[Ericus Rex]

I recently bought a vintage transistor electric bass amp.  It thumps at power-up.  I mean, REALLY thumps!  More than any other amp I've ever experienced and to the point where I'm worried it may damage the speakers in my cab.

Can I tame this thump by adding a thermistor to the power supply?  If not, what do you recommend?

Thanks all!

[richidoo]

The thump means there is DC voltage on the output terminals at least when it starts, but if DC persists after startup then don't connect it to the speakers as it will overheat the voice coils. Thump always means DC offset, whether instantaneous or persistent. Being that loud means it's relatively high voltage DC and something is wrong.

A good tech can easily figure out why it's thumping. It could be a number of things like imbalanced +/- supply rails to the output stage, power supply storage caps old, or the output stage is damaged from previous abuse and got put away by previous owner because it thumped. Good luck with it!

[Ericus Rex]

Thank you, Richidoo!

I will look into biasing and check DC at the outputs.