Baltazar

Neumann BB12 PSU and a Sela T24
« on: February 06, 2019, 04:58:17 AM »
Hello,
In the process of fixing this power supply and need some help to understand this schematic and how the circuit works and how to improve it.

First thing i noticed was that I'm only getting 40-50 volts for B+ instead of 120v.

Someone before me have modified it and put a simple wall transformer instead of batteries, not the greatest "improvement" since there is no filtering at all and goes directly to the filaments.
The voltage from the transformer is more like 5.3 volts than 4v and doesn't seem to be very much filtered, there is a audible high frequency ring from the mic. than i also can see in the scope. Thats probably why the tube must have been damaged, its very microphonic..

So before replacing the very expensive ac701 tube i need to fix this properly.


Can probably figure out how to filter the supply and filament voltage, but can some one here help to understand how transistor and transformer network works for b+?
How can you get 120v from 4.5v?






Gus

Re: Neumann BB12 PSU and a Sela T24
« Reply #1 on: February 06, 2019, 05:48:04 AM »
Looks like it might use three sub C 1.25VDC NiCads (from the .15mA charge rate, 1/10 of 1.5amps)
3 X 1.25VDC = 3.75VDC
It could be the cells used run a little higher in voltage under the filament loading so the voltage could be closer to 4VDC

Nicads in good shape have a low internal resistance

The output on the secondary side of the transformer is a voltage doubler. Maybe this is a hint.

I would not use a transformer to power it needs a regulated voltage for the filaments.


First thing I would try is using the correct batteries without the tube and a resistive load on the B+



Next check C3, C1 and the other parts like the resistors with a meter. It does not have many parts to check

For testing with a load on the output B+ you can try a 120K for 1mA or higher values for less current.

moamps

Re: Neumann BB12 PSU and a Sela T24
« Reply #2 on: February 06, 2019, 02:41:56 PM »
Can probably figure out how to filter the supply and filament voltage, but can some one here help to understand how transistor and transformer network works for b+?
How can you get 120v from 4.5v?

Germanium transistors,  transformer and diodes form a step up DC/DC converter 4V to 120V. I believe the best solution is to build a new modern power supply with precise 4VDC for AC701 and regulated 120V for HT.


 

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