The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all products and all products

    use of thermistor

    Captain Coconut

    Posts : 233
    Join date : 2009-10-13
    Age : 65
    Location : Great White North

    Re: use of thermistor

    Post by Captain Coconut on Thu Jun 13, 2013 8:53 pm

    Captain Coconut wrote:I use a ST-35 amp with a CL-90 thermistor. The voltage is 121-123 at our place. I like the idea of a softer start-up with the thermistor and the fact that it knocks down the line voltage a few volts. However, I have read somewhere that a thermistor is not a great idea to have "running" all the time, and to have some sort of bypass if possible. 

    I know there are thousands of ST-35's out there that have run fine for years without a thermistor, and the tubes have survived hundreds of "hard" startups. Also, the original manual states: " It is intended for use with the 120-volt a.c. power supplied to most homes. Although variations of several volts above or below this figure will have no pronounced effect upon performance or component life ...". It goes on to say that there is no guarantee on the amp if operated above 130 volts. 

    It looks like the 123V at my place is not an issue therefore, but wonder if the thermistor is a good idea anyway. Any thoughts?

    PS - I made a similar post some time ago, but wonder if there's any new insight on the matter. 


    Posts : 790
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: use of thermistor

    Post by GP49 on Fri Jun 14, 2013 12:24 pm

    If it makes any difference, McIntosh used thermistors for decades in their tube amplifiers, in exactly the same application.  The only ill effect I ever saw was when they failed...physically broke apart.  Dead amp, but an easy fix.

    Jim McShane

    Posts : 198
    Join date : 2011-10-19
    Location : South Suburban Chicago

    Re: use of thermistor

    Post by Jim McShane on Fri Jun 14, 2013 4:05 pm

    As well, keep in mind the CL-** series parts are not standard thermistors. They are made with a very long time constant so the resistance stays higher longer than with standard thermistors, giving you increased protection against the initial surge at turn on. They are a VERY good idea, I've used them for 12+ years with great success and zero trouble.


    Posts : 23
    Join date : 2009-12-13

    Re: use of thermistor

    Post by 1973shovel on Mon Jun 17, 2013 1:50 pm

    Both Jim McShane and Eli Duttman recommended using CL-90's in my SCA-35. Interestingly (to me, anyway) they suggested not only placing one in series with the incoming AC line, but also in series with the B+. This serves not only to offer a little extra B+ voltage drop, but also delays the B+ (however slightly), giving the filaments a bit of a head start.

    They're not a game changer, or as good as Roy's time delay circuit, but they're only $2 each, and do what they're designed to. I plan on adding them to both my ST-70 and MkIII's eventually.

    Posts : 1460
    Join date : 2013-04-01
    Location : Michigan USA

    Re: use of thermistor

    Post by sKiZo on Wed Jul 03, 2013 10:26 pm

    Hadn't thought about putting one on the B+ line. Seeing as how ONE will drop the voltage a couple or three points, I assUme TWO would drop it five or six?

    My power averages 122+, so that would put me down to around 117, which I keep hearing is some sort of magic number for tube equipment.

    Sponsored content

    Re: use of thermistor

    Post by Sponsored content

      Current date/time is Thu May 24, 2018 1:53 pm