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High filamant voltage problem?

2/11/2003 7:09 PM
Sam G.
High filamant voltage problem?
Newbie Alert...  
I'm just about finished building a "franken-amp" from all sorts of scavaged parts. I decided to use two Svetlana 6550s for my power section. The amp has a total of three 12AX7s, and one 12AT7 for the pre-amp. I was concerned about the filamant current pull on my power tranny (from a silverface Pro-Reverb, puts out 505vdc on the plates, ss rectified), so I added a filament transformer for the pre-amp tubes. The filaments are wired for AC operation. The center-tap of this transformer is not connected, instead I'm using a 100ohm resistor to ground for each filament side.  
The PI tube is seeing 7.25vac, and the input tube 7.20vac. Isn't this too high? How far above 6.3vac is safe?  
Fiddlin' with the math: The pre-amp tubes are pulling about 1.2A with 7.25vac. If I want to get the voltage closer to 6.3vac, would I use a series resistance of 5.25ohms (R=6.3vac/1.2A)? And...wouldn't I have to divided that resistance in half for a value of 2.625ohms for each filament winding, to be put right before the PI tube?  
The power rating of these resistors should be fairly high, yes? Maybe around 4 or 5 watts?  
Is there a better way to do this?  
By the way, my power tubes' filaments are seeing 6.8vac. Is that within a safe range?  
Thanks for any help.  
2/15/2003 2:30 AM
Mark Lavelle

You've got it figured right, but don't be too concerned about nailing 6.3V. The spec is *always* +/-10%, so anything from 5.7 to 6.9 is totally OK (and 7V won't kill anything).  
Don't go crazy on the power rating of those Rs, either. W = (E^2)/R so you're talking about 1/5 watt (I'd use a 1W).
2/17/2003 2:54 AM
Sam G
Thanks for the "sanity check", I appreciate it!  
4/27/2003 5:13 PM

Here is an easy way to lower your filament voltage by 0.7 volts. Radio Shack has 50piv 6.0amp rectifier diodes. Place a pair of them in series with one side of the filament supply.  
That is, the diodes are wired in parallel with each other, one forward, the other backward. Insert this pair on one side of your filament supply, and you'll have a 0.7 voltage drop that is independent of current draw.  
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