1 Ohm Bias Resistors

[clyde]

I'm having a brain fart here.  I want to install resistors on a quad of outputs, having 2 bias pots, one per side.  I'm thinking I should install one resistor per tube and then one to ground from the junction of each resistor pair.  Measuring from that junction to ground (mv), what will I be measuring?  What is the multiplication factor?  I know it's direct from each cathode with only 1 ohms to ground but I'm getting muddled up with the extra resistor.  Maybe I don't need the separate cathode resistors and could just combine the two cathodes to one common 1 ohm to ground which I know would give me the reading per side but I thought with the extra 2 per side I could calculate each tube's dissipation in case I needed to move them to balance.  Clear as mud?  It is to me!

[shooter]

this may just muddy it more, I would think each tube gets a 1 ohm, measure with 2 meters since you are adjusting parallel pairs but you will see how well those pairs are matched, then adjust for a happy compromise on each pair.  I've only done 1 quad n it was SE but I gave each it's own R so I could tweak the mis-match.

[HotBluePlates]

I want to install resistors on a quad of outputs, having 2 bias pots, one per side.  I'm thinking I should install one resistor per tube and then one to ground from the junction of each resistor pair.

I would simply give each tube its own 1Ω directly to ground. The value is in being able to measure each tube's cathode current independently.

Measuring from [EDIT:each cathode] to ground (mv), what will I be measuring?  What is the multiplication factor?

x1.

Ohm's Law: Voltage = Current * Resistance.

You want to know the tube's cathode current, but measuring current directly requires you to break the circuit and use an ammeter's leads to complete the circuit so that circuit current flows through the meter. Or you have to use the transformer shunt method, requiring power off/power on with clip-on meter leads, or risk arcing as you probe the highest voltage in the chassis. Or you need a specialized current clamp to place around a wire carrying the current to be measured.

So the value of the 1Ω resistor is that you're replacing "Resistance" in the Ohm's Law equation with "1" so that milliamps of current through the resistor directly translate to millivolts measured across the resistor. No specialized equipment required, and no high voltage to probe.

So don't add any additional resistors between the 1Ω resistor and ground. Unless the amp is cathode biased, in which case you have the bias resistor to ground (but in this case you still only measure across the 1Ω resistor and nothing else).

Wattage Rating:
I don't know why people get carried away with thinking they need a high-wattage 1Ω resistor. One form of the Equation for Power is Power = Current² * Resistance. Decide on a maximum safe sustained current for your output tubes. 100mA? 150mA?

Let's say you figure you'll never idle the tubes as high as 100mA. 100mA² * 1Ω = 0.1A² * 1Ω = 0.01A * 1Ω = 1/100th watt.

So any size resistor will work, the smaller the better. You'd probably prefer that resistor burned open before your output tubes melted. You also probably won't find a practical resistor small enough to act as a fuse, so whatever's cheapest should work fine. The only downside to small resistors is the difficulty of manually handling them.

[P Batty]

Doug's one ohm one watt resistors are great- very tight tolerance and very small, they help keep your boards tidy.

[clyde]

Thanks guys, I just I should elaborate more.  I want to have only 2 pots, one per each pair of tubes (stereo amplifier) so that I can balance the sides.  Is this possible?  Yes, the shunt method is my usual bias method on guitar amps but this baby is a little different.  As well, only 2 test points.   

[sluckey]

First you say "quad of outputs" and now you say "stereo amplifier". A schematic would clear up a lot of things at this point.

[clyde]

My bad, it is a fixed bias push-pull stereo amp. 

[sluckey]

My bad, it is a fixed bias push-pull stereo amp.

You still only need 1 resistor per tube. This will allow you to set the bias for the pair of tubes in the left channel and likewise for the pair of tubes in the right channel. Using a resistor for each tube in a pair will allow you to see how well those tubes are matched. Talking about only one channel.

There is no reason to try and match the bias for the tubes in the left channel to the bias for the tubes in the other channel although it will probably work out that way just because the two channels should be identical. If your reasoning is to match the power (or sound level) of both channels just set the balance control to midway. May even be a detent.

[clyde]

Thank-you Steve.  It's an old Eico that I have gutted and am in the process of restoring and installing fixed bias.  I just couldn't get the old noodle around the biasing for each side.