Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Hello everybody,

I am trying to find out what is a device called VR 100 PRESET which I found near a thermistor in a schematics of a transistor amplifier.
I am italian and I don't know what to ask to the electronics shop. They have no clue too. The symbol in the circuit is a resistance with a T crossing it.
Is it a variable resistor or something? Is that 100 the value in K?
Thank you in advance for your reply!
=)

That sounds very much like a trimmer resistor (VR=Variable Resistor).

Like this?
(http://www.best-microcontroller-projects.com/image-files/schematic-symbols-trimmer-capacitor.png)
But a resistor in it instead. If yes, then it is a trimmer resistor (this is a symbol of trimmer cap). Sometimes they depict the same thing with an arrow crossing the symbol instead but that could be mistaken for an ordinary potentiometer while this type of symbol is more specific for a "set-and-forget" trimmer. If it's actually a letter "T" then it means a thermistor.

The "100" sounds like a component designation but if the schematic lacks them in general then I would be inclined to think that it in fact indicates the resistance. In that case the value would be in Ohms. If you are uncertain you should actually measure it.

If you go shopping for a trimmer resistor make sure you get a one that is physically identical to one you have in the amp. These things come at least in three or four different forms of pin placement that are not interchangeable. Physical sizes vary a little as well. So, if you shop "blind" then Mr. Murphy will make sure that you get a replacement part that doesn't fit to the PC board.

Post the schematic (or the relevant part of it) to be sure, but probably it´s a 100 ohm preset. Usually they are part of the output transistor biasing circuit.