12V regulator IC that can power 6 preamp tubes (900mA) FROM COLD STARTUP?

[Wombaticus]

If there's anything worse than making a mistake, it's making the same mistake twice.

The micpreamp I'm working on right now has a 12.6V regulated DC supply for the heaters. It's a dual, with three tubes a side, so a total of six tubes, drawing a total of 900mA of current.

I designed a 48V/12.6V board for this thing. But the f*ing 12V regulator I specced (a Micrel 29300-12) can't get over the thermal hump to warm the filaments enough before going into current limiting, even though it's a 3A part. It simply can't fire with 6, 5, or even 4 tubes. If I have only three, it lights right up, and I can then plug in the remaining ones and go on my merry way, so this is definitely the issue. The cold filaments draw waay more current for the few seconds until they heat up enough so the reg is unhappy, and just can't get going.

Those of you with a good memory will remember that I confronted exactly this same issue a couple of years ago with a different build (albeit a different part). But somehow this whole episode was not taken into account when I designed this board.

Anyway, last time I solved this by fitting the voltage regulator with a BJT power transistor pass device helper, using something pretty close to Merlin's recommendation in his power supply book. This worked. But this time, I'd really really like to avoid a redesign.

Soooo... does anybody know of a 12V regulator (preferably three pin) that can, no questions asked, light up a string of six * 150mA @12.6V preamp tubes *from cold* without locking up? Maybe I'm looking for an older device with less sophisticated over-current protection?

For what it's worth, the voltage multiplier / TL783C adjustable regulator on the same board for the 48V phantom supply worked a charm... so that's something.

[raiken]

Have you considered one of those little modular switching regulators? They usually have soft-start and current limiting, and are much higher efficiency than a linear regulator.

Here are some 3-terminal ones with short-circuit protection, automatic recovery, and a 330kHz switching frequency that should keep your noise to a minimum:

http://www.digikey.com/product-detai...718-ND/1828611

http://www.digikey.com/product-detai...181-ND/2352140

http://www.digikey.com/product-detai...046-ND/2256226


Here's a nice five-pin 600kHz synchronous one from Murata:

http://www.mouser.com/ProductDetail/...%252bB0x1tcUeu

[olddawg]

If it lights up 3 tubes, why can't you just use 2 regulators?

[Alan0354]

I scan through the data sheet quickly, one thing stood out is you are dropping too much voltage......From 48 to 12. That is 36V. So for the first few seconds, you dissipating too much power. Even at .9A steady state, the power dissipation is 36X.9=32.4W. That's a lot of power in one chip. do you have adequate heat sink? I would consider a switching regulator.

[trobbins]

Assuming a 2C/W package for that device, and 48V in, you've got 64C junction temp rise to case at the very least - you must be sinking that case to a huge heatsink just for starters! What was your basic thermal design parameters?

[jazbo8]

Why run them on 12V, all with the same regulator? String 4 together to run on 48V, and just 2 on 12V.

[Alan0354]

Why run them on 12V, all with the same regulator? String 4 together to run on 48V, and just 2 on 12V.

That would be a good idea, but how sure are you the voltage drop is even between tubes?

[jazbo8]

Don't need to... as long as the filaments are good, the voltage drops will fall within the manufacturing tolerance, Peavey has been doing this for years.

[Enzo]

I agree, let the same 150ma feed several tubes wired in series.



My own opinion, for what it is worth, is that especially in a preamp the need for regulators on the heaters is questionable. The mains variation is not going to affect them hugely, and there won;t be any sag issues with all the class A circuits of a preamp.

[Mick Bailey]

I've come across this problem a few times. I use a 3-terminal adjustable regulator with a simple soft-start circuit that tracks the rise of a capacitor charge to control the output voltage. I have a Hi-Fi amp that's been running with this setup for years.

Here's an example;

[Merlinb]

A further possibility is to put a resistor in parallel with the regulator to allow some current to bypass the regulator. This may be enough to warm them up to the point where the regulator comes out of shut down after a few seconds. Line regulation will be reduced of course, but modern regulators are so good that this may be neglibible.

My own opinion, for what it is worth, is that especially in a preamp the need for regulators on the heaters is questionable.

The OP is talking about a mic preamp, which needs to be quiet above all. This makes heater regulation pretty much mandatory.

[Mick Bailey]

Another thought is to put a thermistor in series with the regulator output. Say, 10 ohms. Depending on the spec it can drop to about 0.1 ohms after 100 seconds. Cheap, too.

[Wombaticus]

I scan through the data sheet quickly, one thing stood out is you are dropping too much voltage......From 48 to 12. That is 36V. So for the first few seconds, you dissipating too much power. Even at .9A steady state, the power dissipation is 36X.9=32.4W. That's a lot of power in one chip. do you have adequate heat sink? I would consider a switching regulator.

Gents,

Thanks for the many helpful replies. There is evidently at least one misunderstanding here though. To be clear, I am NOT powering the 12V regulator from a 48V supply. The 48V supply is an independent circuit. And since it's based on a voltage quadrupler, there is absolutely no way that I could source enough current from that to power a heater string. It's intended solely for providing a maximum of 20mA or so for a couple of phantom-powered mics. The 12V regulator is being run from a full wave bridge rectified 12.6VAC supply.

EDIT: Added schematic Roundhead_mic_pre_DC_power_board (2).pdf

[olddawg]

Gents,

Thanks for the many helpful replies. There is evidently at least one misunderstanding here though. To be clear, I am NOT powering the 12V regulator from a 48V supply. The 48V supply is an independent circuit. And since it's based on a voltage quadrupler, there is absolutely no way that I could source enough current from that to power a heater string. It's intended solely for providing a maximum of 20mA or so for a couple of phantom-powered mics. The 12V regulator is being run from a full wave bridge rectified 12.6VAC supply.

EDIT: Added schematic [ATTACH]27271[/ATTACH]

Then just run a Pi filter off of the bridge and a Zener diode to ground at the DC voltage you want. Problem solved.

[Wombaticus]

Then just run a Pi filter off of the bridge and a Zener diode to ground at the DC voltage you want. Problem solved.

Sorry, no, I think there's far too much current involved for such a crude solution to be practical.