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Re: 6,3V center tap or artificial ground?

3/14/2005 2:27 PM
Greg SimonRe: 6,3V center tap or artificial ground?
If you have a center tap on the filament line, then use it as it is easier. If you don't, then you can create an artificial one using two 100 ohm resistors from each leg of the 6.3v line to a common point. This CAN be ground. Then you end up with 0 v at this center tap reference point. BUT, if you offset it with a positive voltage like 40v for example, then it moves the noisy AC filaments away from ground, which is very close to where the signal from your guitar is. You signal is only small millivolts, so any noise close to that will interfere with your signal. Moving the AC filaments away from there with 20v to 80v DC makes a noticeable noise/hum reduction in the amp. You can do the same thing with actual DC filaments, but it is often MUCH harder and certainly more expensive to do that successfully. You don't want to go with more than around 80v on this center tap point because there are limits that the tubes have as far as heater to cathode voltage and what not.  
 
What I do and have done is to wire up the amp with the center tap to ground. Once the amp is up and running and I have the voltages for each stage set where I want them, then I either find a tap that has the 40V DC that I want, or I will create it with a voltage divider. Once I have that, I hook the center tap of the filament winding to this positive voltage and I'm done. You can sometimes get it from the cathodes of the power tubes, or from a preamp tap with a voltage divider. Just make sure if you make a voltage divider that you use the larger resistor closest to the voltage and teh smaller one to ground. The reason for this is that if you reverse it you will have a current drop across the first resistor in the divider and your voltage on the stage you tapped from will go down. I've done this on a Sunn 200S amp I rebuilt and modified and on my brother's Silvertone 1484. I used a 1M or 1M5 resistor as the first in the string and a 470k or 270k depending on the voltage I was after and the voltage I started with. Best approach is (Be careful!)to use some insulated clips and sub in resistors into your divider until you have the voltages you want. Hope that helps?  
Greg