"Phil Allison" wrote in message
...

"Arfa Daily"
"Phil Allison"

**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.


So what's your theory ?


** I thought it was obvious.

The filament wire is not made from tungsten, as in all the others.


Well yes, that much is obvious, but if the heater current ends up at the
same as for other makes when it has reached temperature, then whatever they
do use must have a negative temperature coefficient, and I'm not sure that
any metals suitable for making a valve filament out of, do. Any theories on
what the material might be, and why they would use it over a cheap material
like sintered tungsten ?




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.

** Been there, done that.

12.6 volts at 160 mA = 79 ohms.



What, both of them when they have full volts on ?



Duhhh ..............


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 ?

** Same valves previously sold a "Sovtek " far as I know.


BTW: Ever see a " Sovtek " amplifier ?

Like the MIG100 or MIG 60.

The couple I've seen were amazing pieces of work - no PCB, laced wiring,
ceramic sockets, heavy as hell and full of MIL high parts.

Like something built for the Russian military.



... Phil

No, never seen a Sovtek amp here

Arfa