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In the Forum: Melquiades Amplifier
In the Thread: Bias help
Post Subject: You are OK.Posted by Romy the Cat on: 4/12/2018
Gintaras, basically it is not bias voltage problem but voltage drifting +-10mV with preamp disconnected. The 10mV is absolutely irrelevant voltage. You see, the gas tubes are voltage stabilizers but they are not as precise as the SS stabilizers. They stabilization is greatly contingent of the current you drive through them and even at optimum current of I think 15-25mA it is still not perfect. Also, it would be different if the gas tube not broken in as well as wary from tube to tube. If you have stable voltage when the preamp is connected then I do not think you need to worry about it. I do not know what preamp you use. Some DC preamps has DC offset secretly at outputs that might kill your drifting. What you need to understand is that when the preamp is connected and the left side of the grid bias resistor is shorted to ground by preamp’s output impedance then the positive bias become irrelevant. In fact you might even pull the positive gas tube out and shot down the positive supply. Do not disconnect the preamp wires while you are doing this as it will send a huge pile to you al and will blow your speakers. So, the positive supple is what you need only for amp to work stable in disconnect more. You can even not to measure the voltage at the amp’s input. Make it somewhere near 0 and then plug in and out the cable from your preamp. You will hear a characteristic clicking from speakers. Then adjust the VR1 to the point when connecting and disconnecting preamp will produce no clicks. 
 
The voltage on the input creeps because your positive and negative supplies are discharging with different rate. Hypothetically, you can adjust it by bleeders R13 and R14 (be careful as they set the minimal current over the input chokes) but I find it absolutely no necessary. The voltage at input creeps only with disconnected preamp and it is absolutely irrelevant what voltage is there if the circuitry is not complete. 
 
The measuring the gas tube current draw you depicted correct, the current is set by VR and R9 resistors. I do not think you should worry about it. If I am not mistaken it should be 15-25mA but I know that the schematic shall be well calculated. If you would like to be very anal about it then you can fix for your type of gas tubes the best current, when it produce the best regulation, but I would not do it for positive tube as the positive supply is less relevant. You need a gas tube there to have “soft buffer” that is about it. 
  I think you are in good shape and if I were you I would not worry about anything.

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