Setting your bias using the shunt method for
a specific plate dissipation.
(See disclaimer below)
1) With the amplifier unplugged. Measure both halves of the DC resistance of the primary of the output transformer and write down your findings for each side. Let's say the resistance is 41 ohms from the center tap to pin 3 on both sides. There may be some difference between the two sides so don't worry.
2) Plug the amp in and power it up. Let the amp warm up for at least ten minutes to stabilize the operating points. Now measure the DC voltage on both plates(Pin 3)to ground and write down your findings for each tube. Let's say the measured DC voltage was 450VDC.
3) Let's say that you want to bias the amp for 70% of the maximum plate dissipation of the power tube(KT66 and EL34's plate dissipation is 25 Watts maximum). Take 25W times 70% equals 17.5W. This is the plate dissipation that you will use for the remainder of the calculations.
4) Now we need to find the current draw needed to get 70% plate dissipation at 450VDC. Take 17.5W divided by 450VDC and you will get 0.0388A(38.8mA). This is the current draw per tube needed to get 70% plate dissipation with 450VDC on the plates.
5) Now we need to find the voltage drop across one half of the primary needed to give the 0.0388A of current. Take 0.0388A times 41 ohms equals 1.59VDC. This is the voltage you will be setting the bias pot to read while measuring across half of the primary of the output transformer. Check both halves of the transformer to make sure that both tubes are reading close to each other. Some transformers have a larger resistance difference between the two halves and will have to have the bias setup as a compromise(balance)between the two tubes. One tube will be set at a higher plate dissipation and the other will be set at a lower plate dissipation than the original calculated plate dissipation.
6) After setting the bias you will need to go back to step 2 and recheck the voltage on the plates to see if it has changed. If it has, follow steps 2-5 until it has stabilized. It should only take a couple of times if needed at all.
7) This page is setup to give an example of how to bias output tubes to a particular plate dissipation. You can bias the tubes to dissipate more or less than the example I have given. It is a matter of taste and you should let your ears be the judge. I would not recommend exceeding the maximum plate dissipation of the output tube though.
This procedure must be performed with the amp
If you are not comfortable working on an amplifier have a qualified technician
perform this procedure for you. There are deadly voltages inside of an amplifier that can kill you.
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