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    CL - 80 or 90 Thermistors vs Time delay circuit?

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    Analog Man

    Posts : 56
    Join date : 2013-09-01
    Age : 54
    Location : St. Louis, MO - On The Hill

    CL - 80 or 90 Thermistors vs Time delay circuit?

    Post by Analog Man on Sun Sep 08, 2013 9:19 am

    Back in the day, Thermistors were used to smooth "power on" inrush current on tube amps before time delay circuits were developed. Although Amperex had a time delay relay tube which I have never tried.
    I used a CL - 90 thermistor in a ST - 70 rebuild and everything worked great and I never had any issues.
    My question to you all is, Has anyone out there experimented with these and if so, what were your findings?
    I am planning to use a thermistor in my VTA - 120 as opposed to a solid state time delay circuit. I don't feel that a time delay circuit would withstand the heat of the amp and possible short life of relay contacts.
    I am kinda old school and set in my ways. Your input would be greatly appreciated.Question 

    arledgsc

    Posts : 330
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: CL - 80 or 90 Thermistors vs Time delay circuit?

    Post by arledgsc on Sun Sep 08, 2013 11:57 am

    The relay board Potter & Brumfield relay is rated industrial and can be used safely at ambient temps of 85 deg C.  It also is designed for wave solder with a recommend temperature profile of 270 deg C for 10 secs.  The datasheet states typical applications of boiler controls and garage door openers.  So it should be right at home inside a very warm amp.  I can't help you on the thermistor but the relay board normal operation should not be any concern if that helps.

    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: CL - 80 or 90 Thermistors vs Time delay circuit?

    Post by PeterCapo on Sun Sep 08, 2013 2:10 pm

    I've used a CL-90 in my Stereo 35 to good effect.  However, you might consider changing the CL-90 in your Stereo 70 for a CL-80, as the CL-90 is rated for 2 amps while the Stereo 70 has a 3 amp fuse.  The CL-80 is rated for 3 amps.  There's another line of thermistors called Ametherm and even others.  I am currently using an Ametherm SL22 12103 in my Stereo 70, and it seems to be a nice, soft startup.

    I tend to doubt that the temperature inside the amp would really be a problem for the relay board.  You could always check the temperature ratings for the relay and its associated parts.  The relay's specs might also include the number of cycles that can be expected from the relay, presumably at its maximum temperature rating, during its lifetime.  (Noticed after first posting that arledgsc quoted a temperature rating for the relay)

    Of course, the purpose of the thermistor vs. the relay is not exactly the same.  As I understand it, the purpose of the time delay relay is to warm up the heaters before the B+ is applied in order to avoid cathode stripping, especially in amps without a tube rectifier.  The time delay relay also provides protection from power line dropouts/fluctuations/etc. that could cause undesirable, rapid power cycling of the amp.

    The thermistor does not protect against fast power-cycling, as you would need time for the thermistor to cool down any time power is cut.  I work around the fast power-cycling part by using a GFCI power strip, which trips on line drop-outs and stays open until manually reset (and also offers personal protection from ground faults, especially since it seems that many of us lift the chassis safety ground to get rid of hum).

    On the other hand I would not think that the relay would ease the initial inrush in the B+.  But, this might not be an issue unless you are using solid state rectification (or indirectly-heated tube rectifier?) along with significantly increased capacitance in the power supply, or if your power supply capacitors don’t have a lot of headroom for their voltage ratings.  You could put a thermistor in series with the B+, but you’d probably want to give a good deal of thought as to which thermistor would be best suited for this location.

    Another potential benefit of a thermistor in the PT primary is that it drops the line voltage some, with the amount depending on the individual thermistor and how much current the amp draws.

    In any case, if you go with a thermistor in the PT primary, I’d get one with a maximum current rating at least equal to the line fuse.

    Regards,
    Peter


    Last edited by PeterCapo on Thu Sep 12, 2013 1:55 pm; edited 1 time in total

    Analog Man

    Posts : 56
    Join date : 2013-09-01
    Age : 54
    Location : St. Louis, MO - On The Hill

    Re: CL - 80 or 90 Thermistors vs Time delay circuit?

    Post by Analog Man on Mon Sep 09, 2013 6:28 am

    PeterCapo wrote:I've used a CL-90 in my Stereo 35 to good effect.  However, you might consider changing the CL-90 in your Stereo 70 for a CL-80, as the CL-90 is rated for 2 amps while the Stereo 70 has a 3 amp fuse.  The CL-80 is rated for 3 amps.  There's another line of thermistors called Ametherm and even others.  I am currently using an Ametherm SL22 12103 in my Stereo 70, and it seems to be a nice, soft startup.

    I tend to doubt that the temperature inside the amp would really be a problem for the relay board.  You could always check the temperature ratings for the relay and its associated parts.  The relay's specs might also include the number of cycles that can be expected from the relay, presumably at its maximum temperature rating, during its lifetime.  (Noticed after first posting that arledgsc quoted a temperature rating for the relay)

    Of course, the purpose of the thermistor vs. the relay is not exactly the same.  As I understand it, the purpose of the time delay relay is to warm up the heaters before the B+ is applied in order to avoid cathode stripping, especially in amps without a tube rectifier.  The time delay relay also provides protection from power line dropouts/fluctuations/etc. that could cause undesirable, rapid power cycling of the amp.

    The thermistor does not protect against fast power-cycling, as you would need time for the thermistor to cool down any time power is cut.  I work around the fast power-cycling part by using a GFCI power strip, which trips on line drop-outs and stays open until manually reset (and also offers personal protection from ground faults, especially since it seems that many of us lift the chassis safety ground to get rid of hum).

    On the other hand I would not think that the relay would ease the initial inrush in the B+.  But, this might not be an issue unless you are using solid state rectification (or indirectly-heated tube rectifier?) along with significantly increased capacitance in the power supply, or if your power supply capacitors don’t have a lot of headroom for their voltage ratings.  You could put a thermistor in series with the B+, but you’d probably want to give a good deal of thought as to which thermistor would be best suited for this location.

    Another potential benefit of a thermistor in the PT primary is that it drops the line voltage some, with the amount depending on the individual thermistor and how much current the amp draws.

    In any case, if you go with a thermistor, I’d get one with a maximum current rating at least equal to the line fuse.

    Regards,
    Peter
    Thank you very much for the info/education! I really appreciate your input and knowledge. I guess I have more to learn and yes, I will probably get the TDR board.

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