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Audio and Video Talk
Vinyl
Build Your Own Tube Amplifier - workshop in 2011
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<blockquote data-quote="leonsound" data-source="post: 104572" data-attributes="member: 1295"><p>After some discussion with Karel I added a "soft start" circuit to the amplifier. </p><p></p><p>Essentially now there is a 68k Ohm resistor (10 Watt total) between the rectifier and the capacitor bank.</p><p>In parallel with the resistor is a mains "Delayed start" relay, that shorts the 68k Ohm resistor out of the circuit after a predetermined time.</p><p>I also added a fuse in line to all this just for additional safety. </p><p></p><p>From cold start it takes about 10 seconds to gently climb to around 90Volts. Not sure why it then hovers around there (I measured with no power tubes installed, but the pre-amp tubes were in). Currently the relay is set to 15 seconds, after which the voltage jumps up to 400-and-odd Volts.</p><p> </p><p>I found it interesting that if you test this with the valves already warmed up, you can hear sound while it's only at 90Volts, (although it is soft). </p><p>Also as I suspected, if the relay kicks in when the valves are already conducting (in other words not a cold start) , there is sometimes an audible click on the speakers. I will shorten the time delay to 10 seconds and see how much that helps.</p><p></p><p>Lastly I installed a rotary power switch, so now the next step is to find a nice face, so I can cut the shafts to length , and find screws that look nice enough to mount the name plate.</p><p></p><p>Regards</p><p>Leon</p></blockquote><p></p>
[QUOTE="leonsound, post: 104572, member: 1295"] After some discussion with Karel I added a "soft start" circuit to the amplifier. Essentially now there is a 68k Ohm resistor (10 Watt total) between the rectifier and the capacitor bank. In parallel with the resistor is a mains "Delayed start" relay, that shorts the 68k Ohm resistor out of the circuit after a predetermined time. I also added a fuse in line to all this just for additional safety. From cold start it takes about 10 seconds to gently climb to around 90Volts. Not sure why it then hovers around there (I measured with no power tubes installed, but the pre-amp tubes were in). Currently the relay is set to 15 seconds, after which the voltage jumps up to 400-and-odd Volts. I found it interesting that if you test this with the valves already warmed up, you can hear sound while it's only at 90Volts, (although it is soft). Also as I suspected, if the relay kicks in when the valves are already conducting (in other words not a cold start) , there is sometimes an audible click on the speakers. I will shorten the time delay to 10 seconds and see how much that helps. Lastly I installed a rotary power switch, so now the next step is to find a nice face, so I can cut the shafts to length , and find screws that look nice enough to mount the name plate. Regards Leon [/QUOTE]
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Audio and Video Talk
Vinyl
Build Your Own Tube Amplifier - workshop in 2011
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