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