Problems with Concert Bass

[Walt-Dogg]

So I'm still yet to take my Concert Bass into my tech for a SECOND time. But! I was playing it today and it would seem that it works until it gets it heats up to much then stops producing sound, it stays on and the fuse doesn't blow. And the heat sink )"plate" around the transistors) is searing hot. So can anyone tell me what the problem is? Four transistors were replaced last time I took it in.

[george]

First off have the tech check the emitter resistors on the PA board - make sure they are still within spec. and there are no cold solder joints. I suspect that the bias is not set correctly - some early Concert Bass PA boards don't have bias trim pots too. Once the heatsink gets too hot, there is a thermal switch that sits on the HS rail, the switch opens up and disables the mains voltage. But wait, you say the amp stays on (pilot light stays on??). The thermal switch should be wired in series with the circuit breaker and power switch. Check for bad solder joints...........

[loudthud]

The amp should not run that hot, especially if it's not being played. There is a thermal cutout attached to the heat sink that will shut off everything except the pilot light if it gets too hot, then reset when the amp cools off. Take the amp back to your tech and check the transistors again.

Note to tech: The 4 output transistors should be matched as close as possible, within 5% for beta. Also beware, shorts can develop in the driver transformer. Check that the output voltage is near zero with all four transistors removed. There is no known replacement for the driver transformer, you'll have to get it re-wound if it's bad.

[Walt-Dogg]

Alright. We'll see next time I take it into him.

EDIT: There are 5 transistors, so I'm guessing the 4 that were put in don' match the one that was left in. So I'll have him fix that, go over the WHOLE amp and check the diver transformer. Also, if the driver transformer is jacked, how much would that cost me to repair?

[Isaac]

One of the 5 transistors is the driver for the other 4. Only 4 are output transistors.

[Walt-Dogg]

Which is which? Do you think they're the source of the problem or something else?

[george]

Let's see the driver transistor is the one at the left end of the heatsink when you are looking into the amp with the front panel in front of you. The transistors can be checked with a DMM - not the most accurate but you can detect a short.

[Walt-Dogg]

Well I'm taking it in today, hoping for the best...

[Walt-Dogg]

Do you guys think it's at all possible that the Ampeg SVT 8x10e cab I'm using just needs too much power to be driven and that my Concert Bass doesn't really produce that much power so it's just overheating from being pushed to hard to power my cab? Because I was using my Concert Lead though my 8x10e for S&Gs today and it started heating up and it was awfully quiet at a high volume when I know through my 412L at a relatively low volume, my Concert Lead just screams.

[loudthud]

Your 8x10e may have one or more dead speakers, and they aren't very efficient to begin with. They can sometimes fail with open voice coils so you can still see them vibrate but they are just reacting to the sound inside the box from the other speakers.

[Walt-Dogg]

Meaning that the cab doesn't have the right ohms?

[Walt-Dogg]

I ran my buddy's GK head through my 8x10 twice yesterday, first time it didn't get too hot, second time it seemed to get really hot. But that second time was in a much smaller room that was baking as we practiced. So I don't know what's up.