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Post subject: 74' Twin Reverb Noisy Vibrato Channel
Posted: Fri Nov 07, 2014 8:08 am
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Roadie
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My 74' Twin has a noisy vibrato channel. I dime the master volume, using the channel volume as the main control. Hum and hiss begin around 5. The electrolytic caps are new and I've tried changing preamp tubes. The normal channel is quiet and sounds great. I need some guidance regarding where to look next.


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Post subject: Re: 74' Twin Reverb Noisy Vibrato Channel
Posted: Sat Nov 08, 2014 2:38 am
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Joined: Fri Feb 05, 2010 4:31 am
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Location: Province de Québec, Canada
1- Did the amp was always noisy on vibrato channel or it start do noise ?

2- Any mods in the amp ?

3- Did you replace the right tubes by a know good ones ?

These amp may need some lead / wiring dress to be quiet , moving wires around vibrato circuit may remove the noise.

Please answer to questions :)


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Post subject: Re: 74' Twin Reverb Noisy Vibrato Channel
Posted: Sat Nov 08, 2014 8:45 am
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Roadie
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Joined: Sat Mar 09, 2013 12:27 pm
Posts: 207
The amp has been restored, New caps throughout and many resistors replaced with nos carbon comps, but it is stock. The leads from the preamp output, 220k resistors to the 1watt 100k power resistors hum when I touch them. Everything looks grounded properly. The black master volume leads are grounded, one ground wire is soldered to 1watt 33k resistor and the other to the 1meg resistor in the reverb section. I suspect as ground loop somewhere.


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Post subject: Re: 74' Twin Reverb Noisy Vibrato Channel
Posted: Sat Nov 08, 2014 9:54 am
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Joined: Fri Feb 05, 2010 4:31 am
Posts: 14049
Location: Province de Québec, Canada
Please answer questions. We can't run every where for good trouble shouting


Any picture inside the amp ?

if some wires have been moved when restored this may do noise , hum......Wires must run properly at the right place .
Lead/ wire dress is very important to keep amp quiet

Many things can make noise.


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Post subject: Re: 74' Twin Reverb Noisy Vibrato Channel
Posted: Tue Nov 11, 2014 7:42 pm
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Roadie
Roadie

Joined: Sat Mar 09, 2013 12:27 pm
Posts: 207
The trouble was in the master volume circuit. There is a 12k resistor, a .01 cap, 1 meg resistor and 1k resistor in series connected to the mv shielded cable next to the reverb. A previous owner had the wrong resistors installed. The vibrato channel is quiet now. The added resistance must add a load to the circuit even without the mv engaged.


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Post subject: Re: 74' Twin Reverb Noisy Vibrato Channel
Posted: Wed Nov 12, 2014 2:21 am
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Joined: Fri Feb 05, 2010 4:31 am
Posts: 14049
Location: Province de Québec, Canada
+1 Krankster,

You should tell to us first is it a new buy amp and it did make noise first time you get it .

Unless that we are thinking the amp was always working well and start to make noise .

Not same story for trouble shouting.

We must alway check for mod , poor job.


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