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Odd tube behavior
Odd tube behavior
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Old 1st April 2004, 05:00 PM   #1
woody is offline woody
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Unhappy Odd tube behavior

I have a pair of ECC 83 Telefunken tubes that measure fine
but one of them has a filiment that glows VERRY bringht for
the first second that I power it up then it works fine. I swapped
tubes and it,s problem followed the tube. On my tube tester they both measure the same. The cold filiments have the same ohm reading. Now if I turn the power off then back on a second
or two later it warms up normaly what,s up.

Thanks in advance,

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Old 1st April 2004, 06:09 PM   #2
djmiddelkoop is offline djmiddelkoop  Netherlands
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Don't worry, there is nothing wrong.
It just shows the inrush current of the subjective tube. The filament is just better visible in this tube.
If it troubles you, add something to reduce the inrush current.
This is always better to prolongue life, as the switching on current surge can destroy the filament just like a lightbulb fails when switching on after many operating hours.

Treasure those nice sounding Telefunkens !
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Old 1st April 2004, 06:39 PM   #3
ErikdeBest is offline ErikdeBest  Switzerland
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Odd tube behavior
If it troubles you, add something to reduce the inrush current.
You mean putting a resistor in serie with the filament!?

Supply voltage should be higher than 6,3V (or 12,6V) to compensate the voltage drop at the resistor. Or maybe you can live with 6V (12V) at the filament, even prolonging tube life
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Old 1st April 2004, 08:37 PM   #4
Sch3mat1c is offline Sch3mat1c  United States
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Mullards, and Teles I guess (possibly rebranded Mullards?), always do that. Part of their design.

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Old 1st April 2004, 09:17 PM   #5
fdegrove is offline fdegrove  Europe
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At one point in time all heaters exhibited this behaviour regardless of manufacture.

With some tubes such as the ECC/ 12**7 series this is just more visible as part of the heater is outside the cathode sleeve.

Later on US tubes with controlled heater warm up filaments were developped and these received the suffix A, e.g. 12AX7A, 12AU7A and so on.

Europe adaopted the same type of heaters without a change in nomenclature.
The only manufacturer I'm aware of that stuck with the uncontrolled heater warm up type of filaments till they went under was RFT.

IOW, this nothing to worry about.
It's not even sure wheter the controlled warm up prolongs the tubes life anyway.

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Old 2nd April 2004, 03:09 PM   #6
woody is offline woody
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Default Thanks for the replys

Thanks for all the replys.

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Old 2nd April 2004, 03:43 PM   #7
Alastair E is offline Alastair E  Wales
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Default odd heater

Ive seen this so many times! many years ago, Decca 30 CTV used a PL508 in the frame stages, These things lit up like lightbulbs for a second or two, and after a year or two, the heater would go O/C--This used to be quite common!.

the cause was generally the fact that the heater at the point where it attaches to the lead to the pin ( maybe even weakened during the welding process),-- isnt coated with silica insulation, and as such has no heat-sinking effect and warms up faster, the rest of the heater is coated and most is heat-sunk to the cathode and in the instant of turn on the effective resistance of the uncoated section increases much faster, and dissipates more than normal, particularly in DC heater systems.

A good idea is to incorporate a 20% drop in heater supply at switch-on, maybe via your standby switch, this should minimise the stress at turn-on--You could even use a thermistor having a high resistance at turn-on, which lowers as it heats up--This was a method used in some old CTV's like the Philips G6, but the heat-up time is considerably longer! They were rated at 300mA as I remember, which would be just right for your 12xx7 series.

Hope this helps!
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