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BF Princeton Reverb. Add 100 Ohm Heater/Lamp Resistors?


 
8/30/2003 4:11 AM
Spencer
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BF Princeton Reverb. Add 100 Ohm Heater/Lamp Resistors?
I have a '67 BF Princeton Reverb (Tube chart AA764, GZ34 Recto)that came bone stock without the pair of 100 Ohm resistors tied to the heater /lamp as shown on the AA1164 layout.  
The amp sounds great but hums a little when I turn the volume past 4.  
I added the 100 Ohm resistors. The hum stopped, but I noticed that the amp now gets very hot to the touch. The chassis, the faceplate, even at idle the PT is almost too hot to touch with a bare hand.  
Plate voltage is 433V at ~20 mA, and remained constant before and after adding the resistors.  
Naturally I prefer the amp with the least amount of hum.  
Is the additional heat anything to be concerned about?  
What are the consequences, if any, of adding 100 Ohm resistors if the amp already has a center tap?  
Thanks.
 
8/30/2003 3:55 PM
Daver
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If you add the resistors you need to remove the center tap connection to ground. The resistors form a virtual center tap. It's almost like you're grounding the entire filament winding now. Fix this before you fire up the amp again. This can't be good for the power transformer and is probably the source of your extra heat.  
Daver
 
 
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8/30/2003 3:58 PM
Mark Lavelle
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quote:
"Is the additional heat anything to be concerned about? What are the consequences, if any, of adding 100 Ohm resistors if the amp already has a center tap?"
I'm not qualified to say what the consequences are, but everything I've read says you should never have both a center tap and a pair of 100-ohm resistors.  
 
If the amp is noticeably running hotter, that's a strong indication that you're drawing a lot more current somewhere, and it seems obvious that this change would be the cause. Since Leo wasn't exactly known for over-spec'ing parts I'd worry that you're going to fry your 6.3VAC secondary if you don't disconnect and tape off that center tap...
 
8/30/2003 4:37 PM
Bruce /Mission Amps
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The filament suppy is all AC and isolated from chassis ground by the way it's used.  
The center tap of the filament supply has zero volts, hence there no current to ground even if the center tap is grounded.  
The additional current drawn by the grounded 100 ohm resistors across the outside of the 6vct windings is very very low current.  
At about 3.0vac to 3.5vac each and 100ohms,  
3.5v/100ohms = 35ma.  
It adds around 70ma of current drain to the 2a - 2.5a filament supply.  
The resistors aren't really needed if the PT has a decent filament winding center tap and it's grounded but it willl not hurt anything either.  
 
I suspect as others, you've shorted something else to ground or you just never noticed the heat.  
 
Bruce
 
8/30/2003 4:58 PM
Spencer
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Please excuse my ignorance, but how do I tell (by looking at the schematic or the amp) if I presently have a center tap on the heater winding?  
Thanks.
 
8/30/2003 5:52 PM
Bruce /Mission Amps
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Out of the power transformer, two green wires and one green with yellow stripe wire that is chassis grounded.  
Bruce
 
8/31/2003 6:02 AM
Spencer
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The AA1164 and AA964 layouts show a Red-Yellow wire and a Green-Yellow wire tied together, grounded to the chassis.  
Instead, my amp has a Red-Yellow wire and an solid colored Orange wire tied together, grounded to the chassis.  
Does this offer any clues?  
Thanks.
 
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