Neumann M49 PSU and Neumann U67 PSU

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distorter

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Hi guys, what is the difference between Neumann M49 PSU and Neumann U67 PSU?
I know the filament voltage is 4v and 6.3v. However, when I measure the M49 psu, I am reading 35v unloaded and 4.5v loaded. When I measure the U67 psu, I am reading 6.3v unloaded.
Why the PSU for M49(ac701k) measures differently when loaded and unloaded? Shouldn’t it be constant 4v when measure unloaded?
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Moby

www.mobytransformers.com BV.8, Bv.11, Bv.12, T14/1
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Hi guys, what is the difference between Neumann M49 PSU and Neumann U67 PSU?
I know the filament voltage is 4v and 6.3v. However, when I measure the M49 psu, I am reading 35v unloaded and 4.5v loaded. When I measure the U67 psu, I am reading 6.3v unloaded.
Why the PSU for M49(ac701k) measures differently when loaded and unloaded? Shouldn’t it be constant 4v when measure unloaded?
View attachment 99087
View attachment 99088
You should use Ohm's law to calculate the voltage drop across the series resistors.
Also, Nu67 should give you -15v unloaded. Unless you have one of the DIY fully regulated heater PSU
 

Tubetec

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Remember the NN48a originally came with Nicad stabiliser cells , later these were deleted and replaced with a transistorised regulator by Neumann . The older supplies use a large resistor, 115 ohms to drop the right voltage on the heater , in the event the rail is unloaded and without the stablyte cells 35 vdc from a 25 ac supply sounds about right .
 

distorter

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You should use Ohm's law to calculate the voltage drop across the series resistors.
Also, Nu67 should give you -15v unloaded. Unless you have one of the DIY fully regulated heater PSU
Thank you Moby! Yes, my U67 PSU is a regulated one from Andreas Grosser. Is it safe to switch my NN 48u to a fully regulated heater psu for my M49?
 

distorter

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Remember the NN48a originally came with Nicad stabiliser cells , later these were deleted and replaced with a transistorised regulator by Neumann . The older supplies use a large resistor, 115 ohms to drop the right voltage on the heater , in the event the rail is unloaded and without the stablyte cells 35 vdc from a 25 ac supply sounds about right .
Thank you Tubetec! May I ask what is the right way measuring my NN48u when loaded in order to make sure the voltage feeding my ac701k is 4v? Is it a better choice to switch to a modern fully regulated PSU for my M49?
 

Moby

www.mobytransformers.com BV.8, Bv.11, Bv.12, T14/1
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Thank you Moby! Yes, my U67 PSU is a regulated one from Andreas Grosser. Is it safe to switch my NN 48u to a fully regulated heater psu for my M49?
So, if the U67 PSU is regulated, heater should be 6.3-6.5v. Since it's regulated. ;)
Of course it's safe to switch to the regulated PSU but you should consider the possible sound diference. It's subtle but existing.
I prefer R/C ones, possibly with zener "preregulation" as NU67, but it's up to you.
 

distorter

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So, if the U67 PSU is regulated, heater should be 6.3-6.5v. Since it's regulated. ;)
Of course it's safe to switch to the regulated PSU but you should consider the possible sound diference. It's subtle but existing.
I prefer R/C ones, possibly with zener "preregulation" as NU67, but it's up to you.
Very useful information to me. Thanks a lot! 🙏🏻
 

Tubetec

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Its been said already , you really want to measure across the tube socket to get the true
heater voltage , the cable itself will drop a small voltage , so if you measure at the PSU end with the mic connected you might see +0.25v above what you expect .

Ive certainly heard a few very knowledgeable people say passive RC filters sound different to regulated supplies in tube mics and pre's . All the mics I have are regulated heater supply and its not easy to make that into an RC filter because of the voltage drop needed to make it quiet . The older Neumann PSU also uses a 1HY choke in the heater supply . Im planning on making a battery based tube mic supply soon , no regulators , 4 Nimh cells for 6 volt heaters ,3 cells for 4v heaters . NICAD cells could give slightly different voltages depending on if you want your heaters underrun . The one worry I have is heater inrush current from batteries at switch on ,owing to the low internal resistance .
 
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