Help with exploding tubes!

[carbondated]

So I put the final wiring together on the RH84 amp that uses an ECC81 into an EL84 and powered it up - all the heaters glowed nicely and, when connected to a source, played nice sweet music. After tying down the wiring I powered it up again and the ECC81 exploded like a firecracker. I checked for shorts, etc., and then put in a new one. This one glowed bright like a lightbulb before settling down into its usual orange dim glow. And it played music just fine.

So I suspect the filaments, but the EL84, which is connected in parallel, behaves itself. I tried another ECC81 in the same socket and I get the bright flash upon switching on. Bad batch of tubes? Or something wrong with the filament wiring?

[db!]

Did the glass envelope actually rupture or was it an internal explosion? If it was the latter, the tube likely had a short...

[Miles Prower]

Originally Posted by carbondated

This one glowed bright like a lightbulb before settling down into its usual orange dim glow. And it played music just fine.

So I suspect the filaments, but the EL84, which is connected in parallel, behaves itself. I tried another ECC81 in the same socket and I get the bright flash upon switching on. Bad batch of tubes? Or something wrong with the filament wiring?

From the spec sheet I looked at from Frank's, it looks like this bright glow is normal for the ECC81. The spec sheet recommends the inclusion of current limiting when using this type in a series string. It looks like this type has an unusually low cold resistance.

I've seen the same thing with duals where the two heaters are internally connected in series with just the thin heater wire connecting both sides. (Construction on the cheap here.) That thin wire will really heat up in a hurry before the heaters have a chance to warm up and increase the heater resistance.

[carbondated]

Thanks, guys. Is there a way to slow start the heaters? I'm just using AC heating directly connected to the 6.3V tap and the ECC81 is the first tube on the parallel ladder. Would putting a capacitor across the filament wiring or switching to DC heating help?

The glass envelope stayed intact. It was a NOS tube -- but I had powered up the heaters when I wired that up earlier, but it was on the third switch-on after it had been playing music for about 45 minutes that it died.

[tomchr]

Add some resistance in series with the filaments (or at least the one for the ECC81). Limit the current to maybe 2x the rated filament current. Then short the resistor out with a relay after a delay.

You can also get in-rush limiters, but I doubt they'll help you much in your application.

Another alternative would be to go with DC on the heaters using a regulator with a slow-start feature.

Or just accept the circuit as is. ECC81 is a current production tube...

~Tom

[sepolansky]

Some tubes exhibit this behavior. They are called "lighters". Seen them work fine for years.

[carbondated]

Really newbie question: I "paralleled" the two halves of the ECC81 by shorting pins 4 and 5, the filament outs, and then connecting the voltage to that and then to pin 9, the common. This is acceptable practice, yes?

Or, as Tom says, I can get some new (and hopefully more robust) production tubes and see how it goes. I forgot to mention that it's the RH84 circuit, and when it's not blowing up tubes it sounds sublime!

[r2k]

Hi


here's a really simple constant current regulating circuit, the voltage regulator keeps constant voltage over the resistor and you get I=U/R(+ref current, but that is usually in microamps and thus negilable), just pick the correct resistor and regulator combo for your current needs and put it in series with your filament. that will ensure the current will never exceed the nominal. obviously the voltage of your supply must be appropriately of higer voltage and there will be a loss of power across the regulator

[godfrey]

Originally Posted by carbondated

I "paralleled" the two halves of the ECC81 by shorting pins 4 and 5, the filament outs, and then connecting the voltage to that and then to pin 9, the common. This is acceptable practice, yes?

Yes, but maybe you could slow-start the tube by first connecting the two halves in series to 6.3V, then switching over to the parallel connection after a short delay. That would allow the heaters to warm up a bit before full power is applied.

Switching could be done either with a relay connected to a delay circuit, or with a 3-way power-switch i.e. off/warm-up/on.

[DF96]

Almost all Mullard/Philips dual triodes do this bright switch-on. It is caused by a short length of heater which is not cooled by the cathode. It doesn't seem to do any harm.