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AC30 PT wiring problems

4/8/2005 7:21 AM
AC30 PT wiring problems
I'm scratch building a AC30 using an aluminum chassis,Hoffman AC30 board kit and Mercury Magnetics transformers.The power transformer is a MMP-V30W.  
It has a center tap for the filament winding and a center tap for the B+ both of which are green.There is also a shild wire.  
When I ground the center taps the fuse blows and when I lift them I get I no voltages on the B+ rail.  
Should be very simple but has me stumped.Everything is wired correctly with an on switch and standby switch off of pin 8 on a GZ34 rectifier.  
4/8/2005 3:07 PM

Without really considering things which are specific to Vox amps:  
The center tap on the 5V winding is for the rectifier heaters. With most rectifier tubes this winding will rise to the B+ value as the tube heats up, so you do not want to ground this center tap ever.  
Since you already have it wired to draw the B+ from pin 8, you just disconnect this center tap from ground and carefully insulate it or tape it off.  
Alternatively, if you were only going to use *directly heated* rectitfer tubes, the entire 5V winding for the rectifier heater will also be carrying the B+ so you could draw the B+ from this center tap like it was done on some antique instrumentation. That would be instead of drawing the B+ from pin 8, you can only choose either the center tap or one of the rectifier pins. You can not do both or the fuse will blow almost as quick as it did with this center tap grounded. Since most PT's do not have a center tap for the rectifier heaters to begin with, this is not a very common error. So my suggestion to just tape off the center tap is the way you would wire it with a PT when there is no tap on the rectifier heater winding.  
Now the other center tap for the HV B+ winding is generally supposed to be grounded unless you have a full-wave bridge rectification scheme. I assume that with a tube rectifier you do not have a bridge so you should keep this center tap grounded.  
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