Electro Harmonix Valves ??

[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.



.... Phil

 

[Phil Allison]

"Arfa Daily"

"Phil Allison"

So what's your theory ?

** I thought it was obvious.

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

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.

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

 

[N_Cook]

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

 

[Phil Allison]

"Arfa Daily"


** What are pins 4 and 5 ??

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,

** FFS read my post before you reply with such nonsense !!!!!!!!!!!!!

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

** What are pins 4 and 5 ??

FFS read my post before you reply with such nonsense.

The resistance goes UP from 32 to 79 ohms when power is applied.

Shakes head, over and over............


..... Phil

 

[Phil Allison]

"Gareth Magennis" >

"Phil Allison"

I have quite a few Svetlana "Circle C" ECC83 here with 37 ohm heaters.
All others read the lower resistance range.
Except I have 2 Marshall branded ECC83, one reads high, one low, the
higher looking identical to the Circle C.

The 3 EH ECC83 I have all read high and look nothing like the Circle C.

** I have a few Sovtek branded 7025s with small, silver anodes - pulled
from an amp.

Heaters all read 32 ohms.

Despite a long search this morning, no pic of one quite the same is on the
net.

Seems incredible that Russians have found a new ( and suitable ) high
conductance heater wire to use in the last few years that is cheaper than
tungsten.



..... Phil