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Old 25th June 2013, 06:28 PM   #1
fabioab is offline fabioab  Brazil
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Default 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
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Old 25th June 2013, 06:56 PM   #2
GoatGuy is offline GoatGuy  United States
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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
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Old 25th June 2013, 07:25 PM   #3
fabioab is offline fabioab  Brazil
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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
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Old 25th June 2013, 07:32 PM   #4
fabioab is offline fabioab  Brazil
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I will try a core of 10 resistors (5W) 1,8Kohms in paralel
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Old 25th June 2013, 07:40 PM   #5
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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.
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Old 25th June 2013, 07:52 PM   #6
fabioab is offline fabioab  Brazil
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Quote:
Originally Posted by funk1980 View Post
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
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Old 25th June 2013, 07:53 PM   #7
fabioab is offline fabioab  Brazil
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So, 10 resistors of 5W 1,8K wold work?!
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Old 25th June 2013, 08:44 PM   #8
GoatGuy is offline GoatGuy  United States
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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
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Old 25th June 2013, 09:23 PM   #9
llwhtt is offline llwhtt  United States
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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
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Old 25th June 2013, 09:47 PM   #10
fabioab is offline fabioab  Brazil
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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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