Heater voltage filtering.

[abbey road d enfer]

I had marginal success replacing the fifty year old cells with custom voltage NiCD packs from an RC car battery supply outfit in the Bay area.

Can you elaborate on that? What didn't work right?

 

[Tubetec]

Might be some data on the original Neumann patent here ,
https://worldwide.espacenet.com/publicationDetails/biblio?CC=DE&NR=975865C&KC=C&FT=D#and here
https://worldwide.espacenet.com/publicationDetails/biblio?CC=DE&NR=925959C&KC=C&FT=D#The original documents dont seem to be available on that site .

 

[Tubetec]

I've experimented using a disposable camera flash unit to generate a small amount of HT current to run a tube. Normally you get around 300 volts off the filter cap , There is a bit of acoustic noise bleed out of it at switch-on ,but I generally found once the circuit had time to stabilise any noise reduced considerably . Of course extra RC filters were used to get the HT to the voltage I required for an ECC83 stage in this case .

I wonder could you do away with the need for a Hv winding in a mic psu completely and just tap off the 1.5 volts from one of the heater cells to ramp up the HT via a circuit like shown below ,
The circuit I used in my test was many years ago now , it seems the newer Kodak circuit has some refinements .
Once a few large caps are charged it probably has enough energy stored to run a microphone Ht circuit for many minutes before a top up is required , although some small fall off in the voltage is inevitable , its effect on the performance could be made negligible .

Obviously the flash bulb and step up coil can be done away with , we dont need thousands of volts , so its safer just to unsolder those .

I might try the chemist down town and see if they have a box of used disposable cameras going to the recycle , in general you find a high quality branded 350 volt capacitor in those of 50-100uf or more in value , perfectly suitable for tube circuits with years of usefull life left in it .

 

[Tubetec]

I found a few photos of an original NKT psu here with the Nicads still in place .
https://m221b.com/NeumannNKM/NKM.html
The link to the Schem is missing though

 

[Tubetec]

Thats the different variations on the battery regulated Neumann units , Im still trying to find the model NUK .

 

[moamps]

..I did discover one more stablyt based supply which seems destined for the U67 called the NUK , but I cant seem to find the schematic though , maybe someone here has it ?

After looking at the pictures on radiomuseum.org
https://www.radiomuseum.org/r/neumann_netzgeraet_nuk.html#I don't think that NUK power supply had NiCd batteries. Ge transistor TF80/30 is used to regulate the heating voltage.

 

[abbey road d enfer]

I don't think that NUK power supply had NiCd batteries. Ge transistor TF80/30 is used to regulate the heating voltage.

However, the description in the radiomuseum link specifically mentions "Special "stabilyt" voltage reference (in fact, a nickel-cadmium cell, a Georg Neumann patent) used in the heater voltage stabilizer. "
There is no evidence of presence of stabilyts on the pictures, though.
The mystery remains...

 

[Tubetec]

I was able to dig up the NUKa schematic although its poor resolution . I added it along side the others above , nothing to do with batteries in there .
I found this variation as well ,
Kassetten-Netzgerät NUKa-930-00 Power-S Neumann, Georg,
again nothing to do with batteries .

Mo,
If you scroll down on the link you posted on the NUK is does in fact mention the stablite cells , again as better transistors became available the circuit evolved adding a new regulated sub board on the LT and getting rid of the batteries .
.

 

[Lawrocker]

Can you elaborate on that? What didn't work right?

The filtering could have been better and they wouldn't last very long before hum crept in. About a year. I think the properties of the modern batteries aren't the same as the original nicads. My other unit's had already been converted by someone to just have a 7805 circuit there. Not optimal, in my opinion.

 

[Tubetec]

I think battery regulated mains LT supply beat the early attempts at transistor regulation , but a pure battery supply without mains may well do much better noise wise , downside is maintenance. ie. ability to inspect and maintain the individual cells . Its extra work for the 'roustabout '
and who wants to have to desolder depleted cells every year in their tube mic supply?

You could have one set of cells in the psu followed by another set of smaller capacity cells in the mic itself , if it lies unattended for years your left with chemical seepage , which might destroy everything it comes in contact with . we just arrange that the cells are easily removable .

 

[LevinGuitar]

I'm planning to use a 7908 voltage regulator (-8v) and CRC filter for U67 bias mics. Using a calculator I find that I would need around a 0.5W resistor(s). So would be safe to use a 1W in a long term to drop 2v in one or two resistors?
Thank you!

 

[rock soderstrom]

I'm planning to use a 7908 voltage regulator (-8v) and CRC filter for U67 bias mics. Using a calculator I find that I would need around a 0.5W resistor(s). So would be safe to use a 1W in a long term to drop 2v in one or two resistors?

I would rather use an LM317/337 plus CRCRC than the 7908. Take a look at this project as an example

https://groupdiy.com/threads/diy-pr...ps-like-preamps-buffers-or-microphones.84324/


I find 1W resistors for 0.5W continuous load too little. I don't like hot resistors, I would rather use 2W or 3W.

 

[abbey road d enfer]

I'm planning to use a 7908 voltage regulator (-8v) and CRC filter for U67 bias mics. Using a calculator I find that I would need around a 0.5W resistor(s). So would be safe to use a 1W in a long term to drop 2v in one or two resistors?
Thank you!

What voltage are you starting with?
How much do you want to drop in the passive filter?
You can't just answer with any figure there. The more filtering you want, the more capacitance or series resistance you need.
What is your target in terms of resisdual hum on the heater rail?

Regarding resistor power, the heater current of the EF86 is 200mA; it means that for each volt of voltage drop, the dissipation is 0.2W. A 4.7 ohm resistor would drop 0.94V and dissipate 0.188W. I think a half-watt resistor would be properly dimensioned. 1W is slight overkill.
In your scenario, you would need to drop 1.7V, which could be achieved with a 8.5 ohm resistor (next standardized value 8.2) that would dissipate 0.34W.
If you want to use an RCRC filter, you could use two 4.3 ohm resistors.

 

[LevinGuitar]

I was thinking to use two 4.7R 1W (guess the 7908 will give me a bit more than -8v? Otherwise I don't mind to go 5.8v or even less for the EF86) to be safe and to not overcomplicate the psu. For now I was using Ruud way (reply #20) of using the original 7806 "ground" pin to have a negative voltage, with a 2.2R resistor (+ 220uf) I was getting somewhere around -6.3V from the original -6.8V that goes in the standard chinese psu. But this involves cutting some big traces and it's not so nice looking (at least for the brick psu that have the PCB flipped on its side close to the cage vents). So want to try the other way and btw have a some filtering too. The noise was ok with the Ruud mod (no change when turning off the psu and using a filter cap after the voltage divider for the grid bias that is not present in the u67), want to see if this one will work also that good.

I learned a lot in this thread, thank you all!

 

[abbey road d enfer]

7808 needs at least 11V in order to function properly. Are you sure your unregulated voltage is adequate?

 

[LevinGuitar]

Haven't thought about it, will check, thank you!


Edit: 12.6v before regulator in my Retro Tube II PSU