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Old 13th April 2012, 04:37 AM   #1
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Default SSE possible transformer problem.

So my Edkor package arrived today. I threw caution to wind and hooked up everything. I gave it a quick try, with no input, and I had no smoke or apparent problems. Next I hooked up some speakers and input source, sound came out, but it was very soft and heavily distorted. After about a minute I smelled something getting hot and then immediately pulled the plug. I checked around the board and determined that the bias resistors were starting to get hot. So I went though all the wiring again and could not see a posible problem. So I decided to go back to sqaure one, where I should have started to begin with. I pulled all the tubes and measured my voltages coming out of the transformer. I got the following values.

669 AC, 334 at the tap, on a transformer that's supposed to be 600 VCT
7.25 AC on the filament.
6.13 AC on the rectifier.

I check this with both a newer digital meter, and my 20+ year old RAT Shack analog VOM and got the same results. Since I'm somewhat new to tubes, I'm wondering are these values right, and I'm just measuring peak values instead of RMS, hence I did something wrong in my soldering, or did I posibly get a transformer rated for a lower input voltage. This is the transformer I was supposed too get. https://www.edcorusa.com/p/791/xpwr172_120-220

Any help or ideas are much welcomed.
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Old 13th April 2012, 05:06 AM   #2
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Laid in bed, and suddenly had brain flash. I wired the preamp wrong. So I got up a quickly tried it again. This time sound coming out fine, amp works. Bais resistors are still getting very hot though. Now wondering if Mouser really sent me 5 watt resistors. Going back to bed, will look at it in the morning.
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Old 13th April 2012, 01:18 PM   #3
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Quote:
669 AC, 334 at the tap, on a transformer that's supposed to be 600 VCT
Over voltage transformers are pretty common today. Your numbers were measured with no load, which will give high readings.

Quote:
Bais resistors are still getting very hot though.
I am assuming that you are referring to the cathode resistors. If they are white ceramic, thats the right ones. They will get quite hot, too hot to leave your finger on for more than a few seconds.

Measure the voltage across each one and tell us what you get. What output tubes are you using.
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Old 13th April 2012, 03:20 PM   #4
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Tube are 6n3c, voltage across resistor is 27.8 and 30 on the other. The resitors I got from Mouser were not the square ones. They look like normal resistors but they are wirewound and suposed to be rated for 5 watts. Should I replace them. I'd post pictures but they method of doing so without a link is alluding me at the moment. I have used the amp for a few hours this morning everything is working fine, sounds fantastic. They only thing that somewhat worries me is the heat of the resistors.

Thanks James,
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Old 13th April 2012, 03:24 PM   #5
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Oh, I should add the resistors I used are 430 ohms, 5 watt, 5% tolerance.
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Old 13th April 2012, 03:41 PM   #6
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I'll ad one more thing, I measured the voltage across my filter capacitors and got 378. So if I understand this right this is my B+?.
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Old 13th April 2012, 04:02 PM   #7
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There will be slightly more voltage on C1 than C2. The voltage on C2 is your B+.


Quote:
I used are 430 ohms, 5 watt, 5% tolerance.
30 volts across 430 ohms is 69.7 mA. This gives 2.1 watts of heat in the resistor which will make it quite hot, but shouldn't fry a 5 watt resistor. Post the Mouser part number so we can check the specs. (look at max temp, it's probably pretty high like 175C or even 225C)

The tubes will see the B+ voltage minus the cathode voltage (minus the loss in the OPT). Neglecting the OPT loss and assuming your B+ is 378 volts the hottest tube is dissipating about 24 watts. ( 378 - 30 * .0697 = 24.26 W)

I have never used the 6n3c, so I don't know if this is OK. Look at the tubes in a dark room. The plate (outer metal element) should not have any orange glow on any surface. If it is glowing, you need to use a larger cathode resistor (470 or 510 ohms). If it's not, I think your amp is OK.
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Old 14th April 2012, 03:10 AM   #8
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I did measure the 378 voltage across C2. The tube do have some orange glow to them. I found the data sheet for the resistor, but the graph for the temp confused me. http://www.vishay.com/docs/28730/acseries.pdf

I do have some 40 ohm 5 watt resistor lying around, I guess I could wire them in series with the 430's and see if the glow goes away.

Here's some pics,
Attached Images
File Type: jpg 1.jpg (204.2 KB, 168 views)
File Type: jpg 2.jpg (289.2 KB, 162 views)
File Type: jpg 100_0586.jpg (76.0 KB, 161 views)
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Old 14th April 2012, 03:24 AM   #9
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One other thing, I do have a 5U4 GB sitting around I was going to use till I got the 5ar4, could I use that to drop my B+ till I got a proper resistor. I seem to remember reading that it would drop B+. I realize it gives the output tubes B+ faster but would it be okay as a temporary solution.
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Old 14th April 2012, 04:00 AM   #10
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The only graphs for the resistor that are useful are on page 7 the other stuff is for pulse duty. You are feeding a 5 watt resistor with 2.25 watts. That is about 40% of its maximum rated power. The first graph says that at 40% the resistor could be operated at about 175C without issue. The second graph says that a 5 watt resistor (AC05) will raise its temperature by about 130C when fed with 2.25 watts.

These little resistors are going to get hot compared to a bigger physical size resistor, but they are designed to handle the heat. I would raise it as high off the PC board as you can with the leads it has left. The high heat can discolor the board over time and the more air around the resistor the better it can cool off.

If you have to change resistors, get the white or tan cement ones. The guts are the same (nichrome wire wrapped around a fiberglass rod) and they don'e get as hot. I have hammered the cheap Xicons with 10 watts and only blown 1 in 10 years. It ate about 50 watts when a tube shorted out. The cathode cap can blow up when the resistor dies too.
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