Sag resistor
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 25th June 2013, 05:28 PM #1 fabioab   diyAudio Member   Join Date: Apr 2010 Location: Limeira, São Paulo, Brazil Sag resistor Hello, I need some hint! I wired a sag 180_OHMS resistor and te voltare dropped 20V between the poles of the resistor. This is the exact value I wanted. It's installed in my plexi 100W. The resistor is 100W (yes 100Watt) and installed after the diodes. At the other side of the resistor is wired to the first capacitor , i.e., PT--->Diodes--->resistor--->rectification_capacitor (50uF 500V). Ok, [(20V)^2] / 180_ohms = 2,3W. After play 5 minutes the temperature of the resistor goes up extremely high. I think 100W is sufficient to survive the variations of DDP caused by input frequency of the guitar. Someone could explain waht are happening? Best regards and thanks FAB
 25th June 2013, 05:56 PM #2 GoatGuy   diyAudio Member     Join Date: Dec 2012 Location: SF Bay Area P = E * E / R P = 20 * 20 / 180 P = 400 / 180; P = 2 watts A 100 watt rated resistor shouldn't be getting even barely warm to the touch, Fabio. You've got something else amiss... it could be * the "20 volts" you're measuring is just an average delivered by your multimeter - but an oscilloscope trace across the same resistor might show substantially higher high frequency voltage. -or- * when you're playing your guitar, the voltage rises substantially - because the output to the speaker is taking substantial power to deliver. This wouldn't surprise me. With the above calculations: P = E * E / R P = 60 * 60 / 180 P = 3600 / 180 P = 20 watts Again ... the resistor shouldn't be getting "terribly hot" - if it is 100 watt rated. Nice and warm, but not burning hot. * You could have mistaken the resistance (or mis-stated it originally). Maybe take a measurement of the resistor's actual in-circuit value? If it is substantially LOWER than 180 ohms, of course the heat dissipation (for the respective runtime levels) would increase by the same proportion. Y'all have a circuit diagram of where you actually inserted the resistor? ... and perhaps a rating of nominal output power of the amp? GoatGuy
 25th June 2013, 06:25 PM #3 fabioab   diyAudio Member   Join Date: Apr 2010 Location: Limeira, São Paulo, Brazil Thanks GoatGuy I think the same you, and did some measures again The resistor is 180_Ohms, 20V across the resistor and very hot! It happened with others e.g. 30Watt. I really don't know. The resistor comes direct after the rectification (after the diodes) in the power section. Without the resitor the ampli works well but , I think the plate voltage is so much to the El34 : 480V when biased with 30mA across the tube. I prefer around 455-460V. This is the reason to use sag. Thanks again
 25th June 2013, 06:32 PM #4 fabioab   diyAudio Member   Join Date: Apr 2010 Location: Limeira, São Paulo, Brazil I will try a core of 10 resistors (5W) 1,8Kohms in paralel
 25th June 2013, 06:40 PM #5 funk1980   diyAudio Member     Join Date: Sep 2011 Location: Groningen Just a word of caution, the 100W resistor (probably an extruded aluminium type housing) is only capable of 100W when attached to a suitable heatsink. Otherwise, the power rating needs to be lowered substantially.
fabioab
diyAudio Member

Join Date: Apr 2010
Location: Limeira, São Paulo, Brazil
Quote:
 Originally Posted by funk1980 Just a word of caution, the 100W resistor (probably an extruded aluminium type housing) is only capable of 100W when attached to a suitable heatsink. Otherwise, the power rating needs to be lowered substantially.
This is the resistor! So, what would be the real wattage in air?
FAB

 25th June 2013, 06:53 PM #7 fabioab   diyAudio Member   Join Date: Apr 2010 Location: Limeira, São Paulo, Brazil So, 10 resistors of 5W 1,8K wold work?!
 25th June 2013, 07:44 PM #8 GoatGuy   diyAudio Member     Join Date: Dec 2012 Location: SF Bay Area I think funk1980 got it. No heat sink on the resistor. By itself, it probably only has a rating of 10 to 15 watts with no-heat-sink air cooling. The 10 ea / 1.8K-in-parallel resistors will work fine. Personally, i wouldn't worry about the over-voltage, as long as your tubes aren't arcing or exhibiting red-glowing anodes. They're tough, power tubes are. I think you'd get more traction by simply adjusting the BIAS of the tubes if you're concerned for the total dissipation. GoatGuy
 25th June 2013, 08:23 PM #9 llwhtt   diyAudio Member   Join Date: Jun 2008 Location: SoCal A 100W Marshall pulls about 500ma (1/2 A) of B+ at 100 Watts output, so there will about 45 Watts of power dissipated by the resistor. If you are playing a Plexi properly you SHOULD be well into clipping and therefore even more Watts. Craig
 25th June 2013, 08:47 PM #10 fabioab   diyAudio Member   Join Date: Apr 2010 Location: Limeira, São Paulo, Brazil Thanks all, As first approximation I tried 12 resistors (2w) 2.2k_ohms in parallel resulting in 0,2W dissipation per resistor. The max power of the core is around 21.8W. Playng, the resistor work very hot. If I try Using 12 of 5W it will support maximum 58W. It's almost fine to plaxi! It explain why my core of 12 x 2W is working very hot, and will not support long time!

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