Confused with biasing.

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Confused with biasing.

Post by Mrcads » Sun Aug 12, 2018 3:31 pm

Great Sunday fellas!

I was under the impression that when you bias an amp, you set the negative current to whatever the value is on the matched set of tubes. In my case, that would be 36 mA, with the voltmeter connected to pin 3 and B+, running through a banana plug with a 1 Ohm resistor, using 6L6GC
It's all fine and dandy, but that does not seem to take in consideration the plate to cathode voltage (466VDC in my case, measured straight between pins 3-8, with the meter set to VDC)
If i enter 466 VDC in the bias calculator, that gives me 45.1 mA, meaning that the tubes are about to ice up at 36mA :D

What am I getting wrong?

PS: amp in question is a JCM900 clone.

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Re: Confused with biasing.

Post by danman » Sun Aug 12, 2018 7:46 pm

There should be no 1 ohm resistor attached to pin 3. You will need to take a direct DC voltage reading at pin 3 with your meter set to the highest scale. The 1 ohm resistor should be connected between pin 8 (cathode) and ground. If you are running el34's then pin 1 should also be connected to pin 8 and then to ground. For a bias reading you will set your meter for the lowest dc voltage scale and measure with each meter lead across the resistor. If you installed banana plugs for the 1 ohm, you will plug your leads into those for the reading. Since you are reading current across a 1 ohm resistor, 1 mv on your meter will actually equal 1 ma of current. If you are reading 36mv across the resistor then you are drawing 36ma through the cathode.

Now the numbers will vary. Any number printed on a set of tubes is just a rating number that the seller uses when matching tubes. We don't know what the number actually means because we don't know what the seller used to test the tubes. To determine a safe bias value, you need to know the max dissipation of the tubes you are using. EL34's are a 25 watt tube so we normally bias to 70% of the max. I usually like to bias slightly lower around 60% but anything between 50-70% will be fine. If you decide to use a different tube, the max dissipation may be different so you need to account for that.

You will need to take a plate voltage reading at pin 3 first. With the plate voltage and the tubes max dissipation reading, you can use a bias calculator to figure out what the 70% max should be. With EL34's and a 450v plate voltage, 38-40ma would be about 70%. You can bias lower that that but you really should not go much higher. Keep in mind that when you raise the current draw of the tubes, the plate voltage will fall slightly. You will usually need to measure plate voltage a couple times and adjust your calculations as you raise the current draw with the bias pot. This calculator makes it pretty easy to do the numbers quickly. I usually use the one at the very top of the page...

Just to confuse matter more, there is also a thing called bias voltage. This is the negative dc voltage that you will read at pin 5 of the socket. It is usually between -40vdc and -50vdc. This is the voltage that your bias pot is adjusting. When you raise the voltage, the tube draws less current. When you lower the voltage with the pot you cause the tubes to draw more current and run hotter. You most likely will not need to measure this when doing your bias setting unless there is an isuue when biasing. This voltage should be checked when doing your startup voltage readings and you should set the pot for the highest negative number you can before inserting tubes for the first time.

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Re: Confused with biasing.

Post by RockinRocket » Sun Aug 12, 2018 7:57 pm

466 Volts is the measurement you need to consider with a 1ohm resistor to ground pin 1+8.

El34 make 25W each

To bias at 70%-
x .7 by 25w = 17.5
17.5 / 466 = .037 or 37Ma

To bias at 50%-
x.5 by 25w = 12.5
12.5 / 466 =.027 or 27Ma


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