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DC Filaments

5/23/2003 9:31 PM
Bob DeRosa
DC Filaments
Does running your heaters on DC increase the strain on your PT ?  
For example, I have a Hammond 276X PT with a secondary rated at 6.3vct @ 5 amps.  
I'll be running 6 12ax7 (6*.3 = 1.8a) and 2 el34 (2*1.6=3.2a) tubes for a total load of 5 amps.  
If I go to DC do I need more current capability or is it the same ?  
The reason I asked was that I was reading an article about regulated DC and it said that you need ~40% more "headroom" than with AC. I couldn't tell if that was for all DC or just regulated DC.  
Thanks in advance
5/24/2003 1:33 AM
Hi Bob. In my amps I use a separate transformer to run the preamp tube heaters on regulated DC. I run them at 12v so there is less strain on both the transformer and regulator. The power tubes and driver are run from AC and the amps are totally free of hum. Many others have said it before and it's true; don't run the power tube filaments on DC. It is a waste and has no benefits.  
5/24/2003 2:18 AM

I struggled w/this and couldnt get it to work correctly. Ive heard mesa/boogie runs dc pre and ac poweramp, but have been unable to find a workable schem on this, you got one?  
still pluggin away? rite on my man! yes when converting from ac to dc there is a loss which is converted to heat  
5/24/2003 4:32 PM
Bob DeRosa

This morning I rectified and filtered the heaters. I've got an annoting buzz now...  
Should I tape and stowe the CT or hook it to ground ?  
I think I'm going to go back to AC heaters but try hooking the CT to a ~75volt "standoff" voltage.  
I'll keep you posted if anyone is interested...
5/24/2003 6:56 PM
Bob, you have to regulate the DC if you want it to be quiet. Just using a bridge rec and a cap is not enough and it will be noisier than AC.  
5/26/2003 5:41 PM
Steve A.

you have to regulate the DC if you want it to be quiet. Just using a bridge rec and a cap is not enough and it will be noisier than AC.  
    I've never regulated the DC and if you reference it to ground properly it should be very quiet. The negative terminal of the rectifier should be floated off ground with something like a 0.1uF cap.  
    I wrote up how to do it in my article on modding the Pignose GV40V:  
    Look in the bookmark pane for "DC Filament Supply mod".  
Steve Ahola
5/26/2003 6:14 PM

Steve I tried that mod on the g40v, but couldnt get it to work correctly.
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