I have a tube amp that had one bias pot for two tubes. The bias was recommended to be 125ma. Does that mean that each tube was bias 62.5ma? If a output tube is rated for much higher bias, will increasing the bias up from the amplifier manufacturer recommended value make the sound better or worse?
Jim, it's complicated. That type of circuit expects the tubes to be balanced or close in electrical specs. If they are, then biased for X means each is X/2. But if they drift over time or weren't matched to begin with... you can get very mismatched bias (and sound). Maybe one is pulling 70% and one is 30%? Changing bias has many implications... sound, tube life, death of the amp, operating point and sound quality... and in 99% of cases without you being a very senior engineer knowing why you are changing... is to be avoided. I think you are asking is individual tube bias better, and the answer is yes.
Thanks for the information. What I would like to know is who or what determines how much bias a tube should have. The amp I have is a mono that has two 8417 output tubes. The tube specifications say that two tubes will output 100 watts. My amp is rated at 60 watts. The specs indicate that the tubes can be biased way above the amp manufacturers recommended value. So is amp designed recommending the bias for performance, tube life sonic sound or something else?
>What I would like to know is who or what determines how much bias a tube should have.
Oh, it's not a single thing. It's more like a balance between multiple aspects of the circuit and the tubes capability.
Plate dissipation. Runs Class A up to a certain power level, crossover distortion. Power supply current and ripple regulation limits. Screen grid. Output power. Thermals. Hopefully a point was picked where the amp goes loud enough, sounds good, the tubes last a reasonable time and it doesnt run so hot that other components begin to fail.
So when you turn that single control, imagine 5 - 6 other "dial indicators" are all moving in response. You should probably refit it so each tube's bias is controlled individually 🙂
Oh, it's not a single thing. It's more like a balance between multiple aspects of the circuit and the tubes capability.
Plate dissipation. Runs Class A up to a certain power level, crossover distortion. Power supply current and ripple regulation limits. Screen grid. Output power. Thermals. Hopefully a point was picked where the amp goes loud enough, sounds good, the tubes last a reasonable time and it doesnt run so hot that other components begin to fail.
So when you turn that single control, imagine 5 - 6 other "dial indicators" are all moving in response. You should probably refit it so each tube's bias is controlled individually 🙂
Getting answers about the bias of an unspecified amplifier, and without a schematic . . .
Is like being blindfolded, spun around, and shooting arrows at a target. Where is the target anyway?
Help us help you.
From the original post, it is most likely that you either need very well matched output tubes, or else you need to modify the amplifier to make it more reliable and work better if the output tubes are not well matched.
(matched tube requirement already mentioned in Post # 2, and complicated issues are mentioned in Post # 5).
Please post a Schematic if you can.
Or, at the very least, give the model number, if this is a commercial amplifier, not a hobby amp.
Just my opinions.
Is like being blindfolded, spun around, and shooting arrows at a target. Where is the target anyway?
Help us help you.
From the original post, it is most likely that you either need very well matched output tubes, or else you need to modify the amplifier to make it more reliable and work better if the output tubes are not well matched.
(matched tube requirement already mentioned in Post # 2, and complicated issues are mentioned in Post # 5).
Please post a Schematic if you can.
Or, at the very least, give the model number, if this is a commercial amplifier, not a hobby amp.
Just my opinions.
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Normally, you won't measure current directly when setting bias. You measure the voltage drop on the cathode resistor. Once you get voltage, and the resistance, you get the current. In a great chance, the cathode resistor is shared between 2 output tubes. Thus the current on the resistor will be the total current of 2 tubes combined.
Then you need to solve the question that 125mA is for per tube or for that resistor. Please provide us the output tube model. I believe lot of guys here can give you the answer based on the tube.
Then you need to solve the question that 125mA is for per tube or for that resistor. Please provide us the output tube model. I believe lot of guys here can give you the answer based on the tube.
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tube bas
The amplifier is a Quicksilver 8417 mono using Sylvania 8417 output tubes. I recently had the amps modified to bias each tube separately. My question about tube bias reading for each tube instead of two tubes together has been answered. I was just confused about what the tech said about the bias reading. He set the amps up so I am now reading mill votes instead of mill amps. So I think I have that straight. But I have always wondered why Quicksilver would rate the amplifiers at 60 watts when the tube specifications say that two 8417 tube are capable of producing 100 watts. And the tube bias could be much higher. So would pushing the tubes to max watts and bias be good, bad or make no difference. After all most people think higher watts is good. I personally think the watts rating can be whatever the manufacturer wants it to be. You can buy a automobile amplifier the size of a cigar box rated at 1000 watts. Pardon my ignorance for asking what may sound like a stupid question.
The amplifier is a Quicksilver 8417 mono using Sylvania 8417 output tubes. I recently had the amps modified to bias each tube separately. My question about tube bias reading for each tube instead of two tubes together has been answered. I was just confused about what the tech said about the bias reading. He set the amps up so I am now reading mill votes instead of mill amps. So I think I have that straight. But I have always wondered why Quicksilver would rate the amplifiers at 60 watts when the tube specifications say that two 8417 tube are capable of producing 100 watts. And the tube bias could be much higher. So would pushing the tubes to max watts and bias be good, bad or make no difference. After all most people think higher watts is good. I personally think the watts rating can be whatever the manufacturer wants it to be. You can buy a automobile amplifier the size of a cigar box rated at 1000 watts. Pardon my ignorance for asking what may sound like a stupid question.
Just because the tubes are capable of putting out 100wpc in push-pull doesn't mean the designer built the amp with that in mind. It depends on the design, the way the tubes are employed, the limits of the output transformer, distortion levels, etc. In this case the designer uses the tubes conservatively. If it's an ultralinear design, that would also limit the way the tubes are employed. It would not be wise to exceed the manufacturer's design targets.
Lower distortion. Improved life. Lower total cost (who NEEDS a hundred watts??).
"Nobody" really got 100W out of a pair of 8417. They were developed for Fisher who rated at 75W/pair. Even Bogen, who ran tubes hard in PA duty, rated 75W/pair of 8417 (at significant THD). There were guitar amps with the "100" number but unstated high THD and that market rounds-up. When I hot-rodded my Bogens I got a solid 90W/pair reliable for concert sound at several % THD.
Bias has almost NOthing to do with maximum power. One is like a car's engine idle speed 650RPM, the other is like top speed 113MPH. Jacking the idle RPM up or down has zero effect on top speed; the throttle is nowhere near the idle stop screw when going fast.
Tube amp idle current is, as jjasniew and others say, a compromise between burning-up in light duty and sounding hoarse (crossover distortion) when idled cold. Each case is different. Some amps sure sound best, for soft sounds, with the plate glowing bright (for a day).
You want plate voltage also. Say 400V, at 62mA, is 25 Watts heat at idle. While the tube may be rated for 35W(?) dissipation, it is best to keep a wide margin, especially on HIGH-gain 8417.
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Tube bias
Thank you for your reply. I have learned a lot from my question about tube bias. I am not very audio savvy. I just know what sounds good or bad to me. Thanks everyone for your help.
Thank you for your reply. I have learned a lot from my question about tube bias. I am not very audio savvy. I just know what sounds good or bad to me. Thanks everyone for your help.