Wild volume fluctuations

[jdmmusic]

Hi, Just bought a used Gallien-Krueger RB400-III head and tried it out last night for the first time with some very discouraging results.

I rented an Ampeg SVT-410HLF 4ohm cabinet for the evening to plug into and fill the room. During soundcheck I noticed that I had to crank the input and master volume on the GK full out to get enough volume (and it still didn't seem like enough). We went on about 30 minutes later and when I turned on the amp it was way, way louder. I was able to turn them down to about 9 o'clock and get a comfortable output.

About 3 or 4 songs into the set it seemed to quit on me but I noticed that there was a little sound and then cranked it up again and it was back like at soundcheck. I played like that for about 2 songs and it kicked back in again and I had to turn it way down again.

The soundman seemed to think that it was related to impedance but the GK is rated for 240 watts @ 4 ohms and about 180 @ 8 ohms so I thought that I would be good with the 4 ohm cabinet.

Anyone experience anything like this before? Any idea if it would alleviate it by getting an 8 ohm cabinet, or is the amp fried?

Thanks for the help.

 

[OneWayPunk]

I doubt it was an impedance problem as long as you were only running one 4 ohm cab.
I would guess (educated guess that is) that the amp has been run a too low of an impedance in the past and the transistors overheated and while not completely blown they fail when they heat up. I have seen that exacty type of problem on SS amps in the past. I would take it to a tech and have it looked at.
An 8 ohm cab would not stress the amp as hard and you might not have that same problem again but you should be able to run your amp at 4 ohms with no problem.

I would also be pissed that someone sold me a bad piece of gear.

YMMV of course

 

[fdeck]

In my days of fixing amps, I learned that the great majority of amp problems are mechanical in nature. My best guess is a worn volume pot, or dodgy contacts in the effects loop jacks. Try bypassing the effects loop with a cable.

 

[jdmmusic]

Thanks for the help, guys. I just got off the phone with the repair and thinks that it's most likely some ozidation in the input jacks, specifically the effects jacks.

fdeck, he suggested connecting the send and return with a patch cord as well and seeing if that works. If so, I'll be a happy camper.

This is my first post here and I'm impressed with the amount of knowledge that's readily available. Thanks again.

 

[62bass]

Thanks for the help, guys. I just got off the phone with the repair and thinks that it's most likely some ozidation in the input jacks, specifically the effects jacks.

fdeck, he suggested connecting the send and return with a patch cord as well and seeing if that works. If so, I'll be a happy camper.

This is my first post here and I'm impressed with the amount of knowledge that's readily available. Thanks again.

I had that problem with a 400 RB and it turned out to be the effects loop jacks. They'd start cutting out after the amp warmed up. Jumping it with a short instrument cable "fixed" it. I never got around to replacing the jacks.

 

[jdmmusic]

Final follow up - just in case anyone finds this in the future. I tried patching the effects loop jacks and it worked like a charm.

I had to play for about 40 minutes before the drop in volume reoccurred but I had a patch cord at the ready and plugged it straight from the Send to the Return jacks. As soon as I plugged in the cord it was miraculously 'fixed' and played fine after that.

I think that I'll see how long I can get with this fix. Thanks for the help.

 

[62bass]

Great. Glad it helped. Assuming that is the only problem, the "fix" should last forever.

 

[Espidog]

Getting those jack sockets replaced should be a cheap and easy job (you could do it yourself if you're handy with a soldering iron), and you can splash out on the highest quality sockets you can get. That'll solve the problem for good.