Battery tube mic supply

[Tubetec]

Arising out of the discussion on battery regulated heater supplies and with reference to the Neumann N series supplies with the stablyte cells I wanted to start a thread .

I was wondering about the feasibility of making a tube mic supply with battery backup ,
My experiments using a disposable camera flash pcb and single 1.5 volt cell to generate around 300 volts at a few mA seem to suggest its capable of doing the job ,
The flash units themselves charge a 100uf cap in around 10 seconds which is probably a quicker ramp up than we need in a tube mic . With the customary 4 or 5 RC filters found in tube mic HT's , 5-10k ohms dropper resistors and 470-680 uF caps your ramp up time can be made several minutes . Seeing as we need around half an hour at least for a tube mic to settle down thermally theres no issue , another plus is the caps get a very easy life with no sudden spikes . They dont even need to filter 100/120 hz but more a bucktoothed hashbuzz measured in Khz . Of course the flash unit wasnt designed with low noise in mind , is there any simple modifications you could add to the existing circuit to help clean up the output ? or for that matter helping to prevent any noise appearing across the battery terminals ,
A sweet tin shield might better contain any EM garbage the circuit produces . The circuit itself does emit a small amount of acoustic noise at switch on which gradually fades out as the cap charges up , any who's used a flash camera will be familiar with the sound .

C3 R3 FL S2 and T2 arent needed



As far as the LT circuit goes I note the 'two stage' battery regulation circuit in the Neumann N52a series PSU , choke provides filtering and the required voltage drop from 4 to 3 cells. . I still havent managed to track down the schematic for later Neumann stablyyt supply for 6.3 volt usage.
A five cell chain with a few thousand mAh capacity , then the choke filter followed by four smaller capacity rechargable button cells in the microphone itself was a thought I had , of course the button cells would need to disconnect from the heater when the mic is not in use so at least one relay would be needed in the microphone itself in that scenario .

The other thing I thought about is how much would the heater voltage vary from on charge to battery opperation , obviously if the first set of cells is larger capacity and higher voltage it can easily hold the second smaller capacity cells close to full charge more or less permanently with use or after an hour to stabilise if unused for a longer period.
In other words even as the main cells in the power pack are charging the voltage across the secondary cells/heater should remain fairly constant , that theory I will need to bench test .


There was the mention of the supercap also in relation to the Sony mics , I just wondered if anyone might explain the benefits or pitfalls of that technology , seems to have a shelf life of only a few years and a potential to leave a nasty mess behind if it goes Wiki on you .


I dont have access to a workbench just now so its all theoretical, cobbled together on scraps of paper .
Theres no doubt about it , using batteries introduces a lot of extra what-ifs to the equation even if it does solve some of the shortcommings of parralel RC off the cathode or quiet heater supply , the question of charge management ,ease of maintenance of defective cells etc , my conclusion is make any batteries in either PSU or mic easily accesible ,not soldered in and hidden away so during longer periods of inactivity batteries can be removed and kept maintained on microcontrolled chargers . or at least stored where leakage isnt an issue



Comments/thoughts , up down left right or center welcome as always.

 

[Tubetec]

It could be marketed as the NUK(e)67 , guaranteed to work for (upto)10 hours after the start of WWIII when the power grid is knocked out , weighs as much as a small boat anchor :-,)

 

[Tubetec]

The logistics of trying to swap out and monitor individual cells is a pain in the arse you dont need in your life everyday of the week , besides usual plastic tray panel mount battery compartment would make your mic PSU look like something the Christmas fairy left behind .

Theres any number of 'tough' battery packs available for handlheld radios , The old fashioned style has a really sturdy rail mount and stout spring loaded terminals , what ever voltage and current capacity you want .
If you have the abillity to change over the battery pack quickly and charge it away on an external charger theres no reason at all to be having the mains dumping its garbage into your mic power pack. You save all the associated wiring, the noise, the grounding issues and the need for a mains transformer.

Looks like two of those Icom BP8 type packs could be made to slot in nicely if the IEC connector and voltage selector were removed from the Retro PSU below.
Now you get not only dual redundancy back up /double capacity but also hot swapable on the fly without signifigant interuption in the audio .

 

[Tubetec]

Im a little stuck on the camera Ht unit now ,
Of course I have a fully floating battery generated 300volt supply ,
but most of them rectify the negative spike so you end up with the neg terminal of the supply battery at full Ht volts if you wire the supply negative ground .
Obviously I cant just flip the battery round ,
I cant re-bias the diode to forward and generate a voltage off the positive peak because it would probably upset the transistors and destroy them .
There seems to be at least one or two positive voltage flash generators with respect to its battery negative , but these disposable flash cameras are getting rarer than I thought these days , several chemists I tried said the whole disposable camera thing and film processing had more or less come to an end with covid. Or at very least was shipped out somewhere else .

In any case I'm trying to come up with a simple HT oscillator circuit that runs right way round off a 1.5 v negative ground battery . These flash units rely on core saturation of the transformer to opperate so produce horrendous garbage , several passive RC filter sections can still wipe it out though . Still probably much better to start off with a cleaner sine wave .

The other important point about this is safety , I have no mains connection , no breakers , no ground , how do I protect the user from the potential for shock where chassis has HT leakage voltage appearing ?

 

[Tubetec]

I found a couple of circuits based on an MC34063 IC a Mosfet and few transistors , they run off low enough voltage DC supplies .
The first is a product on Ebay , the second two are from this site
The Hans Borngraber Electronic Battery
I see the way he has incorporated common/differential mode filtering in the third example ,
At 90 volts and 11ma 5 hours isnt running time isnt great
And a 555 based HT generator at the end .
https://sites.google.com/site/evanquirk/Home/ntpsyncednixietubeclock
Clearly the IC based circuit produces a far cleaner square than the old bucktooth betty of a waveform the Flash unit makes. Another plus is that the ascending squeel sound from the transformer underload is eliminated .

The question of opperating frequency arrises and waveform quality , I note in the battery opperated Bruel & Kjaer 2203 SPL meter they derive all supply voltages from a perfectly clean 1khz sine tone , it doubles as cal for the for the unit itself.

 

[Tubetec]

I found this neat little Nixie board with a similar boost circuit , it derives a regulated 6.3 dc and switched HT from single rail mains or battery supply ,

It looks like a really quick way to get a low current tube mic or preamp project off the ground( + added galvanic isolation) ,

An on/off switch , an indicator lamp, an IEC inlet ,a transformer , the PCB , multicore cable/multipin connectors all mounted up in a box should take no more than an afternnoon .

Were all trying to think of ways to save the planet from our own waste and reuse stuff where the possibilty exists
The handy thing about the simple designs above is many of the components are to be found in standard household electrical waste , the HV caps the ,diodes , the inductors, the mosfets and transistors and probably everything else you need are common as muck inside wall wart, inline or computer SMPS's .

A small pcb should cost no more than a few Euros including postage or to have fabricated .
From there its upto the DIY'er , order new components or or get into the spirit of it and skip dive for most of the bits you need , you choose . You also accrue the benefit as far as ease of service goes , in a pinch you'll always be able to find suitable components lying around , even to build one from scratch if the need arrises .


The 6.3v supply could easily be used to run a 6X4 tube if you wanted the gentle ramp up of a tube rectifier in your HT circuit , more costly in terms of power and heat and your signal circuit would most likely require its own supply LT with a different reference voltage to ground . Still an incredibly neat board even if you just use it as HT . A tiny tube regulated HT using a dual triode pentode and some zeners or even Gas discharge tubes might be an interesting possibility with this off the shelf board .

 

[RuudNL]

For some strange reason my question landed in the wrong topic. (Probably my mistake...)
So I will ask my question again:

Is the 6.3 V 'floating' or is there a common connection with the HT output?

 

[moamps]

According to the bottom image of the PCB, HV and heater voltage have a common ground. And this can be concluded also by the fact that the power supply does not use a transformer but only two inductors.

 

[RuudNL]

Thought so!

 

[Tubetec]

Thats obviously no good for a heater supply with a raised reference voltage but maybe ok for your usual anode loaded set up where the heater sits close to ground potential .

 

[RuudNL]

I ordered a pair of these little power supply PCBs and I must say that they are pretty useful.
The 6.3 V output was 6.7 V on the PCBs I received, but this can be adjusted by putting a 820 K.ohm resistor in parallel with R6. That will bring the output close to 6.3V.
Add a transformer, a rectifier and a 7812 with some additional components and you have got a simple and compact tube microphone power supply.
In some cases it could be a problem that there is a common ground and +6.3 and +HT outputs.
A U67 for example needs -6.3 V, because the grid bias is retrieved from the filament voltage.
A transformer with two separate windings and two of these power supply PCBs, will make it possible to have a -6.3 V output and a +HT output, because both circuits are now completely isolated.
In some cases, where you need +120 V, it is inconvenient that you can't adjust the +HT lower than 150 V. (Not much anyway.) But a series resistor and a zener + capacitor at the output can solve this. (I used 4.7K in series and a 100 µF capacitor.) Anode current in a tube microphone is usually less than 1 mA. anyway.

 

[Tubetec]

Thanks Ruud,
Good idea to use two separate windings to isolate the grounds , the other handy thing is common off the shelf LV transformers will do the job fine , no need for a HT winding . We all have usable transformers in the junk box or maybe plucked from the huge pile of municiple electrical waste that surrounds us,that helps keep costs down for experimenters and diy'ers ,and makes projects accessible to anyone anywhere .
reduce, re-use and up cycle where possible .

 

[Tubetec]

https://groupdiy.com/threads/recycled-ht-caps.81410/I wanted to continue this thread but theres a few extras in the above post for anyone who wants to look.

Ive found the Fuji camera board generates a solid 330v DC , its not that noisey either ,very short term clicks/spikes of around 220mv pk-pk are visible at the opperating frequency ,or maybe a multiple of it ,but no acoustic noise is apparent from the unit in opperation . The circuit will run for days on a single AAA cell .

I'm about to try and tackle those noise spikes and see how they respond to filtering , I'm thinking of trying a common mode choke and some foil caps before the traditional RC/LC sections .

In the end the common mode inductor and supression caps made matters worse in noise terms with a buzz saw wave form appearing , The pulses Im seeing are around 14uS appart , it seems to be an anomily with the scope itself ,not the invertor at all ,it come outs at 70khz , most likely well above the opperating freq of the simple invertor circuit on the flash PCB . The circuit is almost identical to what you see below only it uses a UTC 2D2740L transistor and replaces the neon with a led .

 

[Bernard75]

Hello,
reading this thread I had a thought (possibly a stupid one being no expert...)
given the amount of energy needed is low some small size batteries could be used.
If one put a sufficient number of them in serie to generate 130 v (or what is needed -the total batterie size might be affordable ) and derive some current through an adapted resistor to drop the tension to 6.3 v
don't we have what is needed for a battery operated mic ?
(with 0 power supply noise)
Good for the bin or worth a thought ?
Kind regards

 

[Khron]

Hello,
reading this thread I had a thought (possibly a stupid one being no expert...)
given the amount of energy needed is low some small size batteries could be used.
If one put a sufficient number of them in serie to generate 130 v (or what is needed -the total batterie size might be affordable ) and derive some current through an adapted resistor to drop the tension to 6.3 v
don't we have what is needed for a battery operated mic ?
(with 0 power supply noise)
Good for the bin or worth a thought ?
Kind regards

While it's true that the B+ only needs minuscule current, keep in mind that the heater will need 2-300mA @ 6.3V, depending on what tube you use.

A plain resistor dropper is very much not an option, if you recall your Ohm's law (130v - 6.3v = 123.7v; 123.7v x 0.2A = (at least) 24,74W that you'd need to literally burn off with a resistor). And it's also worth remembering that whatever high-voltage supply you use, would need to supply ALL that current/power.
That's pretty much how the Neumann U47 works, but that tube has a 55v heater and single 105v supply, so there's less power needing burning off, but it's just as inelegant / a brute-force approach.

You'd be hard-pressed to find a buck-regulator with quite so high of a supported input voltage, so it makes loads more sense to start with the heater voltage that you need, and use a boost-converter (perhaps with a voltage multiplier ladder on the output) to get a high-voltage low-current supply for the tube plate.

 

[Tubetec]

Bruel & Kjaer 2804 microphone PSU .
Its powered from three 1.5v cells it produces 200 v /28v .
Its not too different from the photo flash PCB in terms of what it does ,
more details here including circuit .
http://yehar.com/blog/?p=4341

 

[Khron]

The only disappointing / annoying part (for me, anyway) is the custom magnetics...

 

[Tubetec]

Its not actually a tube mic supply the 2804 , its produces 28v power and 200v capsule bias for the transistorised mic ,
there is a 6 volt battery input but that seems to be destined to heat the mic preamp in cold climates ,
Still though the simple 3 transistor circuit its based on might be worth further examination for a battery based mic HT supply .

How difficult could it be to wind a small pot core inductor ?
Agreed getting it right might take some experimentation , with materials and coil config ,
Thor.Z seems to know a lot about doing switchers quietly , for several reasons using frequencies up into the MHZ region seems smart , mainly the reduced size of the caps and inductors and less chance of any interference in the audio path .

 

[Khron]

I've had a similar thought in mind (even made a little board for powering active pick-ups for guitars from a lithium cell), but I'll have to experiment a bit with the boost-converter chip i used there...

 

[rmburrow]

Hello,
reading this thread I had a thought (possibly a stupid one being no expert...)
given the amount of energy needed is low some small size batteries could be used.
If one put a sufficient number of them in serie to generate 130 v (or what is needed -the total batterie size might be affordable ) and derive some current through an adapted resistor to drop the tension to 6.3 v
don't we have what is needed for a battery operated mic ?
(with 0 power supply noise)
Good for the bin or worth a thought ?
Kind regards

9 volt batteries are relatively inexpensive and D cells are relatively inexpensive. Four D cells in series can run the tube heater, and one dozen (12) 9 volt batteries in series are good for the mic's HT. My only comment is to put a 100 uF 12 v capacitor in parallel with a 0.01 uF mylar capacitor across the LV output, and a 25 uF 250 v capacitor in parallel with a 0.01 uF 200 v mylar capacitor across the HT output. (The small value capacitors are for RF filtering.) The battery holders can be found at Mouser or other sources. DO NOT LET any batteries or contacts touch each other on the HT side with the series connection!