Gallien Krueger 250ML (Unit 2)

[Enzo]

Oh the lid is tremendously important. There is no fan or anything, and that heavy aluminum lid is a large part of the heat sinking.

[sweatyk]

That's what has been rolling around in my head the last couple of days. This entire casing is the heat sink.

[sweatyk]

Another thought....now that this has been resurrected from the dead and all seems well, would it be advisable given its age to replace all of the electrolytic capacitors for extended life? I know there are people on both sides of the fence on something like this but wanted to get your opinions on this. I mean were talking around 35 years old.

[g1]

You are correct.....the + side is getting over worked by design.................................................. ........

No, it's just doing some work like Enzo mentioned. Sorry if I'm just nitpicking about your choice of words.
1A current for that reg. would be over-worked and the feed resistor will not allow that. The -15V reg. is just loafing. If you measure the voltage across that 10R feed resistor, you can calculate the current and I'm sure it is not near working hard yet. Voltage dropping or regulation usually involves converting voltage into heat. 5W resistors get very hot when they are only dissipating 1W.
Wait til you see how the outputs get when running around full power output. A lot hotter but still not over-worked by design.
I will allow that the heasinking of these are not great, but that is related to the output transistors when being run hard. I may have mentioned earlier that lot's of people used to run them with a fan pointed at the back due to this issue.

As far as a cap job, sure, if you want it to go another 35 yrs., you are going to have to do them at some point anyway.

[The Dude]

I usually just ESR test them. Poor ESR readings will usually tell you if caps are starting to go bad, but each to his own. I won't get into a battle about it.

[sweatyk]

I usually just ESR test them. Poor ESR readings will usually tell you if caps are starting to go bad, but each to his own. I won't get into a battle about it.

I appreciate and agree with that. What esr meter do you guys use? I know there are the old school ones but what do you think about the new ones that are available these days? Pros / cons........

[sweatyk]

One other thing I will have to do to complete this amp is source a pair of speakers. Other than finding a donor unit the originals are very difficult to find being they are rated at 16 ohms. GK gave me a link to some that are rated at 8 ohms but said I would need to disable the external speaker outs.

This is a series 1 amp, I've noticed that the series 2 amps have a switch on the rear panel to select internal or external speakers where as series 1 does not. Would it be that simple to mount a switch that kills the internal speakers when you want to plug into a cab? All the switch does is disengages the brown and orange positive speaker wires between the amp and speakers.

[g1]

Yes that would be fairly simple. A DPDT switch that takes the + output from each side either to a speaker, or to an 'ext.spkr' jack.
That way the jacks don't work when the speakers are engaged. This will prevent loads of less than 8 ohms per side.