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Old 15th March 2009, 01:17 AM   #1
jetbat is offline jetbat  United States
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Default Resistors overheating.

Hi, I'm relatively new at electronics so this may be a simple thing I have overlooked.

I did'nt know if I should put this in Tubes or Solid State since it is a hybrid amp. I picked SS since the power supply has a diode rectifier.

I'm having a problem with some voltage dropping resistors overheating. I have power transformer that I need to lower the voltage from 500V to 300V. It's a hybrid guitar amp that I am switching from 6L6GC tubes to 6V6GT and the plates voltage needs to be lower. Using power resistors to lower it and I calculated what I should need to achieve this. The current is 104mA, voltage drop is 200V, which equals 1923 ohms and 20.8 watts resistor. I had some Mills MRA 12 watt wirewound resistors and thought I would run some in series to divide the heat dissipation among them. Since the values of resistors I had were kind of small, I ended up with 6 in series (mind you that this was a test to find the proper values before I order larger power resistors) with a total of 1850 ohms. I figured it would at least run somewhat cool since each resistor is only dropping the voltage a little. The largest is 510 ohms, thats 53 volts dropped at 5.5 watts which should be nothing for the Mills to handle. But when I turn on the amp, after about 15 seconds you can feel hot air rising from the resistors and around a minute you can see smoke just starting. I''ve been turning the amp on, get some readings on the meter, and shut it down to cool off. Everything seems to be working as it should except for the heat. Here is the reading I got with 1850 ohms with 104mA. For the voltage drop the meter reads 195V which is good. The watts should be at 20, but they must be higher.

With the other tubes in there, since there were 4, 6L6GC, the max current for the amp then would have been around 500mA. I don't know if the fact that the transformer is larger then I need has anything to do with this. There are now 2, 6V6GT tubes in there and there is a new output transformer that is rated for those tubes.

Any help would be appreciated.
Thanks.
Scott
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Old 15th March 2009, 01:32 AM   #2
SA-12 is offline SA-12  Canada
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http://www.mcmelectronics.com/product/50WM220

should work fine and solve heat issue
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Old 15th March 2009, 01:26 PM   #3
djk is offline djk
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Even the 50W resistor will nead a heatsink.

Wirewound power resistors are usually specified to have a deltaT of 250C over ambient. In other words, they will be well over 500F at full power.

A 50W resistor run at 20W will be about 40% of this, still over the boiling point of water (unless you add a heatsink).

You may want to use a plate choke if the supply doesn't have one.
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Old 15th March 2009, 05:26 PM   #4
SA-12 is offline SA-12  Canada
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this is tru, this resistor has mounting flange so adding extra heatsink should be easy. Good advise thanks.
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Old 15th March 2009, 10:40 PM   #5
jetbat is offline jetbat  United States
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Thanks for the replies.

I was planning on getting a couple of those 50 watt power resistors after I tested the amount of resistance I needed. Now that I know I would need a heat sink too, I looked for those and found some that would work perfectly.

As for a choke, I do not have one in the power supply. I looked around for some info and it seemed like they would not lower the voltage very much at all. Then I found a post that said placing the choke directly after the rectifier and before the first filter capacitor would lower the voltage a lot and give better voltage regulation.

It said a 300V transformer and a full-wave rectifier would give you 424V. And a choke after the rectifier would give you 270V! I had wondered why my 375VAC through the rectifier ended up being 500V. I had found out why that was happening and thought it was something I had to live with. Now I am going to get a choke and install it. Thanks for pointing me in the right direction. From what they said, I should end up with around 350VDC at the first filter capacitor. That is much better to work with then 500V.

Here is the source. http://www.aikenamps.com/Chokes.html
I still don't know exactly why it does what it does, but I'm glad it does.

Also I would have just bought a new power transformer, but since it is a hybrid amp, no one has what I would need stock and I would have to get one custom wound.

Thanks again.
Scott
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Old 16th March 2009, 12:26 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
the power resistor/s will be at B+. Any heatsink attached to them will also be very close to B+. That is not nice.

Buy a lower voltage transformer to suit your replacement valves/tubes.
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Old 16th March 2009, 12:45 PM   #7
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just because a resistor has a high wattage rating, doesn't mean it will run cool. 20W is a lot of heat to dissipate. i rin a 500W dummy load with some of the bigger amps, most at 250W. the dummy load resisters still get hot enough to turn any oils on them (such as oils from your hands) into smoke. heat sinking will help lower the temperature.
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Old 16th March 2009, 12:47 PM   #8
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yeah, what he said, or see if there's a lower voltage tap on the transformer you have
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Old 16th March 2009, 12:50 PM   #9
djk is offline djk
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Two words: plate choke.
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Old 17th March 2009, 11:21 PM   #10
jetbat is offline jetbat  United States
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For the plate choke, do you have any suggestions on brand/type, or links to information I can read to find out more? When I google I get lots of high frequency chokes which I don't think will work.
Thanks
Scott
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