Hum in an amplifier....

B

bubs_42

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Will an amp hum of the bias is off? I just recieved an amp and unfortunetly the The footswitch was packed inside the back of the combo cabinet. Completely took out the 1 of the power tubes. So I ran out and picked up a set of JJ6v6's because the local store didn't have the stock EH6v6 and I just swapped them out without bias to make sure everything was okie dokie but I have a hum. Just need biased? everything else appears to be unscaved I pulled the chassis and other then dumping out glass no wires appear to be broken and the amp is built damn good.

Oh and I changed ALL the tubes event he preamp tubes. I also switch the stock pre tubes back in and still the hum is there.

Thanks in advance!

 
I am no expert - maybe one of the big dogs will chime in...

In my experience, hum is usually a bad filter cap. I *did* have a few amps that would not bias correctly due to a bad filter cap that had hum, but that's a different problem... /emoticons/default_smile.png

Depending on the amp, filter caps are generally easy to pull and easy to test if you have an analog ohm-meter:

1> Unplug the amp

2> Drain the voltage stored in the caps

3> Remove the cap

2> De-charge the cap (short a wire across both leads - shouldn't be needed if #2 was done, but do it anyways...)

3> Attach an analog ohm meter + to + terminal and - to - terminal

4> Meter should swing to the right quickly and then back to the left more slowly

Normal warning about high voltage in amplifiers even when turned off, etc, etc.

Or - try to bias it first, and if it still hums, send it back. /emoticons/default_smile.png

Hope that he'ps.

Oh - a bad ground can cause hum as well. /emoticons/default_smile.png

 
To me its sound like ground hum but its really consistant. I'll start with biasing then chopstick connections if that doesn't work the seller and I will work something out.

 
veatch said:
Oh - a bad ground can cause hum as well. /emoticons/default_smile.png
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bubs_42 said:
To me its sound like ground hum but its really consistant. I'll start with biasing then chopstick connections if that doesn't work the seller and I will work something out.
Click to expand...
Bubs, is this amp an old (as in original handwired vintage design) unmodified Deluxe Reverb or Princeton Reverb, either Silverface or Blackface? They both came from the factory with polarity switches for the ground, located on the back of the chassis. Just a thought, as I don't know how 'modern' the amp you've got is.

 
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No its a 2013 model, I will say this Fuchs is second to none on customer services. Answered all my questions and offered ideas to correct the hum. I'll get working on it tonight, its a great amp, i'm sure its nothing major the amp is the cleanest amp i've opened up. I was just too busy yesterday to tackle it.

 
I was hoping to get this recified last night. I have the amp biased correctly now, checked all screws and chop sticked (Tapped) on everything but the hum is still there.

 
Ha! No, after biasing the amp with the new tubes the hum didn't go away but the amps tone and balance is back in order. Sounds great and I was pretty surprised that I could really push it without it being ear bleeding loud. Hope we can get this sorted out pretty quick it does sound damn good and I would like to try other speakers but i'm holding out doing anything until the issue is resolved.

 
veatch said:
I'm still betting on filter caps...
Click to expand...
I'm there with you it has to be a component issue, i'm just hoping not the Transformer. The seller is taking care of the shipping and repairs and has been a true gentlemen through the whole process. I am nudging him to take it up with UPS.

Now back to tone, I was not happy with it at all but after the bias was taken care of the bottom end came around and everything tightned up. I brought into work today and let it rip and man it sounds awesome and for 20 watts its loud and proud. I got a great deal on it so even if down the road I pass it on the next guys going to get a deal as well.

 
Preface 1: I know nothing of Fuchs amps.

Preface 2: you likely already know this.

Is this a Class A amp? They're often a bit noisier than their AB counterparts.

Just something that likely isn't the case but occurred to me nonetheless. /emoticons/default_wink.png

 
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Nope,running AB with a pair of 6v6's. With all controls turned to zero and the amp on it should be pretty much dead quite.

 
Does the him change with vol controls? If not, it may be another indication of a bad filter cap...

 
With all knobs at zero and turning one at at time up and down the Gain is the only that makes any difference but i'm sure its adding hiss and not hum.

 
Yes, but it is properly biased now. I receive an RA# from Fuchs so i'll let them deal with it and report back so we can all learn something or prove one us correct.

 
veatch said:
I am no expert - maybe one of the big dogs will chime in...

In my experience, hum is usually a bad filter cap. I *did* have a few amps that would not bias correctly due to a bad filter cap that had hum, but that's a different problem... /emoticons/default_smile.png

Depending on the amp, filter caps are generally easy to pull and easy to test if you have an analog ohm-meter:

1> Unplug the amp

2> Drain the voltage stored in the caps

3> Remove the cap

2> De-charge the cap (short a wire across both leads - shouldn't be needed if #2 was done, but do it anyways...)

3> Attach an analog ohm meter + to + terminal and - to - terminal

4> Meter should swing to the right quickly and then back to the left more slowly

Normal warning about high voltage in amplifiers even when turned off, etc, etc.

Or - try to bias it first, and if it still hums, send it back. /emoticons/default_smile.png

Hope that he'ps.

Oh - a bad ground can cause hum as well. /emoticons/default_smile.png
Click to expand...
Swung by the local shop today to just have them give it a listen before I shipped it to Fuchs just because I didn't want to make an ass out of myself if I was making something for nothing. I turned it on and I swear before it came all the way off of standby they shoots out........ "Cap definitely a cap". I said are you sure and he said definitely and a very minor instance. He said usually its much louder.

 
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