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Safer soft start for your big amp.

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  #1  
Old 09-29-2010, 08:44 AM
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Default Safer soft start for your big amp.


On larger plate supplies the soft start circuit used can make a big difference in the level of protection offered. While there are countless ways to design a soft start circuit, I see them all falling into two basic categories. Time based and voltage based to control the delay between soft start and full on. The vast majority are time based. Instantly powering the primary trough a wire wound resistor usually around 100 ohms. It then shorts the resistor through a second contactor after some pre-determined amount of time to apply full power.

Here is a great opportunity to add protection beyond just the rectifiers and filter caps. This is especially useful when building a new amplifier or testing a tube of unknown condition. On the typical soft start if we have a short anywhere on the HV line, it's still going to apply full power after about a second. If we trigger the contactor that shorts the soft start resistor from the voltage across the primary of the plate transformer, we added more protection.

Since the voltage across the primary is being fed through the 100 ohm resistor, it will take time for the primary to reach 220 volts as the secondary charges the filters through the rectifiers. You can use a small relay with a 220 volt coil connected the primary to fire a larger mercury contactor that shorts the soft start. By adding a resistor to the relay coil or tightening the tension on the spring of the 220 volt relay you can add more delay if needed.

Now if there is a short on the HV line on power up, the primary voltage will be forced across the 100 ohm soft start resistor. This will hold the primary voltage low, keeping the 220 volt trigger relay de-energized and prevent the full power from ever being applied into the short. Adding an appropriate slow blow fuse in series with the 100 ohm soft start resistor will remove all power and save the resistor.

With the voltage triggered soft start you can have a shorted tube installed on power up and still protect rectifiers, filter caps, current meters, glitch resistor, meter shunts, and the plate choke just to name a few. The standard soft start would fail to protect all of these components in a case like this. Something to seriously consider if your plate supply is big enough to cause this damage.


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Old 09-29-2010, 04:50 PM
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Hey thanks shockwave, this is a really good idea.

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Old 05-19-2013, 10:08 AM
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Default soft start

Actually this method is commonly used in the larger ameritron amplifiers. Like the al-82/al-1200 and the al-1500. The circuit is published on w8ji's website. A 100 ohm resistor really isn't appropriate since if the b+ has to much of a load on it from the bleeder resistors it will cause to much of a voltage drop on the switched side of the primary.Which can cause the relay which is paralleled with the plate transformer primary not to engag or a delayed engagement.

For a transformer with a 240 primary a 20 ohm resistor is more appropriate. Its also wise to put a small slow blow fuse in series with the resistor a 3 amp fuse will be more then adequate. If you are loading the b+ down with bleeder resistors by themselves or with a resonant or swinging choke setup. Then a time delayed circuit is more appropriate Since a heavy load on the b+ will cause excessive voltage drop through the resistor. Which will prevent the relay with the 240 coil from engaging . Manipulating the relays spring does not work. Every open frame relay with a 240 coil I have tried closes around 140 volts. If you lighten up on the spring tension the relay will chatter before it engages and will not fully engage any quicker.

Last edited by kb1sel; 05-19-2013 at 10:33 AM.

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Old 05-19-2013, 10:38 AM
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Default another idea

Maybe rectifying the relays coil voltage would work if you had a heavy load on the b+ during startup. But then you have other issues to figure out.

Last edited by kb1sel; 05-19-2013 at 10:57 AM.

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