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Re: 5E7 PT Selection

1/6/2001 2:08 PM
Jim Jones
Re: 5E7 PT Selection
Hey SA,  
You'd probably be able to find a Hammond to do the trick for you.  
I was wondering why you mentioned the 372JX (dual primary voltages), thought maybe a European tour was in the works. :) Then I spotted the bias winding. I wouldn't worry about going for the more expensive tranny for that. You can just tap the bias off the high voltage secondary and drop it with a single resistor "before" it gets to the bias circuit. A 150K-220K off the high voltage secondary should limit things fine. Take a look at an old Marshall schem from to see what I mean.  
As far as voltage/current requirements go...  
For heater current you need 2 x .9A for the 5881's + .9A for the 2 12Ax7's and one 12AY7, so 2.7A total for the 6.3V winding.  
The 5E7 uses a 5U4G rectifier, so you need 3A on the 5V winding.  
For B+ current, figure on a couple mA for each 12AX/AY7 lets say 6mA, hell how 'bout 10mA just to make things nice and round and safe.  
Checking some tube 450V on the plates of a pair of 5881's maximum plate + screen draw is listed as 232mA. So total B+ current is ballparked at 242mA.  
Chances of you running the Bandmaster for several days at full tilt is pretty unlikely so you can conservatively estimate a "realistic" tranny rating. I've seen 80% of maximum rating as a good figure for ballparking, so 242mA x .8 = 193.6mA.  
In my opinion a 200mA tranny would be perfect (if not still a little "beefy".)  
If you're thinking Hammond, I'd go with the 272HX.  
My apologies if any of the above sounds condescending...that isn't my intention! Just wasn't sure if you knew how the breakdown went.  
Hope this helps!  
1/6/2001 2:13 PM
Jim Jones

Oh, one other thing. To ballpark your rectified high voltage multiply your secondary AC voltage (say 300-0-300) x 1.414, so probably 420-something DC. If your voltage sags too much with the 5U4G in there, try a 5AR4/GZ34 instead...a little less voltage drop with that one.  
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1/8/2001 10:25 PM
Thanks guys i appreciate the help ver much.  
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