New power supply problem

  • Thread starter Thread starter PhilGood
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PhilGood

PhilGood

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I am so close to finishing this U47 clone I can taste it.

But I've run into this huge problem. Since I'm using an EF14 instead of a VF14 for the tube I have to supply filament voltage separately. I decided to make a similar PS to what I used on my Ela M clone, which works beautifully, but when I plug the mic in, the filament voltage drops to less than a volt. I've tried a power supply from an MXL V69, since I used the same socket pinout. That power supply with power the filament at +6 volts or so, but the B+ voltage drops to 25v when it should be 100.

An ideas what might be wrong? I think it may have to do with the ground in the PS but I'm not sure.

Anyone have proper schematics for the real modification? I've been very very close to the original U47 schematic and haven't deviated at all except for trying to supply filament seperately.

I can probably draw up what I've done so far and post it if that helps. Here's the PS. I can get the mic scematic, but just imagine I've removed the R3 and R4 resistors (29ohm and 1780ohm respectively) and gone directly from the power supply with ground and 6.3v. It reads over 10.6v unloaded.

Maybe I'm not doing the cathode bias right.

P
 

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Just off the top of my head, it would seem that either the 12v transformer (which, BTW, should read 12 VAC in the schematic) can't pump out at least 100ma, or there is a low (~5 ohm) resistance pathway to ground in the mic's filament circuit.
 
crazydoc said:
Just off the top of my head, it would seem that either the 12v transformer (which, BTW, should read 12 VAC in the schematic) can't pump out at least 100ma, or there is a low (~5 ohm) resistance pathway to ground in the mic's filament circuit.
Click to expand...

I tried a 24VAC transformer with the same results. Do you think if I put back the 29 ohm resistor to ground it might work?

Yeah, the schematic should read VAC. Sorry.
 
The voltages coming off your transformers, is it supposed to read ACV p-p, not VDC?
 
PhilGood said:
I tried a 24VAC transformer with the same results. Do you think if I put back the 29 ohm resistor to ground it might work?

Yeah, the schematic should read VAC. Sorry.
Click to expand...
I'm not familiar with the circuit, but if that resistor is in series with the tube filament, then it's got to be there to limit the current. Otherwise, you're dropping most of the voltage across the PS resistors and the output voltage will be very low, as you have measured.

Edit: If this PS is not the one designed for the U47, you may also need to change the 47 ohm resistor to a lower value, say around 25 to 30 ohms, to get the 6.3v at the output. You may also need to fiddle with the B+ resistors to get the correct voltage.
 
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I agree with crazydoc that you might be dropping too much voltage across the PS resistors, but still it shouldn't drop the voltage to 1V. Can you post a schemo of the mic as you have it? Do you have another tube to try out? What resistance do you measure across the cold filament, both in and out of the mic circuit?

I wouldn't put much stock in the performance of the MXL PSU, that seems to have a filament supply that is not isolated from B+.
 
You might try Dropping the Fillament resistors and useing a Voltage regulator like the LM317 to get your 6.3VDC......

Just a Thought....
 
gut reaction... for me seems trannies are insuffcient current wise...
 
Minion said:
You might try Dropping the Fillament resistors and useing a Voltage regulator like the LM317 to get your 6.3VDC......

Just a Thought....
Click to expand...

Could be too much heat for the LM317 to drop by itself, at least it would need a fat ass heat sink. If there is a problem in the mic circuit such that the supply drops to 1V, that means that power in the PSU resistors could be nearly 2W.

Really, the issue is having to shed 6V off a 12V winding. That ain't a job for a regulator alone. And if the tranny is short on current, or if there is a problem in the tube or mic circuit, the regulator ain't gonna help.
 
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