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Author Topic: N52a question  (Read 209 times)

Offline J.J. Blair

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N52a question
« on: August 21, 2018, 03:20:52 pm »
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. 

Offline uwe ret

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Re: N52a question
« Reply #1 on: August 21, 2018, 04:06:27 pm »
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.

Offline J.J. Blair

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Re: N52a question
« Reply #2 on: August 21, 2018, 11:56:15 pm »
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. 

Offline klaus

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Re: N52a question
« Reply #3 on: August 22, 2018, 10:26:42 pm »
It’s located exactly midpoint between the two big silver transistors, sitting upright to be adjusted from the sides rather than head on.
Klaus Heyne
German Masterworks®