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power supply question ok idea really

5/3/2000 2:28 AM
Bryan James
power supply question ok idea really
ok got my hands on a power supply that puts out about 230vac (@256ma) and 6.3 for filments.  
the problem is no bias supply or ground tap on either secondaries.  
here's an idea to make it work. looking for input.  
i know the trick of hum rejection on the filments by tieing 2 100ohm resistors to ground. now i did just to make sure measure the filment supply it's about 6.6 volts when measureing from ground to each point is about 3.3volts.  
now can i do the same thing on the 230vac secondiary. tie it to ground via 2 resistors thus give me 115-0-115 therefore i can do a full wave rectifer and also tap of the 115-0 for the bias?  
this is for el84 amp  
is my theory correct here or am i off the mark?  
or are there an other better ideas to get both bias and b+ of this PT?  
5/3/2000 3:05 AM
Bryan James

or should i say 120v neon lamps instead of resistors because of the wattage
5/3/2000 3:15 AM

I could be off base here, but...  
I think what you'd want to do is to use a full wave bridge rectifier on the 230vac secondary and tie the NEGATIVE side of the rectifier right to ground. The positive side obviously becomes your B+ rail. For the heaters, the (2) 100 ohm resistors to ground is fine for ground ref. the heaters. Finally, for the the Bias supply, using one leg of the high voltage AC secondary, connect a 150k-200k resistor (check on that value) and to that connect the diode and the rest of the bias supply. As for the Filament supply voltage, it will go down when under the load of the actual heaters.  
Hope this helps,  
5/3/2000 2:41 PM
Don Symes

Connect that diode with the cathode toward teh transformer. ... you knew that already.
5/3/2000 2:52 PM
Bryan James

you know i swore i replyed to this las night.  
but here we go.  
i did wire it up that way to begin with but wasn't getting good -voltage from the power supply.  
what i thought was that the ground on the rectifer is DC ground and not ref to the ac. so if you pull the bias off the main ac there is ac ground referance.  
the other option i can up with is run the bias in parrellel with the B+ supply but then the question is how to properly ground it. would you just run both grounds to chassis and just insert the -vbias back into the circuit?  
all the circuits that i've seen that pulls the bias off the secondary shared with the B+ supply there is a ground tap on the secondary.  
so the options are either run them in parrellel and figure out the grounding issue or and a ground ref to the secondary via 2 neon 120v lamps  
of course the third is to get a ne transformer and use this one for something else  
5/3/2000 4:45 PM
Dave H.

"all the circuits that i've seen that pulls the bias off the secondary shared with the B+ supply there is a ground tap on the secondary."
You can generate the bias voltage from the B+ secondary and use a bridge rectifier. Check out the VOX AC50/4 and AC100/2 in AMPAGE schematics and Iíll send you a schematic of the amp I made. The bias circuit is a bit simpler and the voltages are closer to what you have.  
5/3/2000 6:01 PM
Bryan James

all right thanks for the info. well check it out
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