Gallien Krueger GMT 600B amp

[Prairie Dawg]

Hi folks. I've got a G-K GMT 600B bass amp here, an old one, that belongs to a friend of mine. It went up in smoke a number of years ago and I agreed that I would try and get it up and running for him. Here's some pics of the PS board.





It's possible I may have the information on a schematic I located for the power amp.

I'm a solid state noob, completely, but it appears to me that a good start here would be to validate all the intact stuff, replace the 'lytics, and replace the burnt stuff. Just to get a baseline of known good parts.

the rest of the amp looks ok-the preamp board's fine. I've got to power it up and take some preliminary voltage measurements too.

Anyone with any experience on these things is welcome to comment, offer schematics and/or service information, kibitz, observe, yawn or pity me.

My friend's emotionally attached to this amp, and if I get it running the prize is a quad reverb.

[pdf64]

Email GK, I remember that they are happy to send schematics. Maybe not this week though.

[Prairie Dawg]

To tell the truth, I got to looking at the 600B power section schematic I found on the net and it actually is the schematic for the board I have plus the power transistors. I am in the process of making a large scale road map because (wouldn't you know it) the components aren't numbered and a couple are crisped. Once I get the board rebuilt then maybe I can start to figure out what went wrong with it in the first place.

I've got a request in to G-K for service information so we'll see what's up next week.

So fellows, email me if you need a copy.

[g1]

Is this the schematic? It appears there has been work done before, maybe it was not successful. You will need to recheck several things. For starters, I would check all the mica insulators and transistor hardware insulators. Anything that has been replaced should be double checked. Do the metal can xistors (and others) match the part numbers listed on schem? You will want to check the orientation of the transistors for proper EBC placement. Not much on the power xistor board, .33R10watts, 2 diodes and the power xistors. There is a zener shown as D1 on the schem. with no voltage or part number listed, you would have to find that out from GK. Looks like most damage on this board was related to the driver circuits so I would assume at some point there were blown outputs that took out drivers with them, with any luck it could be a fairly straightforward repair.
That's some vintage GK stuff there (button and metal can xistors, are the chips on the preamp metal can?). I can see why the owner wants to get it going. As far as GK goes it is probably somewhat collectible.

[Prairie Dawg]

That is the schematic I have, and you are right, it had been worked on before. Part of the problem may have been that the slightly cooked metal film resistor was probably the wrong value by a country mile. At this point I am redoing anything on the board that looks even slightly dodgy and the intention is to bring it back to print as a starting point. I was wondering about the significance of the damage and you may have pointed me in the right direction. All this information is very helpful as this will be my first significant solid state repair job. Lucky for me I've got time on this one.

As to the significance of the mica insulators, what's that about? It looks like a couple of the driver transistors had been changed at one time-they're slopped with bird poo-and I acquired a new set of them. I'm assuming they're important but I do not know in what way.

[g1]

The metal tab or case is often connected to the collector of transistors. Therefore they need to be insulated or they short to the heatsink. This is done with mica (or other type) insulators and plastic insulators for the screws. Sometimes the mica's get cracked or the screw insulators get forgotten and things get shorted to the heatsink.
The bird-poo is heatsink compound and you will need some for the new transistors.

[Prairie Dawg]

Got it. This is all new stuff for me.

I noticed Mouser has install kits that have the screw and nut, the plastic washer and the mica washer-I'll have to order up a few. My intention is to get the board back to print and then proceed to evaluate the power transistors. How? I don't know yet but I should be able to figure it out.