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Topic: Power tube question |
Paul Sutherland
From:
Placerville, California
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Posted 3 Oct 2016 4:28 pm |
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My Twin has been sounding like crap. I suspect the power tubes. I checked the bias and they read quite low. I can't adjust them any higher, the adjustment is already at the max. Is that normal for power tubes that are ready to be retired?
PS: New power tubes have been ordered, plus the phase inverter tube.
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It don't mean a thing if it ain't got that swing. |
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Brad Sarno
From:
St. Louis, MO USA
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Posted 3 Oct 2016 5:09 pm
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when you say the adjustment is at the "max", what do you mean?
B |
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Paul Sutherland
From:
Placerville, California
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Posted 3 Oct 2016 5:28 pm
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I can turn the bias lower but I can't turn it any higher, and it's not high enough.
_________________
It don't mean a thing if it ain't got that swing. |
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ajm
From:
Los Angeles
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Posted 4 Oct 2016 6:50 am
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What do you mean when you say it "sounds like crap"?
Both channels or just one?
Did this come on all of a sudden, or has it been deteriorating over time?
Bias: How are you checking the bias?
What does it read? |
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Paul Sutherland
From:
Placerville, California
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Posted 4 Oct 2016 8:45 am
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I'm using a VHT bias meter per their instructions. The highest reading I get is 24 milliamperes, and one tube is down at about 20. The tubes were a matched quad and they used to read around 30, which is what the amp tech set it at and it sounded wonderful.
I haven't tried the normal channel as I only use the vibrato channel for the reverb.
The sound quality has been declined over time. This is not a sudden change.
When I say sounds like crap, notes seem to die out (loss of sustain), and a loss of sweetness in the high end. The amp just doesn't impress me like it use to. Kind of hard to describe.
_________________
It don't mean a thing if it ain't got that swing. |
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John Dowden
From:
Louisiana, USA
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Posted 4 Oct 2016 11:57 am
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| Just out of curiosity, how old is your twin? I'm assuming you keep it up to date on service and all. Have you tried changing out the V2 preamp tube just to see if that makes any difference? |
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Paul Sutherland
From:
Placerville, California
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Posted 4 Oct 2016 6:13 pm
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The twin is a 1970 or 71 and has been well serviced in the couple years I've owned it. The power tubes were installed new when I got the amp. The V2 is probably less than a year old.
My question really is do power tubes steadily go down in output (milliamperes) as they age? I think the answer is yes, but I'm not sure.
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It don't mean a thing if it ain't got that swing. |
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Carl Mesrobian
From:
Salem, Massachusetts, USA
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Posted 4 Oct 2016 7:01 pm
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I think that AJM's idea of trying both channels is a good suggestion; it determines if the problem is in a preamp stage.
Changing tubes is a good, but expensive, first step.
If new tubes don't help, at least you have some spares. If the currents on the tubes are still low, then I'd check the voltages and condition of the capacitors in the power supply and bias supply, and values of grid resistors. Troubleshooting it depends on what circuit (AB763, AA270, AC568, ...) your amp has.
If the new tubes help and all 4 6L6's idle currents seem within tolerance, but the amp still sounds funky, then it could be a cathode bypass cap, since that will give a "dry" sound to the signal.
Other than tubes my experience has been that the most likely components to fail are the electrolytic caps (power supply, bias supply, cathode bypass), grid resistors, and power supply resistors.
EDIT - just saw your newest post. How many hours are on the tubes? When were the electrolytic caps changed?
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--carl
"The better it gets, the fewer of us know it." Ray Brown |
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Paul Sutherland
From:
Placerville, California
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Posted 4 Oct 2016 7:12 pm
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All caps replaced when I got the amp about two years ago. I've played the amp a lot at home and on stage. I doubt the caps are bad. More work done on the amp about a year ago including replacing some resistors and some other things. That's all I know. A reputable local guy did the recent work, Uncle Spot in Lodi (that's the name of his business).
I still want to know: Do power tubes steadily go down in milliamperes output as they age? Stated another way: When you re-bias an amp with it's existing tubes are you almost always having to increase the milliamperes (assuming the amp was properly biased the last time it was biased)?
If the answers to those question are yes, then my old tubes are highly likely to simply have exceeded their useful life. If the answer is no, then there may be something else wrong.
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It don't mean a thing if it ain't got that swing. |
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