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[kingkorg]

Thanks abbey

I've tested the mic with a 6.5:1 transformer (as suggested in the past by Dave from AA) and I'm really not convinced it improves the noise floor! I feel the S/N ratio is the same, but with more gain. Maybe I'm missing something...

Yes it makes sense, i dont know if Dave stated it should improve s/n ratio or just give more output. Maybe depending on a pre you are using using less gain will give less noise. However a lot modern preamps will give you less noise the more gain you use.

 

[abbey road d enfer]

Yes it makes sense, i dont know if Dave stated it should improve s/n ratio

Yes he did. And it's plainly wrong. A tube without cathode decoupling cap has a higher EIN than the same with a decoupling cap, and since it has less gain, the noise of the subsequent stage will be more accountable.
The difference may be tiny, but it's real.
OTOH, the overall increase of sensitivity provided by the lower ratio of the output xfmr allows turning the preamp gain down, but unless it's a very noisy preamp, it should not be enough to compensate.

 

[elskardio]

Maybe this is not the proper thread to ask this question, but I built a new power supply for this mic and I can't get more than 118V on B+ and 6V on the heater... I use an EH 6072A in the mic.
The PSU is from Matador's C12 clone made for the popular Chinese PSU enclosure. (schematic)
Any hint?

 

[abbey road d enfer]

Check the AC voltages at the rectifier bridges. Looks like a 230V xfmr connected to 115V mains...

 

[terry setter]

Look for a zener diode, probably on the output of the B+ R/C filter chain. Actually, there are commonly two zeners in series. If it's Chinese, they're usually silver in color and connect just before the jumper that feeds the pattern select switch. If you find such, lift the negative end (right by the output) and measure the resulting voltage increase. Your B+ will now be unregulated, but you should see more voltage available at the PS output. In my world, I set the dropping resistors so that the B+ delivered to the mic is what I want. Then, I put a zener across the output of the supply that is 10V higher than the B+ value I want the mic to see. The zener only conducts when there is low or no current being drawn. Almost always, that's when someone has turned on the supply when there's no mic plugged into it. With no mic plugged in and no "safety zener", the B+ will go all the way up to the voltage level exiting the rectifier. Not a good thing for several reasons. Again, this "safety zener" only conducts when it's protecting your circuitry, so no zener noise is added inside your PS during normal operation. If you're using a supply with a high voltage regulator, that's a different story, but most of the Chinese supplies only have a single zener (or two in series) as B+ regulation.

And, relative to Abby Road D Enfur's comment about the mains, make sure - if it has a switch - that it is set for the proper mains voltage.

By the way, you don't need a 630V output cap. You can save some space in your mic and some money for future builds....