"Phil Allison" wrote in message
...

**Hi to the amp techs,

there is something odd about the HEATERS in current production EH branded
valves.

If you check a 12AX7EH, 12AT7EH or 12AU7EH, you get 32 ohms between pins
4 and 5.

Check any other brand and the reading is 10 to 14 ohms.

The EH ones still draw 160mA at 12.6 volts like the others.

Similar but smaller differences exist with EL34EHs and 6V6EHs too.

That Bugera I posted about a few days back would NOT have developed the
nasty AC fuse blowing fault it did if EH brand 12AX7s were fitted - as
the start up heater current would be 3 times less.



... Phil




So what's your theory ? It seems a contradiction that the heater can have
three times the resistance, but still draw the same current ? Maybe the
material that they've made it out of has a different resistance v
temperature characteristic that decreases as the temperature increases to
get back to the same running hot resistance as other makes once they have
warmed up and their heater resistance has increased as a result, although
I'm not sure what - if any - metal has such a characteristic. I suppose it
would act a bit like the heater thermistor that we had in the old days of
valve TV sets, to stop the heaters coming up like torch bulbs at switch on.

How about hooking one of each to a variable DC bench supply, and seeing what
happens to the current as the voltage is gently turned up. It would also be
interesting to look to see if there *is* any surge current when full voltage
is applied from cold.

Interestingly, I used to have great faith in EH valves, but of late, I've
found them to not be anything like as reliable as I think they were a couple
of years ago. Maybe they're getting them from a different manufacturer now ?
Do you happen to know anything about who's valves they actually are ?

Arfa