voltage and
current you were operating at. Now draw a straight line from the left
corner (0V, 0mA). *That* is what you were measuring.

Because triodes have decreasing plate resistance, actual Rp will always be
lower than static resistance.

The trick to measuring it is to couple an AC signal through a resistor,
measure the current through it and the AC voltage thus appearing on the
plate. Divide ala Ohm's law and you have your result. Remember to do some
algebra on the parallel resistor equation to remove the plate supply
resistor factor.

This technique applies to *ALL* situations where you wish to measure the
output resistance.

Tim

--
"I've got more trophies than Wayne Gretsky and the Pope combined!"
- Homer Simpson
Website @ http://webpages.charter.net/dawill/tmoranwms

"Joe" wrote

I've obviously got something wrong either in my set up or my
understanding of what the plate resistance is - or both. Any
suggestions or clarifications are most appreciated.


Chris is right, but since you have the set-up handy, try varying Va
slightly (add a few extra k to Ra) and measure again. Then calculate
using the *differences* in Va and Ia (or Ik) between the two
measurements. That should be close to spec for the midway point
between the two Va you have used.

cheers, Ian

"Ian Iveson" wrote in message
...
Chris is right, but since you have the set-up handy, try varying Va
slightly (add a few extra k to Ra) and measure again. Then calculate
using the *differences* in Va and Ia (or Ik) between the two
measurements. That should be close to spec for the midway point
between the two Va you have used.

Unfortunately, Rk adds degeneration and increases it a bit. If you use
fixed bias, this is valid.

Tim

--
"I've got more trophies than Wayne Gretsky and the Pope combined!"
- Homer Simpson
Website @ http://webpages.charter.net/dawill/tmoranwms

"Tim Williams" wrote

Chris is right, but since you have the set-up handy, try varying
Va
slightly (add a few extra k to Ra) and measure again. Then
calculate
using the *differences* in Va and Ia (or Ik) between the two
measurements. That should be close to spec for the midway point
between the two Va you have used.

Unfortunately, Rk adds degeneration and increases it a bit. If
you use
fixed bias, this is valid.


Oops! Thanks. Not paying attention.

Needs a battery and a pot for bias. Then gm and mu can be measured
too, while he's at it.

cheers, Ian

http://www.du.edu/%7Eetuttle/electron/elecindx.htm

or smthng like that

in real life-what you can see and calculate from load lines and graphs is
good enough for making initial circuit

--
--
.................................................. ........................
Choky
Prodanovic Aleksandar
YU

"don't use force, "don't use force,
use a larger hammer" use a larger tube
- Choky and IST"
- ZM
.................................................. ...........................
"Joe" wrote in message
...
| Hi guys,
|
| I tried to set up a small experiment to calculate the actual plate
| resistance for a 12AX7 under given conditions, and compare the value
| to what I found in the plate resistance curve. I think my
| reasoning/understanding is way off though, because my calculation
| doesn't even come close to what I see in the curves.
|
| The set up includes the following:
|
| 1) A 107VDC B+ voltage through a 100k resistor (Ra) into the
| 12AX7 plate.
|
| 2) A 1.5k resistor (Rk) ties the cathode to ground.
|
| 3) A 1Meg resistor ties the grid to ground.
|
| (resistance values are +- 5%)
|
|
| These are the measurements I took:
|
| 1) Voltage drop across Ra = 39V
|
| 2) Voltage drop across Rk = .547V
|
| 3) current (measured between cathode and Rk) = .00037A
|
|
|
| Based on these measurements I calculated what I thought was the plate
| resistance as follows:
|
| 1) Voltage drop between plate and cathode = (107-39-.547) = 67.453V
|
| 2) Plate resistance (Rp) = (67.453V / .00037A) = 182305 ohms
|
|
|
| However, when looking at the plate resistance curve
| (http://www.mif.pg.gda.pl/homepages/f...93/1/12AX7.pdf)
| (PAGE 4), it appears that the resistance should be about 120k when the
| plate current is .37mA. I'm off by 60k!!
|
|
| I've obviously got something wrong either in my set up or my
| understanding of what the plate resistance is - or both. Any
| suggestions or clarifications are most appreciated.
|
| Thanks,
| Joe

Thanks a lot for the great explanations! I think I got it.

Measuring the change in current against the change in voltage I got
this:

72.4VDC - 71.4VDC = 1V

.000406mA - .000396mA = .000010mA

1V / .00001mA = 100000 ohms

That's pretty close to the 125K on the curve, and since my meter could
be off by a micro-amp or two, this seems close enough.


I'm not quite sure what I was calculating before, if anything of
significance, but I know now it wasn't plate resistance.


What is the plate resistance figure used for ?




On Tue, 01 Feb 2005 04:28:47 GMT, Joe wrote:

Hi guys,

I tried to set up a small experiment to calculate the actual plate
resistance for a 12AX7 under given conditions, and compare the value
to what I found in the plate resistance curve. I think my
reasoning/understanding is way off though, because my calculation
doesn't even come close to what I see in the curves.

The set up includes the following:

1) A 107VDC B+ voltage through a 100k resistor (Ra) into the
12AX7 plate.

2) A 1.5k resistor (Rk) ties the cathode to ground.

3) A 1Meg resistor ties the grid to ground.

(resistance values are +- 5%)


These are the measurements I took:

1) Voltage drop across Ra = 39V

2) Voltage drop across Rk = .547V

3) current (measured between cathode and Rk) = .00037A



Based on these measurements I calculated what I thought was the plate
resistance as follows:

1) Voltage drop between plate and cathode = (107-39-.547) = 67.453V

2) Plate resistance (Rp) = (67.453V / .00037A) = 182305 ohms



However, when looking at the plate resistance curve
(http://www.mif.pg.gda.pl/homepages/f...93/1/12AX7.pdf)
(PAGE 4), it appears that the resistance should be about 120k when the
plate current is .37mA. I'm off by 60k!!


I've obviously got something wrong either in my set up or my
understanding of what the plate resistance is - or both. Any
suggestions or clarifications are most appreciated.

Thanks,
Joe

"Ian Iveson" wrote in message
k...
"Tim Williams" wrote

Chris is right, but since you have the set-up handy, try varying
Va
slightly (add a few extra k to Ra) and measure again. Then
calculate
using the *differences* in Va and Ia (or Ik) between the two
measurements. That should be close to spec for the midway point
between the two Va you have used.

Unfortunately, Rk adds degeneration and increases it a bit. If
you use
fixed bias, this is valid.


Oops! Thanks. Not paying attention.

Needs a battery and a pot for bias. Then gm and mu can be measured
too, while he's at it.

cheers, Ian

I think it may be the end of the world. Iverson stated that someone was
"right." Then to top it, he admitted a mistake! There is hope.

west

"west" wrote

I think it may be the end of the world. Iverson stated that
someone was
"right." Then to top it, he admitted a mistake! There is hope.

I would be happier, Mr Werst, if you could avoid extending your
sycophancy in my direction.

I was especially impressed by Tim's use of the word "degeneration".
It's a nice distinction.

cheers, Ian