R/E/P Community
R/E/P => Klaus Heyne's Mic Lab => Topic started by: J.J. Blair 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.
-
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.
-
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.
-
It’s located exactly midpoint between the two big silver transistors, sitting upright to be adjusted from the sides rather than head on.