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Old 19th March 2005, 10:33 PM   #1
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Default Help extend the life of my 805's

Guys my 805's already have a 30sec delay on the B+, but now i would like to give the filaments a soft start also, i was thinking about using this soft start thermistor the SG160 which is 2.5ohms at start then goes to .03ohms when hot, i've never used these before can anyone tell me how long it takes before it gets down to .03ohms? The 805 is 10v / 3.25amp filaments

Link to Thermistor http://www.farnell.com/datasheets/34817.pdf

Link to my circuit http://www.diyaudio.com/forums/attac...amp=1106011421

Cheers George
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Old 21st March 2005, 01:01 AM   #2
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Geez, plenty of lookers, but no one with anything to say? Frank (FDEGROVE) you there? SY you there? got any input on this one.

Cheers George
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Old 21st March 2005, 01:20 AM   #3
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Here is how I would do it (if I knew programming). Have a PIC chip do PWM to a mosfet over a 30 delay and eventually activate a relay to bridge over the mosfet. It could probably be done without a PIC but with a higher parts count.

Then of course you could get fancy and have it drive an LCD display with a countdown and status. Somebody has done it already but I don't remember the site. They even had thermistors to monitor the operating temperature.
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Old 21st March 2005, 04:19 AM   #4
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Oh my.... PICs, PWM, LCDs... geez, that would be more complex than the amp itself!

George, I see that your amp already has DC on its filaments, so why not feed the filaments from a constant current source? That means the current surge at turn on is eliminated, and might increase the life of your valves.
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Old 21st March 2005, 08:25 AM   #5
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Your right Jason, all to complicated, that's why these Surge Gaurd Thermisitors look good to me, and they are cheap only $5aus for the 15amp or $3aus for the 4amp jobs and easy as pie to install. Just wish someone has already tried it, looks as though that someone is going to be me.

Cheers George
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Old 21st March 2005, 12:50 PM   #6
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I've never delayed the filament voltages on an amp, but I have used current source feeds and series heater strings. And I've not had a problem with tube life.

I'm doing something fun on the amp I'm building now- I used a lightbulb in series with the mains as a surge protector/malfunction indicator. It was so cool to watch it light up and then fade that I'm installing one permanently, with a time delay relay to bypass it after 5 seconds.

Time delay relays are easily available at surplus and are the tube guy's best friend.


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