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Old 2nd October 2013, 01:52 PM   #71
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Originally Posted by Defiant View Post
At the voltage levels in this supply I don't see a problem, provided the amp is regularly maintained (i.e. sockets blown clean, etc). At much higher voltages, I would prefer a damper diode with a top cap for the cathode connection. This is probably the most common configuration for higher powered damper diodes. The heater pins are still at the base, well separated from the plate pin. Here, the heater can either be floated or connected to the cathode through a large value resistor. The latter is a better idea, since it avoids potential buildup between heater and cathode if a leakage path occurs between the anode and heater. Such a buildup could cause the heater insulation to fail, which would be A Bad Thing™.

Again, with the behavior of the choke input supply I find such measures unnecessary. If you feel the silicon diodes are more reliable than the tube rectifier, then why use the tube(s)? Similarly, I have found no conditions that would damage the choke. A 660V MOV across the choke will conduct every time the HV is switched on (when the capacitors are at 0V), due to the 850V potential difference across the choke at that time. This will eventually degrade and destroy the MOV. A better transient suppressor is a series RC, which can be placed across the choke, across the rectifiers, or from the choke-rectifier junction to ground.

The variable we don't know is the parallel capacitance across the choke. This capacitance actually damps ringing. If the choke has a very low parallel C, it can ring at high frequency when the load current is below critical. A snubber of, say, 10nf in series with a 20k 5 watt resistor, would keep any transient peaks at bay. A revised schematic is in the first image. The second image shows the transient. In this particular waveform I reduced the parallel C of L1 to nil, and dropped the external load to nil as well. The parasitic spikes were 1.3+KV and several KHz without the snubber. As you can see, with the snubber the transient is a much lower frequency and effectively limited to the peak AC voltage. The cost is a few cents worth of parts and about 1.5 watts of heat in the resistor. As you say, cheap protection
Thanks for taking the time to draw that schematic! What wattage are the sharing resistors across the caps? Can I safely use 500V caps in this supply? Would a thermistor slow the voltage spikes at startup if placed on the primary of the big power transformer?

Thanks again!

Blair
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Old 3rd October 2013, 06:54 AM   #72
Defiant is offline Defiant  United States
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Originally Posted by deicide67 View Post
Thanks for taking the time to draw that schematic! What wattage are the sharing resistors across the caps?
I will spare you the math and give you the answer: 2 watts minimum. That provides a very comfortable safety margin. The resistors also need to withstand the highest possible voltage (425V in this case).


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Can I safely use 500V caps in this supply?
If you run two in series, then yes. Otherwise, no. The total voltage rating needs to be 850V minimum, but if I am running 2 in series I would go with 1kv total for a safety margin.

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Would a thermistor slow the voltage spikes at startup if placed on the primary of the big power transformer?
Voltage spikes are only a real problem during certain fault conditions. Run the snubber I showed in the schematic and you will be fine.
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Old 3rd October 2013, 01:22 PM   #73
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Thanks! Are most resistors not rated at 450V? I've always used carbon resistors from the local surplus store for my projects.
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Old 3rd October 2013, 05:55 PM   #74
DF96 is offline DF96  England
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Many modern resistors are rated at 300V.
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Old 5th October 2013, 12:55 PM   #75
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Wow. I've never even looked! What happens if they are underrated? Do they break down, arc, or something worse?

Where do you source the proper voltage resistors? I looked at Mouser, but most all of them are ~350WV.

Thanks,

Blair
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Old 5th October 2013, 10:53 PM   #76
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I use the BCcomponents PRO2 range, which is a suitably small 'old looking' 2W resistor with a 500V DC or RMS continuous rating to some IEC standard. That gets me through normal electrolytic capacitor bleed applications. For personal use, I'm happy to put that resistor in somewhat higher continuous conditions, although that's pretty rare even in larger power amplifier applications.
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Old 6th October 2013, 12:15 PM   #77
hpeter is offline hpeter  Europe
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Quote:
Originally Posted by deicide67 View Post
Wow. I've never even looked! What happens if they are underrated? Do they break down, arc, or something worse?
used in ef86 circuit a 0,6W 470k carbon (not exceeding power rating, but voltage rating a bit) after few minutes it became several megohms
no magic dust or sparks released..
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Old 24th October 2013, 02:59 AM   #78
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So, I'm about 80% done with chassis work on this. If I proceed with this schematic, I should be good?

Click the image to open in full size.

I think I was a bit, "big eyed" when I looked at these power transformers in the floor. They are huge!!!

Blair
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Old 15th November 2013, 12:59 PM   #79
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Hello Eli, or anyone else that knows

I have the VPL12-4000. The schematic shows two sets of 6.3V windings. Easy enough to understand. Both sets of windings have center taps. Do I need to ground the center taps? Is the center tap where the cathodes of the 6CJ3 ties in?

Never mind. I see that grounding the center tap would be a direct short. Dumb, pre-coffee question.

Still, my question is, "Does the center tap connect to the cathodes?"

Thank you!

Thank you!

Blair

Last edited by deicide67; 15th November 2013 at 01:13 PM.
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Old 15th November 2013, 04:25 PM   #80
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Like this?

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