Regarding your bias problem. This is normal and it is very easy to deal with. Short the amplifier input jack. You should have about minus 3.4V at the driver tube grid or about 200V on plate. That would be depends the quality of your tube and if you have 185-215V on plate then it is still OK. Your bias is set by negative (green) supply chain and resistor R6. The positive supply chain (red) is just compensates the same voltage that negative cleat on the left side of the resistor R5. This is bias resistor and keep it exactly as it is (unless you build filters at input). The values that are given in the schematic are very accurate but they presume that you use gas tubes that give 150V. In reality they never do. Some of them stabilize voltage at 148, or 149, or 151 volts. They are not bad tube, this is how they works normally. So, you might have a positive gas tube give 151.6 and negative gives 148V and then you do not have enough voltage on positive side to set zero at input. Pay attention in negative side we have R10 = 10K and on positive side we have R9 which is hale of it and then trimmer run +/- 10K. So, of you a few KOhm not enough then just change the value of R9. Alternately you can do what I usually do: switch the tubes, positive and negative between each other, or get another gas tube. Between 2-3 tubes I always find a configuration where I can get zero at my input. Sine you find a right set of tube and adjust to zero input it should reminds like this for years. If you use a direct coupled preamp that accidently runs DC at output then you can even to correct if at your power amp input.
Another aspect that you need to recognize is that 8mA at amp input is VERY little and it should not affect sound at all. All that it will do “bad” will be creating auditable click when you connect and disconnect cables. As you understand when your cables are connected then your positive supple (red) is grounded to your preamp output impedance. My preamp has 8R output impedance, so when my cables are connected I might even to pull the positive tube out or turn off positive supply – it will have very little impact to anything. Well, if I disconnect the cable after this then I have voltage burst that might burn my driver tube and might send the speakers diaphragm flying. This is what we have positive supply and balance the input to zero, because we want the amp to be balanced independent from the environment and to be stable.
Rgs, Romy the Cat |
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