Bias ground resistor value?

[Tdale]

It says 15K on the schematic. I have two pots, one for each tube, and I was told to half the value. I only had 10K resistors, and put them in.

Is there any danger in trying the amp with those two 10K resistors on the bias pots?

This is my first build with adjustable bias... how should I adjust the pots when I plug it in for the first time, to be sure nothing bad happenes?

Tommy

[Ed de Jager]

Maybe this will help
http://www.duncanamps.com/technical/lvbias.html

Greetings,Ed

[Tdale]

Thanks.

As for the 10K resistors... I'll just assume that they are a ok value...

Tommy

[Bob-I]

This is my first build with adjustable bias... how should I adjust the pots when I plug it in for the first time, to be sure nothing bad happenes?

Tommy

Your best bet is to adjust the bias voltage to a reasonable value. You didn't mention the amp this is for but for a 6L6 amp I'd go for about -40 to -45 to start. The first thing I do when I power up with tubes is measure the plate voltage and bias.

[Tdale]

Ok.. so I won't risk anything blowing up if it's adjusted wrong from the start..

When you say -45V that's the bias voltage right, not the value measured across the 1 ohm resistor from cathode to ground?

Or is the bias voltage equivalent to the reading I get when I measure across the cathode resistor...

Like if I measure 45 mV (which equals 45 mA) over the 1 ohm cathode resistor, the bias voltage will be -45V?

I thought there wasn't a connection like that... but am I right in assuming that a reading across the 1 ohm resistor, of about 45 mV would give me about 70% of the max output of the tube, and put me in the right spot?

It's a hybrid-a build.

Tommy

Tommy

[Tonegeek]

Forget about the 1 ohm resistor for the moment. Pull the power tubes and turn the amp on. Then measure the voltage at the wiper of your bias pots. You may need a range of -38 to -62 to set bias for a 6L6 so turn the pot both ways and measure the values at each end of travel. If you can't get the proper range, you will have to monkey with the resistor values until you do. If you are close on the range, then set it to the maximum negative value (-60 volts or above). Turn the amp off, plug in tubes. Now- amp back on, out of standby and set the bias using your 1 ohm resistor and the millivolt setting on your volt meter. a common voltage across the resistor would be 35 millivolts to get you in the ballpark but err on the side of lower values till you get the hang of it. I think there is another post earlier about duncanamps.com that will explain how to do the actual adjustment.

[Tdale]

Thanks! Very informative.

I'm always afraid the thing will blow up, or start smoking when I turn it on..due to a wiring mistake.. But I guess that that's pretty rare, as long as there is a speaker connected. I went over all the groundes, and compared to the schematic, and I have all ground connections in place.

Do I just pull the power tubes, or do I pull the preamp tubes too, for the first bias voltage check?

Tommy