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Post subject: 68" Super Reverb Still Works With the Knobs Down
Posted: Sun Mar 22, 2015 6:35 pm
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A buddy of mine was playing through my amp tonight and we discovered that with every single knob turned all the way down, the amp still works. I mean there's sound coming through the speakers, quite loudly too.

Any ideas?


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Post subject: Re: 68" Super Reverb Still Works With the Knobs Down
Posted: Mon Mar 23, 2015 12:39 am
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Just one, forget about it and turn the guitar volume down. :wink:

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Post subject: Re: 68" Super Reverb Still Works With the Knobs Down
Posted: Mon Mar 23, 2015 12:22 pm
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It sounds like bad grounds. Try tightening the nuts on the input and speaker jacks. If that doesn't help, tighten the grounds inside the chassis on the transformer mounting screws. Also, be sure the controls have tight nuts.
While it's unlikely, it could conceivably be bad electrolytics in the power supply. Even with the volume turned down, the input stage is always at full volume.
It can bleed sound through the power supply to other stages if the caps are worn out. The power supply caps provide isolation between stages
If it's an original 68 and hasn't been recapped, I would look under the cover to inspect them for leakage.


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Post subject: Re: 68" Super Reverb Still Works With the Knobs Down
Posted: Mon Mar 23, 2015 4:22 pm
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TimsAudio wrote:
...The power supply caps provide isolation between stages..


I think that you mean "coupling caps", the caps that connect one preamp stage to another, these are not electrolytic. :?:

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Post subject: Re: 68" Super Reverb Still Works With the Knobs Down
Posted: Mon Mar 23, 2015 8:32 pm
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Thanks you guys, very helpful.


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Post subject: Re: 68" Super Reverb Still Works With the Knobs Down
Posted: Mon Mar 23, 2015 9:44 pm
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Coupling caps isolate the DC from the AC signal between stages,
I do mean power supply caps. The signal in the plate resistors causes ripple in the power supply voltage. The filter caps not only absorb 60 cycle hum, but they filter signal out of the power supply buss too. It is DC isolation at work here.
If the caps are not filtering, the signal can travel on the B+ upstream to other stages, where it is impressed on their plates. This results in interstage bleedthrough. Hum doesn't necessarily increase much.

The reason I think its unlikely is that usually the main caps go first because they have more ripple and voltage. But the preamp electrolytics could have a broken lead or bad solder preventing them from doing their job.
I like to try simple fixes first. Tightening grounds is musician easy. Beyond that, it gets much harder.


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Post subject: Re: 68" Super Reverb Still Works With the Knobs Down
Posted: Tue Mar 24, 2015 12:07 am
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I misunderstood when you said "isolation between stages", and I took that to mean the coupling caps which isolates DC from one stage to the next.

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Post subject: Re: 68" Super Reverb Still Works With the Knobs Down
Posted: Tue Mar 24, 2015 6:25 am
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Do you have photos of the caps under the doghouse? Also, has this amp been worked on, before? Maybe, a previous tech's work was not done correctly. I've seen mis-wired tone stacks that lead to all kinds of oddball tone and volume control problems. Was the amp "hot-rodded?"

Also, this could be a grounding issue. Sometimes a potential at ground can cause a problem like this. Or a wire shielding issue.

Photos may help, Good luck! :)


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Post subject: Re: 68" Super Reverb Still Works With the Knobs Down
Posted: Wed Mar 25, 2015 6:56 am
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jnorem wrote:
A buddy of mine was playing through my amp tonight and we discovered that with every single knob turned all the way down, the amp still works. I mean there's sound coming through the speakers, quite loudly too.

Any ideas?

Defective volume , dirty = short ? Use contact cleaner
Or
Wrong pot value , any mods on it ?


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