Open circuit 100R heater resistors?

[JB]

Help me with some weirdness here!  Took my 5E3 with VVR to a rehearsal, had worked fine last time I tried it at home about a week before.  Massive hum/motorboat type stuff happening which varied with volume controls and the VVR position.  Back at home I swapped rectifier and 6V6's but still big hum, less motoring.  Lots of experimenting - pulling driver, measuring volts (all good), lifting output stage coupling caps in case of leakage.  Bypassed the VVR - no difference.  Then I measured the heater supply.  6.3V across the pair but 174VAC from one leg to chassis.  So the reference has gone.  Both 100R resistors are measuring open circuit.  They were 1W metal film.
So - er, why?  They were mounted on the lamp holder base with a flexible wire to chassis earth point.  They don't look mechanically damaged or burnt.  What would have taken them both out?  I can replace them but any thoughts on there being some other problem that took them out, or just some kind of weird coincidence?

[John]

They were mounted on the lamp holder base with a flexible wire to chassis earth point.

I guess it's possible that the connection shorted against something, but then you'd think they'd be burned. Usually, when those fail (I've read here) it's because you had a cathode to heater short. I think that usually happens in the power tubes? Check your sockets and make sure there's no sign of charring anywhere.

[JB]

Interesting thought.  Sockets - ceramic - look nice and white and clean though.  Disconnecting the power supply to the driver and preamp cleared the big hum.  As did lifting the driver to PA coupling caps.  As if the hum was on the driver stage power rail.  Maybe that's a false positive, or maybe it's not just dead heater resistors.

Edit: It was a new to me, used but testing good pair of 6V6 in there that I had only put a few hours on.  Maybe one of them had a wobble, I'll put them both in the valve tester when I get the chance and look at the cathode/heater resistance.

[92Volts]

Heater-to-cathode shorts don't *only* happen in power tubes, but power tube cathodes are more likely to output enough current to fry those resistors. Hum can be caused by floating heaters, but *large* amounts of hum seems consistent with a heater-to-cathode short.

If any heater wire, pin, or element inside a tube contacts high-voltage... main wiring, high voltage transformer taps, supply, etc... the voltage will go to ground through those resistors.

Try replacing them, maybe with a higher resistance so you don't get fireworks.

Even 100k resistors should keep the heater windings close to ground, unless something's pulling them up. If they're not both (roughly) 3.15v relative to ground, the root cause is still there.

Also, I'd test for DC voltage. Assume heaters are in contact with a plate supply or something. They're at 300v DC +/- 6.3v AC. That confuses the hell out of a cheap multimeter.

[JB]

More good thoughts, thank you.  It's a 5E3 so cathodyne PI with the cathode sat at about 45VDC.  Nice old Mullard ECC83 in that position which should be able to handle that no problem, but of course it could have developed a fault.  I can measure/swap out that once once I've replaced the heater resistors.
I've got a basic but decent Fluke MM which should be able to cope with 6.3VAC sitting up on some DC.  Or break out the scope!
I'll get some more hands on time with it in the next day or two and report back here.
 

[sluckey]

Heater to plate short is more likely. May have been just a high voltage "flyback" arc such as might happen when an amp is played hard with an open speaker load. Replace the resistors and see what happens. Half watt is fine.

[JB]

Found the culprit.  Heater open circuit on one 6V6 (put them in my valve tester).  Smoked glass so I didn't see it, and I swapped them out and still had problems (big hum because the 100R resistors had gone).  Didn't imagine it would be two faults.

I haven't replaced the resistors yet but hopefully all will be well when I do.
I guess it didn't enjoy the car journey to the studio!  New experience for me, we learn new things.  Sadly it was my best pair of 6V6's !


Update:  Found a couple of 120ohm resistors and tacked them in, now working fine again.  Just need to get the VVR back in place and tidy things up.  So, as you guys suggested it was a heater to cathode/anode or whatever short, maybe momentarily when the heater broke and that took out the resistors.  You wouldn't have know from looking at them that they were open circuit. 

Thanks all for you help!
JB

[pdf64]

I'm not sure of the exact mechanism, but a common type of power tube failure sends HT current to 0V via the heater circuit.
If there's a heater CT then fine, fault current flows and the mains fuse blows.

But if ~100 ohm heater balancing resistors are used, then they tend to get blown open circuit by that HT fault current, and without that reference to 0V, if the fuse holds, the whole heater line gets pulled up towards HT voltage.

That can cause damage to the heater - cathode insulation of all the other tubes in the amp, resulting in tubes with a lot of buzz.To avoid a power tube short damaging the heater-cathode insulation of the other tubes, I suggest that electrically robust resistor types are used for this application, eg 3 watt vitreous wirewound.

The point is it seems beneficial for those resistors to hold up when there's a power tube short.

That assumes the correct value and type of fusing has been used in the amp; that's a safe assumption, right? 

[John]

going completely off topic: I clicked your "my band" icon. Is that Ed Chambley playing bass?? Because that's exactly how I've always pictured him. 

[sluckey]

Ed is a bit more animated. Well, OK, a lot more animated! Fun guy. Big history!   

[John]

Darn! Another shattered illusion and broken dream. Just like a country music song.

[pdf64]

Haha, no, that's the great Chris Bevington, unfortunately he's moved on as he got too busy with his real band and the dayjob http://www.chrisbevingtonorganisation.com/