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previous: mikey MatthewI am going to try th... -- 3/25/2003 10:13 AM View Thread

Re: A tidbit till the real amphounds show up..

3/26/2003 5:19 PM
Matthew Springer
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Re: A tidbit till the real amphounds show up..
Yes. If you use the two 100 ohm resistors, you should leave the heater center tap unconnected. A trick that I use that somebody else here shared was to heat shrink the end wire and while the heat shrink is still hot, take a pair of pliers and "crimp" the end.  
 
The number 100 isn't particulary magical either, it's just suppossed to be enough to develop equivalent DC acroo sboth legs so the "twisted pair" heater wires have the magic humm cancelling effect. The point at the junction of the two 100ohm resistors is a "virtual center tap".  
 
Also, you don't have to attahc the "virtual center tap" to ground, you can reference it to any good AC ground, but not necessarily DC ground. I usually tie this point to the top of the cathode bias resistor if there is one or to some elevated but filtered DC point.  
One method I think TUT advocates is taking a resistor ladder off of B4 and tying the center tap to this. Just make sure you put in a nce big cap to ground from wherever you tie it.  
 
What wayne said also goes, the resistors need to be close to identical. Also, use some nice, big, flame proof metal film resistors for this, not carbon comps. By it's nature a heater winding can kick out ALOT of current if it shorts to soemthing it's not suppossed to. No sense using crappy resistors.  
 
-Matthew

 
Replies:
Ted Actually, I think I'd rather have t... -- 3/28/2003 4:50 PM
Mark Lavelle [QUOTE]… you can reference i... -- 3/28/2003 10:48 PM