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Old 29th January 2009, 01:24 PM   #1
hunim is offline hunim  South Korea
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Default [Question] Ground Error?

Hi~

I have a question.

When I built my very first tube amplifier(EL34SE), I had a bad experience with a severe arcing in the rectifier tube 5AR4(By Sovtek). When I install two EL34's, the arcing takes place. When I removed one EL34, the amplifier was working okay(In other words, Manaural)

I thought the problem was caused by a bad rectifier tube and replaced with a JJ GZ34. As soon as I installed this rectifier, I have enjoyed a firework in the tube and the tube was dead and had broken fuse.

After investigating the circuit, the grounds from the amplification stage were connected directly to the "0V" tab of the transformer (Before the choke coil) bypassing the grounds of the capacitors after the choke coil.

When I corrected the error, the problem with the arcing was completely gone and I found that the Sovtek 5AR4 was in takt after severe electrical shocks.

Now a I am using the amplifier without any problem. But I would like to know what was the reason why I had arcing in the rectifier tube.

In addition to the arcing, the output voltage from the rectifier was ~20% lower that the nominal output voltage shown in the schematic diagram.

Thank you in advance for your replies.

Regards,

Hun
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Old 29th January 2009, 03:32 PM   #2
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I'm no expert, but I'll take a crack at your question.

The grounding of your 2 schematics looks identical, I don't see how that would make any difference.

I crudely modeled your PS in PSUDII and it appears that you may be on the ragged edge of the max forward current that the 5AR4 can handle with a CLC of 10u, 5H, and 230u. What's the voltage of the transformer secondary? What's the DC resistance of the 5H choke?

Pulling one output tube may have eased the load on the rectifier enough to not arc over.

I would suggest that you download PSUDII and model your PS as accurately as you can. It will let you know if your forward current is too high for a given CLC and load.
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Old 29th January 2009, 04:04 PM   #3
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Reducing the value of the first cap (the one before the 5H choke) will reduce the forward current and the B+ voltage. In my model, changing the first cap from a 10uf to a 5uf eliminated the "forward current exceeded" warning, and reduced the B+ voltage by about 10V. YMMV since my model is crude.
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Old 30th January 2009, 12:25 PM   #4
hunim is offline hunim  South Korea
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Default Thank you for your kind explanation

Dear Boywonder,

Thank you for your help.

When I checked the specification of the choke coil (5H/250mA);
the DC resistance of the coil is 47Ohm.

The AC voltage fed into each plate of 5AR4 is 280Volts. The voltage right after the rectifier is 324 Volts.
B1+ is 317Volts and B2+ is 314volts.

It is really strange. Whenever I connect the ground of the Power Tube to the center tap between the two 280Volts, I have the unstability. As soon as I connect the ground to the ground of the last electrolytic capacitor, everything is okay.


Thank you again and have a nice weekend.

Best regards,

Hun
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Old 30th January 2009, 06:18 PM   #5
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Looks like the problem is due to stray inductance in the DC return. Your modified connection avoids that. When constructing PS's, I try to connect everything to one point as close to the DC return point as possible, and so far, I haven't had a problem like that.

Stray impedances in the DC return are often overlooked, but can cause all sorts of nastiness, such as power supply hum issues. Even a few ohms of stray impedance can translate into some big voltages at the end of the signal chain. Bad news indeed.
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Old 31st January 2009, 11:53 PM   #6
hunim is offline hunim  South Korea
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Dear Miles,

Thank you indeed.

Your expalanation is clear and easily understandable.

This problem happended when I built the amplifer.
After fixing the problem, the amplifier works fine now.


Best regards,

Hun
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Old 14th March 2009, 02:38 PM   #7
hunim is offline hunim  South Korea
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I have found the cause and fixed the problem ultimately.

When I built the amplifier, I soldered a 1K Ohm resistor on the printed circuit board on which electrolytic caps are mounted.

I soldered a filter cap and a resistor from the back side of the
circuit board and a lead of the resistor caused a short circuit
on to a negative elctrode of one of the capacitors.
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