Plexi 6V6 - setting the bias

[Big_Mike]

Hey guys -

I just finished up the Hoffman 6V6 Plexi.  I have not fired it up yet, but I know I need to set the bias.  From Hoffman's board install instructions, it looks like I need to first fire up the amp without tubes.  Then I need maximize the amount of negative voltage to pin 5 on the power tubes.  Second, I need to add the tubes, then measure the milliamps across the 1 ohm resistors going to the power tubes?  And the reading should be in the 25 milliamp range for 6V6s?  Just wanted to check before I start doing my readings.  Thanks.

[sluckey]

Set your meter to measure millivolts across the 1Ω resistors. 1mV represents 1mA.

[Big_Mike]

Thanks for the info.  Also, would it make sense to add a bleeder resistor to the circuit for safety purposes?  If so, what resistor value should I add?

[dude]

Yes, use a 2 watt 220K, from first filter cap to the cans neutral. The bleeding R will run the voltage down pretty fast, about a minute or so when you turn the amp off, but always check first.


al 

[Big_Mike]

Just to be clear, to install a bleeder resistor, would you run it from where point "A" on the layout to ground, or would it be better to run a resistor to ground from point "E" on the layout?

[sluckey]

Point A. I would physically mount the resistor on the cap can.

[Big_Mike]

Good idea, I could just run the resistor from the positive terminal to the negative terminal to ground.

[dude]

Good idea, I could just run the resistor from the positive terminal to the negative terminal to ground.

Just put the bleeding resistor on the first filter cap tab (point A), where it comes from the standby, the first cap can has two, 32uf legs and a neutral tab, a 2 watt 220K from the first 32uf tab to the neutral tab, very short distance. Of course the neutral tabs on both cans also go to ground. 

[cboysen]

For your particular build, may I suggest using a very simple heater elevation circuit instead? it will act as a bleeder resistor, and at the same time decrease hum and relieve some strain on the cathode follower - A three for one deal, only using 2 resistors and a capacitor.

/christian