Deriving tube Heater voltage from +/-15Volts . . .

Hi there,

   I have searched extensively, to no avail! Can I just use a 5V regulator and a diode off one rail?


     Any help most welcome!


   ANdyP

Current!!!

No, seriously... think about the current. -A DC-DC converter might be a better idea for 6V heaters, unless you can gang a few of then up in series, and even then... you have to consider the current.

Keith

In theory you could (I'd use two diodes, to get closer to 6.3V). However you may want to consider regulator dissipation; a common dual triode like a 12AX7 draws [email protected]. Your regulator needs to drop the difference between supply and heater voltage or (15-6.3)=8.7V; at 300mA the reg will have to dissipate 8.7*0.3=2.6W. If your tube allows it you may want to consider wiring it for 12.6V heater voltage.

JDB.
[...but a 12AX7 or similar will still draw 150mA heater current @12.6V, exceeding the power budget of a 500-series module. If you want to stay within spec that means you'll need to design or buy a switching DC-DC converter, preferably one that's not too noisy]

[EDIT: Der Keef beat me to ze punch... foiled again!]

Yeah, you always need to remember that the juice needs to be sunk somewhere unless you're using a DC-DC converter as Keef suggested. A zener and a fat resistor might do the trick simply, but JD's suggestion of series-wired heaters would save you a bit of bother as you'd only need to drop 3V odd.

Keith and JD,
  (and Roddy now too!)


   that's kinda what I thought, but I am so green as to be cabbagelike . . . that's why I asked! Thanks.


    not beyond the power budget of TWO (ie Doublewidth!) API 500-series modules though. Also tube i have in mind is only a 6.3v heater . . .


    Many thanks,



       ANdyP

You might be able to, as others posted it depends on the supply.

Do you a schematic of the +- 15VDC supply and its specs?  Also what current is needed for the circuits +_15 and fil?  Are the existing heatsink(s) in spec for the added loading?

Some +- supplies might not work well with unbalance loads on each side.

It could be as easy as the first post.

Actually,


    it is for the API Lunchbox! Nice ready-made supply giving +/-16V at 130mA per module and +48V at 5mA, apparently . . . .



    Kindest regards,


    ANdyP

Goes back to the question of current then Andy. What valve?

The bigger issue: how to get an adequate B+.

JDB.
[obvious answers: a switcher, low-voltage tubes, starved plate. Pick yer poison]

What is the total + and - rail currents available from the rack supply?

What is the fil current needed for the tube? you don't have to post the number just the current needed.  Frank's tube page is good site for tube data sheets.

Say it is 300ma that would be > two modules of rated supply current(130ma) almost three.  You could use the + or - rail to heat the fil it does not have to be the + rail.  If possible measure the rail currents and tap into the one that has the smaller drain.  I am going to guess the raw + is loaded more by the 48VDC phantom if it is derived from the raw + voltage

If that is too much for the rack using a series dropping regulator circuit with the amount of modules wanted in it you could look into DC to DC converters.  You think in power with DC to DC and % eff.  130ma x 16VDC = 2.08watts

2.08 watts / 6vdc = .347 amps at 100% eff

Search for the Thomas Holly thread on the Phantom tube DI with DC to DC converter

Things to know, how overbuilt is the rack supply?  if a series drop  or DC to DC from one of the rails can the supply deliver the added current, are the PCB traces and edge connector and PCB? back plane able to withstand the added current?  You might want to plug the added tube module in a slot close to were the power is supplied to the backplane.

You might be able to find a stock DC to DC converter possibly adjustable to 6 or so VDC being close to 5VDC something like 15 to 5.

Hi Guys,


  thank you very much for all your replies. Looks like I am stretching the realms of possibility unless i build a THREE module width unit! Just have to build a bespoke case and psu afterall, at least whilst I'm experiment . . .


    Kindest regards,


      ANdyP

Gus makes a good point about just how over-built or under-built the PSU might be. Another thing to watch out for is ripple appearing as you draw more current. I assume this is a regulated supply, but things could potentially get a bit lumpy as you empty those filter caps more quickly.

You could quite easily test the supply with a dummy load though.

if your serious in considering building a tube unit the use with a 500 series supply, then best might be to use a dc-dc converter. That is what the companies that make 500 style tube units are doing well the ones I talked to at AES last year. I don't know about tube tech though, they got their own chassis and so forth.

Hey Andy,

For the 2nd time today I get to refer you to Gearslutz for tech info!!  Who would've thought it eh?

Anyhow I remember Justin Morse from Rolls music talking about the specific implementation of DC converter in his Tubule pre - might save you some leg work.  He's a very helpful fella too.

Cheers,
Ruairi

You don't need to build three module wide.  I maybe was not clear you need about 3 modules rated current if you want to use a series dropping reg, something to add if the + or- supply is well regulated all you will need is a dropping resistor(s) for the fil.

What is needed, the rated max loading of the rack supply.  If you are in the total rated max current with the modules you have in the rack and the extra drain from the fil supply you might be OK, and as jbakker posted what is the B+ powered from?

The parts about the PCB and backplane traces is because I have seen backplane traces burn if a part drew too much current when failing on a card in other rack equipment so it is something I keep in mind.

 I have not worked on or seen inside a API rack.   Whats the rated current of the connector and PCB connector edge traces?

Me again,

I tried various searches on Gearslutz and couldn't turn up the thread I thought I remembered.  Maybe it was Dan Kennedy talking about his 500 series pre? Either way I know they both use a DC-DC converter to get the voltages they need.

Cheers,
Ruairi