Resistor power ratings in valve amps

[Impossible]

Hi all,

I've been researching modifications for my Epiphone Valve Junior for a while and am ready to dive in and start soldering. I'm handy with circuit boards and components but my question is this:

What rating resistors should I be putting in a valve amp? What is easily available to me is 1/4W resistors but when other guys have specified what they use, it's usually 1/2W. Is there some way for me to calculate the required rating?

Thanks for your help!

[Jazz P Bass]

If the amp circuit initially had 1/4 watt resistors, then that is what you may use.
Upping resistors on a PC board can be problematic.
1/2 watt resistors are longer, wider & the leads may be a larger diameter (which may be a problem if the board hole was spec'd for a 1/4 watt).
Point to point is generally not a problem.
The wattage rating can be calculated by Ohm's Law.
Power (in watts) = voltage squared divided by resistance.

[HTH]

maths isn't my strong point (admittedly), but I thought the ohms law power equation was...

Power (watts) = Current (squared) / Resistance

EDIT, actually its...

Power (watts) = Current (squared) x Resistance

right?

urgh, talk about being more of a hindrance than a help, ha ha

[defaced]

From Ohm's Law: V = I * R
and from the power equation: P = I * V
You can derive any number of ways to express the four variables in terms of each other (P, V, I, R).

HTH, it's P = I^2 * R

[Jazz P Bass]

Ohm's Law Circle

Here is the Ohm's Law circle.
This should help in keeping it straight.

[pdf64]

Remember the voltage rating also. Many resistor types are only rated up to 250V or even less. My understanding is that means the total instantaneous voltage, so with say a common cathode 12AX7 on a 300V supply, its plate resistor may have more than 250V across it on the troughs of large signals.
So it's best to use resistors rated close to the amp's V B+, to avoid worrying over the voltage rating of each one.
Pete.

[tubeswell]

maths isn't my strong point (admittedly), but I thought the ohms law power equation was...

Power (watts) = Current (squared) / Resistance

EDIT, actually its...

Power (watts) = Current (squared) x Resistance

right?

urgh, talk about being more of a hindrance than a help, ha ha

I'm so bad a maths that I use the E I R triangle to help me remember Ohm's Law 8-/

With a bit of squinting you can see how it works i.e.: E = I.R or E/I = R or E/R= I.

The formula for power is Watts = Volts x Amps, and since amps is Volts/Resistance, then you can substitute the 'amps' part of the equation thus: Watts = Volts x (Volts/Resistance), which can just as easily be written as: Watts= Volts x Volts / Resistance or P = V^2/R. There you go

[Gtr_tech]

Half watt for cathodes is fine. As long as yer "in there" I would use 1w for plate loads. Cheap insurance. Anything replaced in the pwr supply must be at least the pwr rating of teh original part. More is OK, as long as it fits physically.

[deci belle]

The formula for power is Watts = Volts x Amps, and since amps is Volts/Resistance, then you can substitute the 'amps' part of the equation thus: Watts = Volts x (Volts/Resistance), which can just as easily be written as: Watts= Volts x Volts / Resistance or P = V^2/R. There you go

What does the symbol (^), stand for, tubewell?

[Jazz P Bass]

To the power of two.
Voltage squared.

[deci belle]

Oh! Thank you, Jazz!!

Now I see that he just wrote out what he said in the equation in two ways.

[jrfrond]

The resistor type makes a difference as well. Carbon comp resistors, besides sounding "right" to my ears, take more punishment from transients that will often pop open carbon or metal film resistors, so 1/2W for plate resistors is just fine, otherwise, 1W should be what you use with carbon film.

[greekie]

Remember the voltage rating also. Many resistor types are only rated up to 250V or even less. My understanding is that means the total instantaneous voltage, so with say a common cathode 12AX7 on a 300V supply, its plate resistor may have more than 250V across it on the troughs of large signals.
So it's best to use resistors rated close to the amp's V B+, to avoid worrying over the voltage rating of each one.
Pete.

I agree that overspec'ing is a good thing to do most times for reliability. But the resistor does not care what the voltage is in any other place of the amp than the one it's in. It's the total voltage we're seeing over the resistor that the rating applies to - not to a potential some other place in the amp.

E.g. if the B+ for that 12AX7 is 300V and the resistor is dropping 150V, we've still got 100V on tap.