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Bias troubles and "ungrounded" amp buzz around inputs.

T

TallestCrayon0

Guest
Jun 29, 2015
41
8
34
Montréal, QC
#1
I have been having trouble with my bias on an SVT classic head. The 1st side light is fine, I can get it green but the 2nd one goes from being unlit to bright red after a certain point, no green to be found. What should I do? The amp sounds fine to me, it does have consistent hum to it but I figured that's normal for a big power tube amp. Also when I touch the head near the inputs (0dB & -15dB) or the top of the head on the input's side it's as if I was touching the end of a plugged 1/4" jack. Sound heard through speaker. Is this normal? I didn't notice this at my group's loft but when I brought it home I noticed it immediately. Can the location make a big difference? Switching the polarity switch in the back made it so that the unplugged wire effect was slightly diminished but then I could no longer get a green light for the 1st and still couldn't for the 2nd. The tubes have a slight orange glow. I'm new to Amp/cab rigs, i've been using combo amps.

Thank you!!
 
#3
BassmanPaul said:
There always is a residual hum level on a tube amp but it should not be noticeable a few feet from your speakers. As it appears that you have no experience with tube electronics may I suggest a trip to a tech. It sounds to me that you are having some output tube problems.
Click to expand...
Yes, but before I take the easy and costly way out I want to learn about it! Thanks for the info! The hum sounds fine except when I touch the inputs or the top left of the head it gets louder, almost like it's poorly grounded
 
#9
There are two triangles of output tubes on the SVT-CL. Swap the left three and the right three tubes, if you now can't bias the first light and the second biases properly the trouble has followed the output tubes. One triad of tubes has gone out of range> If the trouble doesn't move and the second still won't bias up then new tubes will be a waste of money and only a tech can help you (and keep you from damaging output tubes).
 
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#10
As for the hum. Go to a hardware store and buy an outlet tester. They cost under $10, plug into the outlet, have three LED's. This will test your home wiring, or wherever you use your amp. It is a good inexpensive tool to have.

The hum could also be caused by a bad ground at the input. Are the jacks loose? Does it do this when the instrument cable is not plugged in?
 
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#11
B-string said:
There are two triangles of output tubes on the SVT-CL. Swap the left three and the right three tubes, if you now can't bias the first light and the second biases properly the trouble has followed the output tubes. One triad of tubes has gone out of range> If the trouble doesn't move and the second still won't bias up then new tubes will be a waste of money and only a tech can help you (and keep you from damaging output tubes).
Click to expand...
Great idea! Thanks! After switching the polarity switch to diminish buzz I wasn't able to achieve a solo green light on the first triad anymore. I would get a green then red immediatley. I will try again and see if it is truly a tube or bias issue. Great thinking B-String!
 
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#12
beans-on-toast said:
As for the hum. Go to a hardware store and buy an outlet tester. They cost under $10, plug into the outlet, have three LED's. This will test your home wiring, or wherever you use your amp. It is a good inexpensive tool to have.

The hum could also be caused by a bad ground at the input. Are the jacks loose? Does it do this when the instrument cable is not plugged in?
Click to expand...
Also a fantastic idea, thanks for the tip! Yes, it only does it when there isn't a 1/4" plugged in! On both inputs. It seems like a ground issue, if I touch the head on the side that the ground wire is screwed to on the inside I hear a change in the buzz.
 

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