Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: worth on June 11, 2012, 11:41:03 am
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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 ?
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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 (http://www.vishay.com/docs/30201/30201.pdf)
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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 :icon_biggrin:
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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.
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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+ ?
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http://www.el34world.com/Forum/index.php?topic=11506.0 (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!
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You actually hook them to the CT
Not on this amp. Just put the zener whereever you had the resistor.
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Oops, sorry. Didn't look at the schem.
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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
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The zener must be connected just after the rectifier and the other end ( of the zener ) to the ground.
Show me a schematic.
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For a schematic using zener diodes, look there ;
http://www.blueguitar.org/new/schem/trainwreck/wreckxpr.pdf (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
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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.
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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 ?