Biasing tubes

[jckid649]

Im about to begin my first express build and have limited knowledge. Biasing is a subject I don't really understand and wonder is it something that you have to do the first time you build an amp or is it something that is done when you change tubes later. What exactly is it in laymens terms and how do you do it?

[xtian]

Yes, for fixed biased amps (like the Express) you have to adjust the bias to your initial operating conditions.

Biasing sets the grid voltage (where your signal enters the power tubes) relative to the plate voltage, in order to set the current that flows thru the tube.

Good reading:
http://www.mackamps.com/articles-mack-a ... amp-tubes/
http://www.aikenamps.com/Biasing.html

[jckid649]

Seems there are tons of methods for biasing. What is the favorite method used commonly by users on the forum and what are the exact steps for this method?

[jckid649]

I got this from Aiken Amps and it sounds like the safest and most accurate, but being a newbie I need to know ABC. Where does the probes go on what part or which lead of what tube and exactly what calculations and how do I make them? Most of these articles assume a certain amount of knowledge. There seems to be no step by step "for dummies" version. I don't get why when you start most of this stuff that it is so hard for these people to understand that you may have readers that simply are beginners in every sense of the word...lol

The cathode resistor method sounds simple enough but I hate the idea that I have to unsolder the ground and solder in a 1-ohm resistor every time I want to bias tubes.

"The plate current can also be measured by first measuring the resistance across each side of the output transformer primary (it will usually be different on each side) with the power off. Make a note of the resistance on each side, and then, with the amplifier on, measure the DC voltage drop across each side of the output transformer. Divide this number by the previously measured resistance, and you end up with the plate current for the tubes on that side. Again, if there is more than one tube on each side, you must divide the total current by the number of tubes. This method is extremely accurate, and much safer than the shunt current measurement method, because a slip of the probe won't short anything out due to the high resistance of the voltage measurement setting on the meter compared to the very low resistance of the current measurement setting. You can also make a safer measurement by clipping the negative side of the voltmeter on ground, and measuring the center-tap voltage of the output transformer and the voltage at the plate of each output tube. Subtract the plate voltage from the center-tap voltage and you have the voltage drop across each side, and can then use this to calculate the current in each tube, again dividing by the number of tubes on each side."

[Firestorm]

The cathode resistor method sounds simple enough but I hate the idea that I have to unsolder the ground and solder in a 1-ohm resistor every time I want to bias tubes.

The 1-ohm resistors are installed and left in place permanently. They do not affect the tubes' normal operation. Aiken's method is safer than shunting the OT, but still requires you to measure the highest voltages in the amp; the cathode resistor method is safer still because the voltage and current involved are both miniscule.

[jckid649]

The cathode resistor method sounds simple enough but I hate the idea that I have to unsolder the ground and solder in a 1-ohm resistor every time I want to bias tubes.

The 1-ohm resistors are installed and left in place permanently. They do not affect the tubes' normal operation. Aiken's method is safer than shunting the OT, but still requires you to measure the highest voltages in the amp; the cathode resistor method is safer still because the voltage and current involved are both miniscule.

Ok so in short on V4 and V5 I will remove the jumper between pin 1 and pin 8 and ground pin 8 with the 1-ohm resistor and leave pin one grounded through the 10-ohm resistor?

Here's a picture of my current set-up on V4 and V5.

[xtian]

No! You're going to leave pins 1 and 8 tied together, and you're going to connect pins 1+8 to ground using either a 1-ohm or 10-ohm resistor. If you've already got a 10-ohm resistor in place, leave it. You'll just be dividing your DC reading by 10.

[jckid649]

No! You're going to leave pins 1 and 8 tied together, and you're going to connect pins 1+8 to ground using either a 1-ohm or 10-ohm resistor. If you've already got a 10-ohm resistor in place, leave it. You'll just be dividing your DC reading by 10.

Oh ok. So I already have what I need with that current set-up?

[jckid649]

I understand the equation on how to obtain static dissipation wattage, but I cannot find an equation that tells me how they determined that you need to set your bias voltage at 38mA to achieve it. They just say, "17.5W/450 volts gives you a bias voltage setting at 38mA." But how do they come to that number?

[jeff12]

I understand the equation on how to obtain static dissipation wattage, but I cannot find an equation that tells me how they determined that you need to set your bias voltage at 38mA to achieve it. They just say, "17.5W/450 volts gives you a bias voltage setting at 38mA." But how do they come to that number?

P- ExI

E
-
I x R

[jckid649]

I understand the equation on how to obtain static dissipation wattage, but I cannot find an equation that tells me how they determined that you need to set your bias voltage at 38mA to achieve it. They just say, "17.5W/450 volts gives you a bias voltage setting at 38mA." But how do they come to that number?

P- ExI

E
-
I x R

Could I have that in English? Again, I'm a newbie and have no idea what any of that stand for....lol

[roberto]

Power = Voltage multiplied by Current

Voltage = Resistance multiplied by Current

[jckid649]

Ok I may just be completely stupid and that's fine cause I have to learn somehow but what numbers from my bias reading am I plugging into that equation?

[jckid649]

Doing the cathode resistor method the numbers I have are PLATE VOLTAGE, CATHODE CURRENT, and STATIC DISSIPATION WATTAGE. These are the only numbers I have. So how am I plugging these in to that equation to come to the BIAS VOLTAGE answer?

[jckid649]

I've read and re-read the article from Duncan amps on this method and the best I can understand is that you will take readings from pin 1 and pin 3 and multiply them. Then adjust your bias until you obtain a reading that is within an acceptable operating range for the tubes you are using. You can set it hotter or colder depending on your preference and/or your tolerance for replacing tubes...lol (So long as you are not greatly exceeding operating parameters.)