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Fan Supply Question


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8/8/1999 5:59 PM
Pat F Fan Supply Question
I want to tap off my 6.3VAC filament supply to build a DC power supply for a cooling fan on my AC-30 clone. I have lifted the ground for the AC filament supply and have 100 ohm resistors from each side going to ground. If I build a bridge rectifier, tap off each side of the AC filament supply and attached the ground of the bridge rectifier to the chassis ground, will this interfere with the phantom ground created by the 100 ohm resistors between the AC filament supply and ground?
 
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8/8/1999 8:55 PM
ken gilbert
quote:
"I want to tap off my 6.3VAC filament supply to build a DC power supply for a cooling fan on my AC-30 clone. I have lifted the ground for the AC filament supply and have 100 ohm resistors from each side going to ground."
 
 
 
 
Sounds great so far.  
 
 
 
quote:
"If I build a bridge rectifier, tap off each side of the AC filament supply and attached the ground of the bridge rectifier to the chassis ground, will this interfere with the phantom ground created by the 100 ohm resistors between the AC filament supply and ground?"
 
 
 
 
Yes. You need to keep the fan supply floating WRT the "center tap" of the heater supply. This isn't really an issue--just connect the - diode bridge terminal to the - cap terminal and the - fan terminal, and do the same for the + node. No other connections are necessary, or wanted in this case.  
 
 
 
You should get about 8VDC output to the fan.  
 
 
 
Check your power tranny periodically to see if it gets too warm to touch, since you will be drawing more current from it to supply your fan.  
 
 
 
~KG~
 
8/8/1999 10:06 PM
pat
Thanks for the feedback, my first fan power supply. Let's clarify here. negative terminal of bridge rectifier goes to negative of main power supply caps and negative side of fan. (the entire amp is star grounded). And positive terminal of fan goes to positive terminal of bridge rectifier, right? Do I need to add and cap filters to this arrangement? I'm assuming no, because no audio is involved.
 
8/8/1999 10:52 PM
J Epstein

Pat, if I followed Ken correctly, the - terminal of the bridge rectifier goes to the - terminal of the fan AND NOTHING ELSE, assuming you leave the 100-ohm virtual center tap connected as you already have it.  
 
 
 
If you are pondering using a DC filament supply, you would need to disconnect the virtual center tap and ground the - terminal of the bridge but I don't think that was what you and Ken were originally driving at.  
 
 
 
-j
 
8/9/1999 2:21 AM
Pat F
Thanks for your reply. No, I don't need a 12VDC filament supply. The current AC supply is sufficiently quiet. You've cleared up my question. The fans (2, one for across the output tubes and one on the power transformer) draw about 160ma at 8 volts, which leaves me 1.6 amps headroom on the filament windings.
 
8/9/1999 12:03 PM
ken gilbert
Yes, J was exactly correct. The whole point here is to leave the entire fan supply floating, with no reference to chassis ground. Believe it or not, the fan supply IS actually referenced to ground, on the other side of the bridge rectifier, through the balance "pseudo-center tap" resistors. Putting in ANOTHER reference would cause massive currents to flow, and something would have to give.  
 
 
 
If you measure the fan supply WRT ground, you should actually see something close to a symmetric supply, with +4 and -4 VDC.  
 
 
 
So looking at it another way, just make believe you have a separate ground for the fan supply, which the - bridge rectifier, the - filter cap, and the - fan lead all connect to. The other rail is the + bridge connection, the + terminal on the cap, and the + fan lead. That's it.  
 
 
 
Good luck, it should work like a charm. I have two 12VDC muffin fans running at 16VDC (a doubler from the filament supply) that absolutely crank out air and keep 12 KT90's cool to the touch. The overvoltage doesn't seem to harm them, and I don't really care anyways because I have a heap of them from computer power supplies.  
 
 
 
~KG~
 
8/9/1999 12:18 PM
Pat F
You are right, it works like a charm. I've kept the voltage deliberately low in an effort to get the air moving but keep the fan noise from getting into the amp audio chain. It seems like I've achieved that. I had it running in my garage last night where the air temp was already 100 degrees F, and I could still touch the top of the rectifier and output tubes. I have one fan drawing air acoss the rectifier and past the OP tubes, and the other blowing against one side of the PT. The PT has pleanty of headroom on both the filament and HV windings, but in the PP class A config, it still gets pretty warm. The cabinet isn't finished yet, but I like to keep the tubes up, and use expanded metal for the front and back grill. I usually cut and bend it to shape, and then have it powder coated. Looks good and keeps thing cool. Thanks again for your input.
 

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