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    Do I need a thermistor?

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    Captain Coconut

    Posts : 209
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    Age : 64
    Location : Great White North

    Do I need a thermistor?

    Post by Captain Coconut on Sat Jan 21, 2012 5:13 pm

    Maybe I am needlessly agonizing, but am wondering if I should add a thermistor to my ST-35. I have around 122V in the winter, but it drops to 118 in the summer. I have been told that a thermistor will drop the voltage by another 2 volts so I am left with about 116V in summer. Will that to be too low for the tube heaters to run optimally? And is 122V in winter on the high side of desirable.


    danskman

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    Age : 57
    Location : Bière, Switzerland

    Re: Do I need a thermistor?

    Post by danskman on Sat Jan 21, 2012 6:30 pm

    Check the heater voltage, in winter or in summer; you'll know if you need to add such a device. IMHO, you should be OK without it. Mains voltage vary often, depending of the area or time of day (lunch time, factory or office opening hours, etc.). I live in Switzerland, we get a bunch of power, but the voltage can vary from 225 to 235 volts, without any damage to anything. Tube electronic circuits are designed to endure this kind of thing. Don't worry about it and be happy!
    Best regards,
    Danskman

    tubes4hifi
    Admin

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    Re: Do I need a thermistor?

    Post by tubes4hifi on Sat Jan 21, 2012 11:57 pm

    I would use a thermistor only because of the fact the ST35 has a solid-state power supply.
    The thermistor will prevent an "instant" turn-on surge and slow it down to 3-5 seconds.
    Heater filaments are nominally rated +/- 10%, so 5.7v to 7.0v is fine, and figure +/- 10% of 117vac is 105-129v,
    it's pretty rare for the power company to have more than a 3-5% variation.

    burnedfingers

    Posts : 31
    Join date : 2009-05-06

    Re: Do I need a thermistor?

    Post by burnedfingers on Sun Jan 22, 2012 8:15 pm

    quote:
    I would use a thermistor only because of the fact the ST35 has a solid-state power supply.


    There is no need to worry about cathode stripping with the power supply voltage. There is no need for s thermister. You have to remember that tube circuits have been around for years and to site an example I have three Wurlitzer 6420 mono block amps with the original tubes. Now, If there were a problem they would not have survived this long they were designed around 110-115 volts AC line and work fine with 110-125. As pointed out in another post the heater voltage is + or minus 10% its ok.

    Captain Coconut

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    Re: Do I need a thermistor?

    Post by Captain Coconut on Sun Jan 22, 2012 9:20 pm

    I was hoping for consensus on this, but what I'm getting is that it's a good idea to add one (according to Roy) but not necessary (according to burned digits and Swiss Cheese)?

    tubes4hifi
    Admin

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    Re: Do I need a thermistor?

    Post by tubes4hifi on Mon Jan 23, 2012 1:16 am

    is that a question? If it hasn't had one in there for 40 years, it's not really NEEDED, but recommended!! Laughing

    Captain Coconut

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    Re: Do I need a thermistor?

    Post by Captain Coconut on Mon Jan 23, 2012 9:41 am

    Thanks again Roy. It wasn't really a question but if someone wants to chime in, that's great. I was thinking out loud; this oversized brain of mine needs a workout from time to time.

    Bob Latino
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    Re: Do I need a thermistor?

    Post by Bob Latino on Mon Jan 23, 2012 10:08 am

    The other thing to consider is that who knows how many thousands of Dynaco ST-35 and Dynaco SCA-35 tube amps are still in use after 40 + years in service? Both amps had solid state rectifiers with NO DELAY. Many of these amps are still out there in service and still working fine after 40+ years of no delay - SO - (IMHO) the "danger" of using a solid state rectifier with no delay and the idea of cathode stripping and shorter output tube life on amps like this may be exaggerated ...

    Bob


    Westy56

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    Join date : 2011-05-09
    Location : Albuquerque

    Re: Do I need a thermistor?

    Post by Westy56 on Mon Jan 23, 2012 12:02 pm

    McIntosh still utilized a thermistor with SS rectification. I can only speak for the 240.

    "To greatly extend tube and component
    life a thermistor in the MC240 limits current
    surges produced when the equipment is
    turned on."



    Steve

    danskman

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    Re: Do I need a thermistor?

    Post by danskman on Mon Jan 23, 2012 2:03 pm

    Captain Coconut wrote:I was hoping for consensus on this, but what I'm getting is that it's a good idea to add one (according to Roy) but not necessary (according to burned digits and Swiss Cheese)?

    Don't forget the chocolate and the cuckoo Wink

    Luddite

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    Re: Do I need a thermistor?

    Post by Luddite on Mon Jan 23, 2012 3:06 pm

    I built a ST-35 (from Dynakit Parts) about three years ago and included a thermistor. I don't know if that made any difference, but figured there was no harm in doing so. My amplifier is working very well.

    Best Regards,
    Charlie

    Captain Coconut

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    Re: Do I need a thermistor?

    Post by Captain Coconut on Mon Jan 23, 2012 6:05 pm

    danskman wrote:
    Captain Coconut wrote:I was hoping for consensus on this, but what I'm getting is that it's a good idea to add one (according to Roy) but not necessary (according to burned digits and Swiss Cheese)?

    Don't forget the chocolate and the cuckoo Wink

    Sorry about my feeble attempt at a stereotype, but I couldn't type as if I'm yodeling. Suspect

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