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Old 22nd Apr 2017, 10:38 pm   #1
Boater Sam
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Default Odd ECC85 behaviour.

Today I found something that to me is not logical.
Picked a rough PAM 713 up at Grappenhall MW,LW.FM.
It is intermittent on AM,a few crumbling Hunts caps, not the prpblem.
But its dead on FM, really dead.
Due the ECC85 having no heater, neither of them. Not seen that before.
Hey Ho fresh one in, lights up. Whilst waiting for it to warm and emit, checked the heater on the original, reads 4 Ohms! Yet not a glimmer, and no overload obvious on the mains transformer.
How can this be? Has senility caught up with me?
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Old 22nd Apr 2017, 11:15 pm   #2
paulsherwin
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Default Re: Odd ECC85 behaviour.

Internal short.
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Old 22nd Apr 2017, 11:30 pm   #3
Boater Sam
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Default Re: Odd ECC85 behaviour.

My immediate thought too, can't see where and both filaments? They are in parallel though.
But there is no sign of excessive heater current, the transformer is happy.
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Old 22nd Apr 2017, 11:38 pm   #4
Skywave
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Arrow Re: Odd ECC85 behaviour.

Try measuring the actual heater current with an a.c. meter, making sure that there is 6.3 v.a.c. across the paralleled heaters when that current measurement is made.

Al.
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Old 22nd Apr 2017, 11:59 pm   #5
Biggles
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Default Re: Odd ECC85 behaviour.

My first thoughts were dirty valve pins, but in this case it seems not. I would have thought that an internal short would fry the transformer, or at least cause low voltage to the other heaters, and a partial short of 4 ohms would at least light part of the heater visibly. I'm not sure whether a loss of vacuum would prevent the heater lighting up?
Alan.
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Old 23rd Apr 2017, 12:00 am   #6
AC/HL
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Default Re: Odd ECC85 behaviour.

Lost vacuum?
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Old 23rd Apr 2017, 7:57 am   #7
Boater Sam
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Default Re: Odd ECC85 behaviour.

Well, if you can be lost in space, we can have a lost vacuum.

No, getter is still there. The 6.3v stays up with the valve in, the other heaters stay on. The current does go up, pulls the volts down a bit, so it's a heater cathode short ON BOTH CATHODES!!!!

The transformer is really gutsy, it's standing it well, I'm not risking measuring the current.

The radio came with the mains tap on 200-210v so presumably that was to keep the AM working!

Never seen a double valve do this before.
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Old 23rd Apr 2017, 8:31 am   #8
Peter.N.
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Default Re: Odd ECC85 behaviour.

Does the valve get warm in spite of there being no visible glow? if so I would concur with Bill and say it's lost its vacuum.

Peter
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