Bias Measurements 50 mA=500mV

[chris1973]

Ok, so it's really that easy?(so easy that I can't figure it out myself apparently!)
380=38x10

Thanks!

[David McGown]

Ohm's law is the most important thing to know when fiddling around with electronics. It is the basic equation for setting bias, for determining voltage drop across a resistor (such as a cathode or plate resistor, or a resistor/choke in a power supply). Anything dealing with DC parameters of a circuit. Know it, it is your friend.

Power law is the other important equation, P = VI (Power (Watts) = Voltage x current). This will let you know how much power you are dissipating across a resistor, tube, etc. Very important for selecting components such a resistors (always select a resistor rated approximately 2 times or more higher that the power dissipation) and for knowing how much you can push that output tube to keep within rated plate dissipation.

Master these equations, and you can deal with 90% of all tube circuit and power supply related design, bias and component selection.

David

[SoundMods]

David hit the electronic bulls eye!

[Pelliott321]

I can confirm what Walt is talking about.
Walt suggested two things which made dramatic and positive differences on my system when he came over for a listen. I had ASL Monsoons driving my Maggy's. He suggested that I reduce the bias from "Manufacturers recommended" 50 ma down to 38 ma (the previous owner ran the amps at 65 ma and replaced the 4 kt88's every year, and swore that the amps sounded best that way) and move the speakers from the "recommended" 4 ohm output to 8 ohm output.
The sound just opened up, far more musical and enjoyable
Since every room and every system is different one has to experiment a little to get the best sound for you. The only rule is YMMV,so try different settings and listen to the changes

[tomp]

Ohm's law is the most important thing to know when fiddling around with electronics. It is the basic equation for setting bias, for determining voltage drop across a resistor (such as a cathode or plate resistor, or a resistor/choke in a power supply). Anything dealing with DC parameters of a circuit. Know it, it is your friend.

Power law is the other important equation, P = VI (Power (Watts) = Voltage x current). This will let you know how much power you are dissipating across a resistor, tube, etc. Very important for selecting components such a resistors (always select a resistor rated approximately 2 times or more higher that the power dissipation) and for knowing how much you can push that output tube to keep within rated plate dissipation.

Master these equations, and you can deal with 90% of all tube circuit and power supply related design, bias and component selection.

David

My most important law when dealing with electronics is don't let your body complete the circuit. Ohm's law is right behind that. This wheel helps explain.

Tom

Attachments

Ohms-Law-Formula-Wheel.png (8.19 KiB) Viewed 20874 times

[dberning]

Higher bias current=hotter tubes=shorter life and less stability. Less stability=more difficulty maintaining core balance in transformer=less stable sound quality. Use the lowest bias current that you can get away with while maintaining acceptable sound quality, otherwise you will be chasing the bias settings every time you listen because the sound quality will be all over the place. My EA-230 used only 3 mA of bias current although I tend to use whatever I need now to have idle dissipation in the tubes of five to ten watts in most cases. This applies to push-pull designs only.

David

[SoundMods]

So -- several of us are on the same page here and it makes one wonder why so many proclaimed "experts" call for heavy bias. I've read where 100-ma is recommended for KT-88s. YIKES!!

My view is that the heavy bias crowd needs a "band-aid" for systems that have issues with hard/sharp reproduction characteristics of other components and interconnects in the signal path. They need to calm things down to get that "tube warmth."

If everything in front of and after the power amps. can get the job done -- dialing in the bias (lower-not higher) and trying different output taps can provide one with audio icing for that audio cake.

[chris1973]

Thanks!

I have some of these formulas written down.

Sometimes I forget to turn to them before I ask a question.

Just one more question that might be silly, but it's the actual question that instigated my post:

Can the Cathode current be measured directly, or is it just reliably predicted by the voltage drop across the resistor, and as so, you can safely assume it's as you intended?

Chris

[SoundMods]

You can measure it directly with a multi-meter. The meter probes would be placed in series with the cathode and ground.

Or -- you can buy a bias meter such as that offered on eBay:

http://www.ebay.com/itm/Quad-Tube-Amp-B ... SwZd1VcVnP

I have one and it was invaluable. You can read the bias setting and listen to music through the adapters. It makes dialing in the bias for the best sound quite easy.

[chris1973]

So, you can tune the bias as you are listening?

Amp manual says you set the bias with volume all the way down and no input signal.

Does the bias meter allow you to do differently?