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Hotbox DC Heater???

6/6/1999 3:54 PM
Bill S. Hotbox DC Heater???
Hi All,..  
I'm looking to build a HotBox,..however,...I can't find one part,..and that is the MP-3510 Bridge Recifier for the filaments.  
Can I use 4 diodes instead...  
What diodes can I use for this? 4007's?  
6/7/1999 6:42 AM

Sure, four 4007's are fine, or any of a number of packaged bridge rectifiers of sufficient VA...  
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6/7/1999 7:12 AM

If you rectify a 6.3V rectifier winding, the DC heater voltage will be too high and you will damage the tubes. With 4007's or a similarly rated rectifier you'll get about 8 V DC after rectification. With two 12ax7's you'll have to put in a 3-3,5 Ohm/5W resistor between the filter cap's + connection and the heaters to drop to 6-6,3 Volts. Aim for 6 Volts.  
6/7/1999 1:16 PM

Just a thought...  
You could use a 9v transformer and a bridge rectifier. 9*1.4=12.6, and run the 12AX7's without the center tap.  
You would need about 150mA per tube at 12.6v, so look for a 6VA transformer or so.  
6/7/1999 8:45 PM
Bill S.
I have heard people use a 10,000 uf in parallel with the heaters, drop the voltage down,..correct?
6/8/1999 3:37 AM

Sorry, not correct,  
the 10000 uF is parallel to the rectifier's + and - to filter the ripple of the rectified voltage. It does not drop the voltage, it just filters it and is therefore a necessary item.  
You can not drop the DC voltage in this setup by putting a capacitor or resistor parallel to the heaters, this changes only the load impedance the rectifier sees, which does not affect the voltage.  
Like I said before put a resistor in series with that heaters, i.e. between the + of the capacitor and the first heater.  
Alternatively, use this setup for better filtering: rectifier --> 4700 uF cap --> 3 Ohm attenuation resistor ---> 4700uF cap ---> heaters.  
This is a nice C-R-C filter which filters out the AC ripple on the DC even better.  
Any more info, let me know  
6/8/1999 6:13 AM
Bill S.
Thanks for the post, it was certainly helpful!  
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