resistor wattage ?

[worth]

I'm using a 175 ohm 10W resistor to drop 30v from the B+. I'd like to raise the wattage rating.. can  I do that by using two resistors in series or parallel ? Two 350 ohm in parallel , or two 90 ohm in series ? Also , what is a good method of using the chassis as a heatsink to dissipate the heat from this sag resistor ?

[sluckey]

I'm using a 175 ohm 10W resistor to drop 30v from the B+. I'd like to raise the wattage rating.. can  I do that by using two resistors in series or parallel ? Two 350 ohm in parallel , or two 90 ohm in series ?

Yes. However, a 175 ohm resistor that is dropping 30V is only dissipating 5.14 watts. A 10 watter should be fine. Just give it plenty of space (air).

Also , what is a good method of using the chassis as a heatsink to dissipate the heat from this sag resistor ?

Most of the resistors we deal with are air cooled. To use the chassis as a heat sink requires using a style resistor meant for that purpose. Look here for an example...

http://www.vishay.com/docs/30201/30201.pdf

[Willabe]

To add to sluckeys words, part of "space" is/can be to lift the R up off the board into the air so air can flow _all_ the way around the R.  

Remember heat rises, so take into consideration how the amps mounted, ie, combo or head style.


                  
                           Brad      

[stratele52]

I'm using a 175 ohm 10W resistor to drop 30v from the B+. I'd like to raise the wattage rating.. can  I do that by using two resistors in series or parallel ? Two 350 ohm in parallel , or two 90 ohm in series ? Also , what is a good method of using the chassis as a heatsink to dissipate the heat from this sag resistor ?

Using a resistor to drop the B + is not good , you'll have too much sag from the amp .The B+ will drop at higher volume .You are better to use some 10 volts zener diodes like 1N5347B , less than $1 us each. You probably need 4 diodes.

[worth]

This is the amp that I want to drop the B+ on... where would be the best place to install the Zeners , between the SS rectifier and the B+ ?

[John]

http://www.el34world.com/Forum/index.php?topic=11506.0

There is a great topic on dropping voltage using reverse zeners. You actually hook them to the CT of the PT in reverse, thereby lowering the "zero" voltage to -30v or whatever you want it to be. Reply #34 has a nice picture by Geezer. The zeners do get hot though, I believe that thread also has good heat sink info. Hope this helps!

[sluckey]

You actually hook them to the CT

Not on this amp. Just put the zener whereever you had the resistor.

[John]

Oops, sorry. Didn't look at the schem.

[stratele52]

The zener must be connected just after the rectifier and the other end ( of the zener ) to the ground. Low voltage 5 watts diode don't need heatsink.

Like I write before , I use 1N5347B

[sluckey]

The zener must be connected just after the rectifier and the other end ( of the zener ) to the ground.

Show me a schematic.

[stratele52]

For a schematic using zener diodes, look there ;

http://www.blueguitar.org/new/schem/trainwreck/wreckxpr.pdf

Look at this version on the middle of all these pages;
 
The Blues Express
Version A5h

[sluckey]

The zener must be connected just after the rectifier and the other end ( of the zener ) to the ground. Low voltage 5 watts diode don't need heatsink.

Specifically, connect zener between the negative side of your bridge and ground. Or put it in the same place as your resistor.

[stratele52]

The zener must be connected just after the rectifier and the other end ( of the zener ) to the ground. Low voltage 5 watts diode don't need heatsink.

Specifically, connect zener between the negative side of your bridge and ground. Or put it in the same place as your resistor.

You are right , I made a mistake when I write this.  The Trainwreck schematic show the right way and excuse my error ?