peak current 300B heater with start up

[Jaap]

How much Ampere does a 300B filament use before it is on temperature and stabilizes ?

[jeff mai]

It may not be exact, but measure the resistance of the filament when it's cold and then use Ohm's Law.

[Sheldon]

Originally posted by jeff mai
It may not be exact, but measure the resistance of the filament when it's cold and then use Ohm's Law.

It will be a lot closer if you add transformer resistance, and any other R in the power supply.

Sheldon

[kevinkr]

With regulated dc supplies I have measured inrush currents of >3A with cold 300B filaments, where 3A was the current limit of the regulator in question.

(JJ 300B with nominal Ifil of 1.5A, not the 1.2A typical of most 300B)

I now use constant current heating which results in the tube taking I estimate about twice as long to heat to operating temperature as constant voltage mode.

For voltage regulated filament supply just filament dcr may be adequate if the regulator does not drop out , in the case of unregulated supplies or regulated supplies that are in drop out you need to do as Sheldon suggested.

[Brian Beck]

If you use three-terminal voltage regulators for heaters/filaments, it's easy to slow down the turn-on voltage ramp with a couple of extra parts. This prevents the surge of current when the filament is cold, reducing stress on all parts. I do this routinely for even indirectly heated tubes.

[richwalters]

Originally posted by Brian Beck
If you use three-terminal voltage regulators for heaters/filaments, it's easy to slow down the turn-on voltage ramp with a couple of extra parts. This prevents the surge of current when the filament is cold,

Well' not quite. If the psu doesn't droop, then the dissipation has to go elsewhere in the regulator pass transistor. for example I notice the instant cold rush heater current spiking in one half NOS ECC83 is over 1A. This drops fast but the initial surge can still damage chips.
In the case of a common reggy 78 series or 317, these devices have thermal + internal foldback current limiting, so the device is essentially self protective on s/c's solong is heatsinked.. In the case of discrete parts, i.e simple pass transistor and zener config can be easily destroyed without additional protective features.

Amp builders using DC for the big game heaters, 6550/KT series and 6C33 stuff, carefully do the current homework. The instant on current v.s resistance represents a darned good s/c scenario.

A car headlight bulb is a cheaper simulation than a vacuum tube.

richj

[Brian Beck]

You'd have to work overtime to blow out an LM317 or one of its brethren when used this way. The point is that the slow turn-on circuit prevents the current surge from ever happening to begin with (with properly chosen time constant). And LM317-style devices are both over-current and thermally protected. They would shut down in a fault event, but even that won't happen here. The idea is to gradually raise the filament voltage so that the current never does surge. Current will track the voltage monotonically, although not linearly. In the case of the 300B, it would gradually grow to its final value of 1.2A. BTW, that's just under the 1.5A limit on the standard LM317. It would probably work fine, but the LM350 can be used for more margin. During the short time when most of the supply voltage is dropped across the LM317, the current is not yet at full value, and even if the momentary power dissipation is high in the LM317 due to higher voltage drop, the thermal inertia will carry it through the ramp-up safely, unless you've chosen a very long delay. I've built a number of these delayed heater supplies, many driving a large number of tubes simultaneously, and I don't ever recall an LM317 failure. Likewise, I've used them to regulate 1000V with nary a glitch. You gotta love this little part.

[richwalters]

true true....the 317 is very versatile and with protection diodes and offers excel' ripple reduction and is bomb proof. However my favourite brew is pnp pass instead of npn darl in 317 to reduce in/out differential, hence dissip.

[Jaap]

I asked the question because I bought 2 switched PSU's very cheaply in a surplus shop. They are 5 volt/4A and are protected against overload. I wonder if it is safe to try them. Perhaps I can mount them in a tin box to prevent HF hash (?).

[Brian Beck]

Oh well, in THAT case... You will have no easy provision for a slow start, but the current capability ought to be fine. I would double-check that the output voltage is isolated from the AC ground, since the filament voltage is your cathode voltage. It probably is isolated, but I would still check. Noise is then the question. Yes, I would mount them inside metal boxes, and perhaps consider placing these outboard of your main chassis. You may find that you will have to add LC filters after the supply to get rid of switching noise. This can vary a lot from supply to supply depending on the original application it was intended for. A ferrite clamp on the output wires might be useful.