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Author Topic: Help with Gallien-Krueger 200GT guitar amp  (Read 2098 times)

g1

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #15 on: March 04, 2019, 10:26:44 PM »
That sounds right.  Now on the schematic there is a straight line down from the E of Q26 to the pot.
So if there is -9V at Q26 E, it must be on the pot as well, or there is a missing connection.

smackoj

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #16 on: March 05, 2019, 05:41:15 AM »
Yes, pin 3 of the Current Lim. pot has -8.85 vdc

g1

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #17 on: March 05, 2019, 04:07:46 PM »
Still have no idea how you could have had -30VDC on that pot if Q26 and it's circuit is ok.
Have you tried the amp now to see if it works?

smackoj

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #18 on: March 06, 2019, 10:59:40 AM »
Yes the amp is working, however, with it sitting on the bench outside of the enclosure it sounds perfect. Very quiet outside the wooden and tolex covering. But when I put it in the case it starts humming badly. It is not one uni-piece chassis. there are 3 pieces; the power chassis (both PT and OT, along with the ss rectifier and the power amp board) that mounts into the back of the cabinet. 2nd is a flat piece of aluminum that the reverb tank is mounted on that slides in from the front and sits on the bottom of the cabinet. And 3rd the faceplate - preamp conrols that slides into the front of the cabinet. There is a 5 wire harness that connects from the power amp board to the preamp board and a 3 wire harness from the reverb pan to the preamp board. All 3 pieces touch metal to metal.

I thought maybe there was insufficient ground between the power amp and the preamp so I ran a ground wire between the 2 which did not cure the loud hum.

g1

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #19 on: March 06, 2019, 01:41:30 PM »
All 3 pieces touch metal to metal.
And you have all these contacts made when testing on the bench?
Does the reverb control affect the hum at all?
If all electrical connections remain the same as on the bench, then it is more likely some kind of EMF due to positioning of components or lead dress.

smackoj

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #20 on: March 06, 2019, 06:02:00 PM »
I have not been testing with the aluminum plate and reverb tank sitting in posistion. I put it together outside the box just like it would be inside. Lots of hum. I disconnected the reverb leads from the preamp board. Still humming. I moved the alum. plate so it was not touching the power amp chassis. still humming. I removed the plate/reverb pan completely so not touching the preamp faceplate/board or the power amp .... dead quiet. The last piece to be moved from touching the reverb plate was the preamp faceplate. That is when it stopped humming

phatt

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #21 on: March 07, 2019, 03:56:03 AM »
Check the reverb wiring, you may have re-connected the tank backwards which would induce the hum symptoms you describe.

Reverb circuits are normally wired with only the pickup side grounded to the Tank Case. (Not always but most often)

If the tank connections use RCA plugs then swap them over and see if the hum improves.

Input and Output are often marked on the tank case but some folks get them mixed up.
Phil.

smackoj

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #22 on: March 07, 2019, 02:19:06 PM »
Hi Phatt; the wiring is correct but I found out where the faceplate meets the aluminum reverb pan is the noise problem. I put some non-conductive strips between the 2 pieces and it is all quiet.

thanks a million,

Jack D

g1

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #23 on: March 08, 2019, 01:34:19 PM »
Reverb circuits are normally wired with only the pickup side grounded to the Tank Case. (Not always but most often)
I think you are right that something has changed with regard to the tank.
Other possibilities are that the tank was replaced with same type but different jack grounding scheme (5th character of accutronics tank code), or there may have been rubber mounts insulating the reverb tank from the aluminum plate it is mounted on.

Anyway, good to hear smackoj found a workaround and it's all working fine now.

smackoj

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #24 on: March 09, 2019, 10:11:50 PM »
I am not sure about what changed with the grounding but it is playing nice and the reverb sounds real nice too.
Thanks for all the help amigos.

Jack D

phatt

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #25 on: March 09, 2019, 11:06:26 PM »
Answer; You just removed the ground loop. 8|
If you ground the case there are two ground paths which will cause hum. >:(
When looking at schematics the ground paths are hardly ever shown as they are all assumed.

Translating that drawing onto the actual circuit can be a challenge especially if you don't have a good understanding of ground paths.

I fixed a circuit years back and forgot one small fiber washer and hum was a problem.
I had created a ground loop as the fiber washer isolated the PCB from chassis.

i've seen reverb tanks where the rubber grommets had rotted away and gave intermittent hum problems as the tank would make contact with chassis as it vibrated.
Stuff like that can drive you insane if you don't know about grounding techniques.
Phil.

smackoj

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #26 on: April 25, 2019, 10:14:30 AM »
Yes I would like to know more about proper grounding techniques. I have problems with stomp boxes because of poor grounding. Ugh

phatt

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #27 on: April 26, 2019, 04:09:44 AM »
Yes I would like to know more about proper groundin g techniques. I have problems with stomp boxes because of poor grounding. Ugh
Well if you define your actual problem we will see if we can help :tu:
Phil.

 

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