Rehabbing an old amp

  • Thread starter Thread starter Thurgood
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Thurgood

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Hello all. I am considering restoring an old (60's) gretch amplifier i believe it is a Model 6150 or something like that. I cannot say for sure as the paint etc has worn off. I did use it years ago and was able to put up with the hum but it did sound fantastic. I have the open back cabinet with two 12's one of which has a broken connection plate so a new speaker is probably in order. Nonetheless the amplifier is in good shape jsut probably needs a recap and general cleaning. Do any of you folks have suggestions for the method to bring her back to life. Btw the amp has bass, Gtr Hi and Lo and Accordian inputs!!!. Tube set up is 2 6l6 power state and looks like 4 preamp tubes. It runs well but humms like a bitch. thanks in advance. Will.
 
When someone asks how to repair an amp, the canned response seems to be, "If you have to ask, take it to someone who knows." I think this is going to be the same situation.

Sounds like a cool amp. Have any pictures?
 
Thurgood said:
Hello all. I am considering restoring an old (60's) gretch amplifier i believe it is a Model 6150 or something like that. I cannot say for sure as the paint etc has worn off. I did use it years ago and was able to put up with the hum but it did sound fantastic. I have the open back cabinet with two 12's one of which has a broken connection plate so a new speaker is probably in order. Nonetheless the amplifier is in good shape jsut probably needs a recap and general cleaning. Do any of you folks have suggestions for the method to bring her back to life. Btw the amp has bass, Gtr Hi and Lo and Accordian inputs!!!. Tube set up is 2 6l6 power state and looks like 4 preamp tubes. It runs well but humms like a bitch. thanks in advance. Will.
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It needs to go to the doctor. Plunking around in a tube amp's innards when you don't know what you are doing can get you seriously shocked - maybe even killed.

As to the humming, it probably needs its high voltage filter caps changed out. You don't want to try this at home.
 
I am aware of the lethal voltages lurking inside adn I do have a pretty good grasp of electronics little Diy stuff and have built an ax84 P1. But to be on the safe sideI'll pass on the repair. I bet the old girl needs to be recapped. The amp sounds amazing but it just buzzes like mad. It definitley needs a proper 3 prong ac cord. I just hate seeing just collecting dust. I will prob give it over to a local tech who is very good. You can see leaky goo that has oozed out of the caps. Anything with an accordian input is cool. Thanks for the reply I will let ya'll know how it comes out.
 
Not so fast. If the you'll-kill-yourself ninnies here took their own advise, they would be having someone else record their stuff for them- they (presumably) learned home recording by doing, there is no reason you can't learn tube amp repair the same way. Yes, capacitors can kill you- even long after the power has been unplugged. So, get some good instructional manuals, maybe another, less important (to you) amp to practice on, follow all safety procedures, and go to it! Recapping a tube amp is NOT a difficult thing to do- capacitors are easy to identify, easy to discharge, easy to replace.
 
RejectionNinja said:
When someone asks how to repair an amp, the canned response seems to be, "If you have to ask, take it to someone who knows." I think this is going to be the same situation.

Sounds like a cool amp. Have any pictures?
Click to expand...

What if you took it to someone who repairs them and they said it can't be fixed?I have a hughes & kettner attax200 head i bought on ebay and knowing that it kept blowing fuses i went ahead and bid on it.I know,long story short is that i had one stolen and being this was a half hour from where it was stolen i was suspicious and couldn't get the police to put a hold on the auction in time to verify it wasn't mine.

I won it,it wasn't mine and it still blows fuses.Power turns on but no sound comes out of it.I'm a mechanic so i feel anything can be fixed it's just when it costs more to fix than it's worth it's just best to say goodbye.It's a shame cause i really liked the sound of these amps.It's in my shed.
 
stevieb said:
Not so fast. If the you'll-kill-yourself ninnies here took their own advise, they would be having someone else record their stuff for them- they (presumably) learned home recording by doing, there is no reason you can't learn tube amp repair the same way. Yes, capacitors can kill you- even long after the power has been unplugged. So, get some good instructional manuals, maybe another, less important (to you) amp to practice on, follow all safety procedures, and go to it! Recapping a tube amp is NOT a difficult thing to do- capacitors are easy to identify, easy to discharge, easy to replace.
Click to expand...

Only if you enjoy the smell of ozone, arc welding and burning flesh.
 
stevieb said:
Not so fast. If the you'll-kill-yourself ninnies here took their own advise, they would be having someone else record their stuff for them- they (presumably) learned home recording by doing, there is no reason you can't learn tube amp repair the same way. Yes, capacitors can kill you- even long after the power has been unplugged. So, get some good instructional manuals, maybe another, less important (to you) amp to practice on, follow all safety procedures, and go to it! Recapping a tube amp is NOT a difficult thing to do- capacitors are easy to identify, easy to discharge, easy to replace.
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Not so fast with the pejoratives, OK? A lot of us "ninnies" are experienced with amp repair and/or electronics in general and know that trial and error is not the best way for a novice to get started tinkering with tube amplifiers.
 
So say it is the cap.
What's the best way to discharge them?

I've read up on using the screwdriver with a rubber or plastic handle and how it works just fine.
Then in the next search, they're sayin...No, a screwdriver is NOT a good idea.

:confused:

I ask cuz I also have an older amp (like ThurgoodDude :D) and it has leaky caps.
 
Dogbreath said:
So say it is the cap.
What's the best way to discharge them?

I've read up on using the screwdriver with a rubber or plastic handle and how it works just fine.
Then in the next search, they're sayin...No, a screwdriver is NOT a good idea.
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The best way, IMO, is to use a wirewound high current resistor and a clip wire. With the amp plugged in (but powered off, of course) clip one end of the clip wire to the chassis and the other to one of the wires of the resistor. Hold the resistor by its ceramic case and put your other hand in your pocket, and touch the unclipped wire of the resistor to every node in the circuit that could hold a charge.
 
arcaxis said:
Why with the amp plugged in and running the risk of accidently touching the resistor clip to something with incoming AC?
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Having the amp plugged in provides the path to ground for the charge you bleed off the caps. As long as you don't touch the hot side of the power switch, there is nothing with live AC voltage, and if you don't know where that is well enough to avoid it, then you shouldn't be poking around in there at all.
 
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Okay, sorry about the name-calling. I'll correct myself here- SISSIES. I meant to say SISSIES, not NINNIES. Please forgive me.:)

Really, I guess I should make more use of smilies, so everyone knows I am not pickin' on them.
 
Damn. I did not believe that such furor would erupt. Anyway, I will let the pro folks deal with the issue. BTW I truly do appreciate the help with safety. for real.

On a side note: To add insult to injury, my Mesa mark iii jsut had a melt down. I pulled the chassis to take a look and voila!!! smoked resistors in a few places, looks like three .47u caps went ........you get the idea. I am mad. I think its the jinx. It got jealous. So what did any desperate amatuer do... Built a mini Ruby amp. It will get me by since my Marshall and my Fender are on loan to Gf who is in Portland. (she is coming back, I have her dog) Well got to go. thanks all.
 
If hum is your problem with this old tube amp I think there are 2 possible causes , Old electrolytic caps become leakey over time and will cause a hum so replaceing all the electrolytic caps is a good start , even replaceing them with a bit larger Values would be a good ...

second issue is that many old tube amps use AC heaters , useing AC on the heaters will increase the noise floor and inject hum into your signal , luckilly this is pretty easy to fix , if the heaters are run off of 12v AC then a 1n4007 Diode will rectify the AC to about 16v DC and a 15 ohm 5w resistor will drop it to 12v DC and a 2200uF 16v Capacitor will filter the DC , a good fix for lowering the noise floor ....

Cheers
 
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