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In Reply to: importance of biasing tubes posted by ooocella on April 11, 2007 at 08:33:50:
i just return the bias, ac and dl bal settings to what appear to be "factory default" since it is sort of the medium setting and is marked. Sound is good but I am trying to tweak it by ear with headphones. The bias of the power tubes seems fine, now im messing with ac bal and dc bal...
Follow Ups:
Do some research on what you have. Read the owners manual if you can find it somewhere online. It should tell you the basics of how to set balance and bias. There are some real experts, not me, on most of the vintage amps in this forum if you let them know what you have.For balance and bias on most amps: all you really need for most amps is a relatively cheap DMM with ac and dc voltage reading. Most amps designers/manufacturers make it easy to do bias and balance by reading the voltage drop over specific resistors. Most even provide pins on the side of the chassis for your convenience to attach your meter leads to. The owners manual usually gives a designed range for the bias setting.
I don't know your skill or knowledge level, if any of that seems confusing then you need to bring it to a tech or do some quick learning. But be careful, the capacitors can store a lot of potentially dangerous energy, some even hold the charge for a while even when the unit is not plugged in, that can be discharged through you if you are not careful.
thanks,I do have a radio shack multimeter but the manual is in Japanese and I would have to special order it from Luxman. I do see the pins though.
I can always bring it into my tech but would rather avoid it.Right now I put all 6 dials for bias, ac and dc balance and the middle setting. The sound is OK but for some reason the bass has become flabby. I'm going to try to adjust by ear a little now if I can.
i think adjusting by ear for the other settings besides bias as harder to hear (?)Anyway, I'd love to do this on my own but as the manual is not that easy to get I suppose I should bring it in? I care about the amp and tubes and don't want anything to get damaged.
Not sure what to tell you. My heathkit W-5M manual is easy to get online and it has balance adjustment. YOUR AMP MAY BE DIFFERENT! In the W-5M the balance is adjusted to zero volts to compensate for differences between the two KT66 output tubes and the bass distortion is minimized. The voltmeter is just connected to the two balance connectors on the chassis and the pot is adjusted to zero volts. This is usually done after the amp has warmed up for a few minutes.
One note: if your amp is vintage, there may be some components that may need to be replaced. Old capacitors in the power supply are especially known to fail. If that is the case and you cant do the repairs yourself, make sure that you get a tech that knows your amp and is known for doing good work. I have heard of guys getting their amps messed up by sloppy techs.
If your manual has pictures and schematics you can figure out if your bias is measured as a voltage drop across a resistor. The bias current is equal to the voltage you measure divided by the resistor value. If you have two sets of bias adjustment pots then you most likely have bias adjustment for each EL34 and that is good. Set each voltage reading to the same number for the bias pots if you have two. If the bias current is too high then you will overheat the tubes and shorten their life. Really depends on the amp design but at least keep them lower than the top range for EL34, go look up the data sheet for your EL34 and get the safe operating range numbers for it. Problem is that you need to know what your plate voltages are in order to know the power through your tubes. The power, voltage x current, is what will heat them up real fast.
In some vintage amps like my Dynaco Mark 3, the power supply has dropping resistors that depend on a certain amount of bias current in the output tubes to keep the power supply caps below their maximum operating voltage, the workaround for that is a redo of the caps in the power supply to handle higher voltages, so biasing SOME amps too low to protect the tubes and you could run into trouble. So much depends on your amp circuit design to know how it was designed.
This is about as far as I can take you without seeing the schematic. There are others here that know much more and may have experience with your amp.
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