Welcome to Solid State Guitar Amp Forum | DIY Guitar Amplifiers. Please login or sign up.

February 24, 2024, 05:16:40 PM

Login with username, password and session length

Recent Posts


Fuse or resistor?

Started by Diablo_IV, June 18, 2019, 04:21:33 AM

Previous topic - Next topic


I am restoring an old Randall RG50-112 amp.  When I opened the chassis I saw this component in the picture.  It was mounted vertically on the pcb board with one of the leads either clipped intentionally or blown.  I am trying to figure out if this is a simple fuse or a thermistor or a resistor fuse.  Anyone recognize one of these?  No ohmage and no connectivity reading.


Looks like a fuse,blown

Inviato dal mio Redmi Note 5 utilizzando Tapatalk

Jazz P Bass

Here is the schematic for the RG50T.

It shows a fuse on the AC mains inlet.

You will have to trace out the path of where the component goes.


thank you for the schematic, the amp I have is the solid state version.  Here is the schematic. I see a strange resistor symbol with a circle around it where B (58vdc) comes out towards the transistors.   I am wondering what that symbol is.


There is a temperature dependent resistor connected to the base of the Tip30, if that is what you mean.
Where is the mystery part connected to on the schematic?
That green coating looks like heatshrink, cut it off and see what is under there.


The resistor within a circle /oval is the symbol for a thermistor, and is part of the temperature compensation for the bias network. It should - but is not always - be mounted on the heatsink for the output transistors. Unfortunately there is no value given for it.


yes, thank you, it is connected to the collector of a tip31c transistor.  The collector is acting as a heatsink, being throughbolted and soldered to the same trace as the component in question.  The solder joint for the heatsink/collector is melted out a bit as are some of the other transistors collector joints.  I cut the thing and half and included a picture.  I am still not sure what it is. 

Jazz P Bass

My take on it is it is a 'rod thermistor'.


inside the amp i found a big 15k resistor in a bag.  It seems like the original plan was to replace the thermistor with a 15k resistor.


A few more pictures.  You can see the transistor collectors acting as heatsinks which melted out the solder joints on the bottom.  would it be safe to say these transistors should be replaced?