Cheap power supply for AC701 tube microphones

[Khron]

If it's a vintage M49, you'll first of all need to replace the regulator for the heater voltage with a 3.3v one plus a diode (to get about the 4v a AC701k needs).

 

[Papa Tango Charly]

https://groupdiy.com/threads/m49-microphone-from-6s6b-v-to-ac701k.81028/post-1043190

 

[Papa Tango Charly]

Does any body have a schematic of a M49 with a 6S6B-V tube? Thank you

 

[Papa Tango Charly]

 

[kingkorg]

Does any body have a schematic of a M49 with a 6S6B-V tube? Thank you

You can use this one, 6s6b is obviously triode, but nothing is changed. You might want to adjust the plate/cathode resistor to get 1/2 of B+ at anode if necessary.

https://www.tab-funkenwerk.org/ami-parts/mic-kits/m49a-kits/m49-schematic/

 

[RuudNL]

Or the cathode resistor.

 

[Papa Tango Charly]

Hello Ruud thank you fro your answer. The plate cathode resistor is in the mic or the PSU?

 

[Papa Tango Charly]

I know it must be pretty basic but I’ve got to start from the beginning…

 

[Khron]

I know it must be pretty basic but I’ve got to start from the beginning…

https://en.wikipedia.org/wiki/Triode

 

[RuudNL]

The cathode resistor is (usually) inside the microphone.
Higher value of the cathode resistor = more negative on grid = higher anode voltage and v.v.

 

[Papa Tango Charly]

Ok the Heater is 6V

 

[Papa Tango Charly]

This week a Neumann M269 microphone came in.
The owner knew that the microphone worked, but he didn’t have a power supply for it.
So, if I could build a correct power supply for him…
Because it is a lot of work to create a single power supply, I investigated the possibilities to use a simple ‘Chinese’ power supply, like the ones used with the Apex 460 for example. Those power supplies can deliver +120V HT and ~6.7 V (7806 + diode) for the filament. The AC701 wants only 4 V, so this would need a modification. Since we don’t have something like a 7804 regulator, I used the well known LM317 in a pretty standard configuration. But with a little extra: a ‘slow start’ to hopefully extend the life of the filament in the (expensive!) AC701.
Because I wanted the circuit to be 100% safe, I also added a crowbar circuit.
In case the output voltage becomes higher than 4.1 V (adjustable), the thyristor is triggered and blows the fuse. I have build the circuit on a small PCB, that can easily be fitted inside the 460 power supply.
The original transformer delivers 9.5 Volts AC, this results in about 12 V after rectification and smoothing.
To reduce the dissipation in the LM317, I added a 47 ohm resistor where the 7806 regulator used to be. (Between input and output.)
This works fine. Filament current is only 100 mA and everything stays cool.
Here is the schematic:

I don't have the skills to do that on my own. But i was wondering if your circuit would work M49 with a AC701 Tube.

 

[Papa Tango Charly]

Could this little guy would be an option too? Ultra Low Noise LDO Voltage Regulator TPS7A4700-LM31XG
https://www.ldovr.com/product-p/tps7a4700-lm31xg.htm

 

[Papa Tango Charly]

I don't have the skills to do that on my own. But i was wondering if your circuit would work for e M49 microphone with a AC701 Tube.

 

[kingkorg]

Don't get me wrong, you have to get your basics in place first, you don't want to burn your house down. I suggest building some kind of kit, or proven working project with existing support thread.

 

[Papa Tango Charly]

Thank You! wise suggestion..... i will dig this one. best

 

[RuudNL]

For a M49 with a 6S6B you can use a generic power supply.
There is no need for an extreme low noise filament voltage.
An 'ultra low noise regulator' would be overkill.

This would be a different situation for a KM56 or a U67 microphone (for example), where the grid bias is derived from the filament voltage.

 

[Papa Tango Charly]

For a M49 with a 6S6B you can use a generic power supply.
There is no need for an extreme low noise filament voltage.
An 'ultra low noise regulator' would be overkill.

This would be a different situation for a KM56 or a U67 microphone (for example), where the grid bias is derived from the filament voltage.

"You can use this one, 6s6b is obviously triode, but nothing is changed. You might want to adjust the plate/cathode resistor to get 1/2 of B+ at anode if necessary."

I understand
Would i have to lower the voltage of the power supply to +120V
I use a 56V zener and a 62V zener in series, that gives me a voltage close to +120 V. R4 seems to be the resistor of the plate and R3 seems to be the resistor of the cathode?

 

[kingkorg]

"You can use this one, 6s6b is obviously triode, but nothing is changed. You might want to adjust the plate/cathode resistor to get 1/2 of B+ at anode if necessary."

I understand
Would i have to lower the voltage of the power supply to +120V
I use a 56V zener and a 62V zener in series, that gives me a voltage close to +120 V. R4 seems to be the resistor of the plate and R3 seems to be the resistor of the cathode?

Exactly. You don't have to lower it, but 120v is "traditional" voltage used in these.

 

[Papa Tango Charly]

Thank You