On 24/08/2013 10:30, Arfa Daily wrote:


"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

Have you ever come across EH EL34 with suspicious inward dimples on the
dome end.? The retainers are ring and spring so probably heat-sink the
outer part of the dome , leaving the central area, which could perhaps
overheat and melt inwards. The dimples are not all the same so unlikely
an imprint from a production jig. I find it highly unlikely that 4
valves should have ended up with much the same dimple.
If the getter is activated with the valves dome down , could there have
been localised glass contact heating , causing small remelts?
All 4 test in normal range. No other signs of the valves overheating in
use, chalky print , green and black is still green and black and readable