Leonardo Capossio wrote:



** PTC100 operates the "mute" function of the power amp by shutting off Q7 and so the base voltage to Q8 - which is a 2mA current source for the input pair Q9 & Q10. This results in no current in the driver stage - transistors Q11, Q12 & Q13 and so no amplification.



Now I don't fully understand how the PTC100 shuts it down. At 25deg the PTC100 has 110ohm, according to some tables, and at 100deg it has a little less than 140ohms.


** There is a device called PT100, a platinum resistance sensor that is quite linear and rises about 0.4% for each degree C. The thing on the schem labelled "PTC100" is not one of them.

It is a PTC "positive temp coefficient" thermistor (aka posistor) with a steep rise in resistance at 100C. The mute circuit in the Fender trips when the PTC device reaches 800 ohms and is rising steeply - so operates with a snap action.

FYI: "PTC100" is not a part number, but a code used on the schem referring to the parts list.


.... Phil