Heater filament resistance question

[Joel]

Hello fabulous people!

Back story:  (I haven't seen the device yet, I don't know the model, I'm only reporting what he's told me so far) 
A friend of mine has an old tube hifi he wants to fix up.  He says that it looks like beeswax has melted out of the power transformer.  He's pulled the 6CA4 rectifier tube, and says that the transformer winding resistances read OK...  We read online that 6CA4 tubes are prone to fail short, which might explain the power transformer overheat.  Break out the multimeter, and we find the heater filament cold resistance is 1.1 ohm.  Yes, I know there is a lot more fault finding and testing to do before he goes plugging any tubes back in.  But now I'm curious about heater filaments and there isn't a whole lot of information online.  This is where you fine people come in.

Now we finally get to the point:
6CA4 heater is rated for 1A at 6.3V, so I'd expect the operating resistance of 6.3 ohms.  I understand that the heater filament resistance changes with temperature, but what can we expect the cold resistance to be?  1.1 ohms seems very low. 

[sluckey]

I've never measured the cold resistance of a 6CA4 (EZ81) but I'm not concerned about that cold low resistance reading.

[PRR]

Cold resistance is 1/5th to 1/10th the hot resistance.

And "beeswax" can melt-out on a hot day, and may not be any problem.

[Joel]

Thank you gents for the quick response. 

It's been known to get warm in South East Queensland.  If it truly is beeswax, then the transformer would only have to heat up another 20 or so degrees for it to melt on a summers day.  (And that's real degrees, not your wacky Fahrenheit degrees!)

[Williamblake]

EZ80 are 3 Euro a piece and dont have a bad name i have heard of. But maybe the filter caps bleed DC?

[sluckey]

EZ80 are 3 Euro a piece and dont have a bad name i have heard of. But maybe the filter caps bleed DC?

I would not replace a 6CA4 with a lower current rated EZ80.

[Williamblake]

Sorry, should have thought about that before. I only now see that 6CA4 and EZ81 and not EZ80 seem to be the exact same tube and you see nobody complaining about EZ81s. Maybe that 6CA4-is-prone-to-shorts thing goes back to one internet post only? On a sidenote: Wouldnt a tube arch before it failed with a short? Eventually? And wouldn't that mostly be the plates arcing to the heaters? I know, the cathode would arch before but it normally runs at HT-potential and one of the few EZ81s i tested recently was arcing from the plates to i don't know where. But it was old. Anyways, hoping the hifi is ok again.

[HotBluePlates]

...  We read online that 6CA4 tubes are prone to fail short ...

... you see nobody complaining about EZ81s. Maybe that 6CA4-is-prone-to-shorts thing goes back to one internet post only? ...

I imagine it could depends on whose EZ81/6CA4 you're talking about.  I've got one in a Hammond AO-44 and a '65 AC10, and neither have failed.  Both are old American & European tubes, not modern stuff (I don't trust modern rectifiers because of the high number of reports of failures).

That said, the original quote was "prone to fail short."  I can't say I've heard anyone complain a rectifier "failed open" which would lead to no damage but also no amp function.  So then "every rectifier is prone to fail short" if it fails at all.  Smart Money then assumes the rectifier will eventually fail short, and installs a solid-state diode between each plate & the high voltage winding.  That pre-rectifies the voltage before it's applied to the rectifier tube, and prevents a shorted rectifier tube from making the filter caps explode.

An example is D6 and D8 in the Princeton Reverb Reissue schematic.