Power supply resistors power ratings

[Nihilist]

After looking at some RCRC networks, I wonder how the resistors used don't burn up. For example :
http://diyaudioprojects.com/Tubes/KT...Pull-Tube-Amp/

Scroll down and look at the PS resistor , a 100 ohm 5W unit.

How is this NOT burning to a crisp ? It is providing ALL the power for a pair of Class A KT88's ! Thats gotta be close to 50W combined on the plates .

Can someone enlighten me , PLEASE ?



As long as we're looking at this schematic, what is the benefit of loading the V2 tubes' screen with a resistor rather than directly tying it to ground as per the Melvin Lebowitz original design ?


Thank you................................Blake

[leadbelly]

P=I^2 * R
For 100 mA, R=100, P=1W
For 200 mA, R=100, P=4W

The screen of V2 is not tied to ground, the grid is. The tube is drawn upside down.

[Nihilist]

"The screen of V2 is not tied to ground, the grid is."

My fault. Any idea why he used a resistor inline there ?

"P=I^2 * R
For 100 mA, R=100, P=1W
For 200 mA, R=100, P=4W"


Never saw that formula before.


Sweet shortcut !



Thanks...................Blake

[leadbelly]

The design uses grid stopper resistors, while the one in the original article does not.

[Nihilist]

"The design uses grid stopper resistors, while the one in the original article does not. "

Yeah, I noticed that it is different than the original. I was wondering WHY he did it.

I thought a grid stopper went from the grid to ground , not in series with the grid.


..................Blake

[chrish]

Originally posted by Nihilist
"The screen of V2 is not tied to ground, the grid is."

My fault. Any idea why he used a resistor inline there ?

"P=I^2 * R
For 100 mA, R=100, P=1W
For 200 mA, R=100, P=4W"


Never saw that formula before.


Sweet shortcut !



Thanks...................Blake

Yeah, it is that esoteric, rarely used, difficult to find thing called Ohm's Law

[Nihilist]

Nobody likes a smarta** .


P=I^2 *R is NOT Ohms Law. It's not even Thevenin's Power Law.

Ohms law is V/I=R or I * R = V or V/R=I .

Amps squared , multiplied by Resistance equals Power is not taught in ANY school or books that I know of.

How did you come to know it ?


Any takers on the Grid Stopper question? A series resistance inline with the grid constitutes a grid stopper ? As I already said, I thought a parallel resistance from the grid to ground was considered a grid stopper. Can anyone clarify ?


.................Blake

[Gordy]

Originally posted by Nihilist

Any takers on the Grid Stopper question? A series resistance inline with the grid constitutes a grid stopper ? As I already said, I thought a parallel resistance from the grid to ground was considered a grid stopper. Can anyone clarify ?

A series resistance in line with the grid forms a low pass (i.e. high cut) filter with the internal capacitance of the tube, and hence helps 'stop' it oscillating at high frequency.

Going back to the resistor / power question; remember that it is the power dissipated in the component / device that is to be considered and it is not going to be the same as that dissipated in the target load.

Power = Current x Voltage
Power = Voltage(squared) / Resistance
Power = Current(squared) x Resistance

The amount of power a component can take varies with ambient temperature (and sometimes with physical orientation).
Always choose a component that can handle higher power than the calculated figure. For example if you calculate a dissipation of 0.8W in a resistor it would be wise to choose a 2W resistor.

[Robert McLean]

A series resistance inline with the grid constitutes a grid stopper

Yes, used to stop oscillations. Hence the name stopper.

I thought a parallel resistance from the grid to ground

that is just a grid resistor, sometimes called a grid-leak resistor ( although I believe that term is not really applicable unless it is being used to provide bias, I may be nitpicking. )
used to drain away charge from the grid.

Amps squared , multiplied by Resistance equals Power is not taught in ANY school or books that I know of.

You can't be serious. I can't think of any elementary book that doesn't give it, and any advanced book doesn't have to.

How did you come to know it ?

It is simple algebra. If power is voltage times current, and if voltage is current times resistance, then power must be current times current times resistance, current squared times resistance

P = I * V
V = I * R
P = I * I * R

[Nihilist]

Thanks for the info. I had "Basic Electricity" (a 1 hour a day course) and "Electronics" ( a 2 hour a day course) in high school , and the variants of the power law using the square of voltage or current were never taught.

I never thought my teacher really knew what he was doing.

That was some time ago, about 15 years, but seriously this is the first time I have seen it used this way.

Thank you.


Here's a schematic of my current setup. Do I need stoppers or "leak" resistors ?

I plan on using CCS later.


http://img353.imageshack.us/img353/1...rrentsetup.png


.......................................Blake