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[J.J. Blair]

Had some random clicking in one of my M49s, so I replaced the tube, and everything is better after a couple days of burn in.  However, my heater voltage is 4.3 VDC .  I don't know if the extra .1 is going to be a problem or not, but I'm wondering if there's a way to adjust the heather voltage in those, without having to solder a resistor into the circuit. 

[uwe ret]

The original N52a did uses two banks of Stabilyt cells for filament regulation, followed by a dual (adjustable) series resistor to provide the desired 4 V filament supply. Stabilyt cell have a life expectancy of up to 2 years and sure have expired decades ago. I assume that section has been modified? When Stabilyt cells were discontinued in the mid 1970s Neumann briefly offered a modification kit SK 33d with Zener diode regulation.  Modern voltage regulator ICs allow for much more effective circuit substitution with precise regulation and tube saving soft filament supply ramp-up. The later version N52t used solid state (transistor) circuits which used selected resistor values to compensate for component tolerances and adjust for accurate filament and plate/bias supply voltages.

[J.J. Blair]

Wow, Uwe!  I've never seen that replacement schematic!  Thank you so much.  I only have the N52a schematic, which is of course useless without your schematic.  So, I guess I just have to locate R5, and that's my heater voltage pot. 

[klaus]

It’s located exactly midpoint between the two big silver transistors, sitting upright to be adjusted from the sides rather than head on.

[kidb]

I've got a N52t on the bench right now. Customer had someone tell him it had to run off 220v. Its been working with 120vac for a while. I don't see a problem as long as voltages are correct/corrected. Any issue running off 120v vs 220v? Schematic says both!?

Thanks

[klaus]

No problem running an N52t with U.S. line voltages. 
But remember:

1.  As far as AC power goes, the N52a also uses a single voltage input transformer (either 220 or 117 VAC, as Kai below is explaining in greater detail). As far as secondary (DC) voltage adjustment go, you need to be sure that the supply in question is actually labeled N52t, rather than one of many factory "t" converted N52a supplies.

2. You must precisely calibrate the supply for the AC701-powered mic it will be used with. The U.S. nominal voltage is not 117VAC but 120VAC. An oversupply error of as little as 0.2 volts on the heater output will shorten the life of the tube significantly. Likewise, an oversupply of B+ which is not regulated in N52a, will put stress on the capsule.

[Kai]

I've got a N52t on the bench right now. Customer had someone tell him it had to run off 220v. Its been working with 120vac for a while. I don't see a problem as long as voltages are correct/corrected. Any issue running off 120v vs 220v? Schematic says both!

The N52t is by no means a “universal voltage”-design as you often find with todays switched mode PSU’s.

The schematic even shows, there’s no 2nd tap on the transformer to switch to the other mains voltage.

So, it’s either 117 V or 220 V.
Running a 220 V model on 117 V just won’t work.
Running a 117 V model on 220 V (or todays EU 230 V) overvoltage would at least blow the fuse, or break the PSU.

As Klaus mentioned, calibration is mandatory and denoted in the schematic.

Both, anode and heater voltage are regulated in the t-model.
Anode V by the Zener GR3, heater V by the active two-transistor circuit.

So, once calibrated, output voltages are independent of mains voltage variations within reasonable range.

[kidb]

Thank you Klaus and Kai for clarifying the transformer primary label is an OR not an EITHER. I was confused as well how it could operate with 120vac. The mic actually does work with 120vac, the heater is down .3V and the B+ is at 100vdc. (which the customer was doing..) I expected much worse. Someone put an IEC on the psu case and no label! These PCBs are labeled N52t. Always amazed to see these supplies with original caps and still completely working. I too am on a quest for saving all AC701k tubes, adjusting heater to be safe and regulating if not already.