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Old 19th Jan 2022, 12:44 pm   #1
Ambientnoise
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Default Switching off EM34

Trying to preserve the EM34 in my Philips 170A. Is there any problem simply switching off the heater supply to the EM34 only, for most of the running time of the set ? Thinking of fitting a hidden switch.

Tks

Ken
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Old 19th Jan 2022, 1:23 pm   #2
Jan Zodiac
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Default Re: Switching off EM34

Hi Ken,

There is no problem in that at all. It only serves as an indicator, and can be switched off or removed without altering anything in the set.

Regards,

Jan
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Old 19th Jan 2022, 2:48 pm   #3
kalee20
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Default Re: Switching off EM34

If you have a switch, I'd make it a double-pole switch and cut the HT also.
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Old 19th Jan 2022, 3:57 pm   #4
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Default Re: Switching off EM34

Quote:
and cut the HT also
Why bother? The EM will be fine and only low voltage switching needed.
 
Old 19th Jan 2022, 6:39 pm   #5
Ambientnoise
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Default Re: Switching off EM34

Thanks for the replies. I was concerned that maybe having ht etc present but no heater would cause some deterioration

Ken
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Old 19th Jan 2022, 7:32 pm   #6
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Default Re: Switching off EM34

Could there be a problem just after the heater is disconnected?

The HT will be present but the cathode will be cooling down.
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Old 19th Jan 2022, 8:17 pm   #7
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Default Re: Switching off EM34

I've got a "VI103" [essentially the RAF version of the 6U5G] magic-eye in my PCR communications-receiver, and to prolong its glowy life I fitted a 3-position switch in the HT circuit. It's a centre-off switch with one position spring-biased back to the centre, the other a normal position.

Up [against the spring bias] - Eye comes on so long as you keep the switch toggle lifted.
Centre - off
Down - Eye stays on.
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Old 19th Jan 2022, 8:57 pm   #8
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Default Re: Switching off EM34

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Could there be a problem just after the heater is disconnected?
Not for long if there was one, there isn't.
 
Old 20th Jan 2022, 6:28 am   #9
Graham G3ZVT
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Default Re: Switching off EM34

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Originally Posted by Silicon View Post
Could there be a problem just after the heater is disconnected?

The HT will be present but the cathode will be cooling down.
I always thought it was the opposite scenario that should be avoided, ie heater on, but no HT, because of the possibility of so-called cathode poisoning.
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Old 20th Jan 2022, 8:11 am   #10
kalee20
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Default Re: Switching off EM34

I have a tape recorder which uses an EM34 as record level indicator. It has HT switched on only on record. It has the original EM34 which is good and bright, no indication of cathode poisoning at all.

Certainly, many hours of heater with no HT has not caused a problem here.

The other way round - I was just thinking of the visual effect of switching off heaters and leaving HT on, the display would fade out... Personally, I'd rather see an abrupt switch-off, hence the suggestion!
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Old 20th Jan 2022, 10:33 am   #11
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Default Re: Switching off EM34

I was aware of cathode poisoning being mentioned in connection to switched ht in vhf front ends, hence my switched heater suggestion. It’s also easier to do so I will go that route.
I am making an led “substitute” but modern bar graph drivers ( not NS/TI) don’t seem to meet the data sheet specs !
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Old 22nd Jan 2022, 6:22 pm   #12
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Default Re: Switching off EM34

I have a Graetz Melodia. It also uses an EM34. It has a switch on the front that allows me to switch it off as and when.

Not sure if it cuts the 6.3 or the HT, or both, but it's definitely a good idea.

If you want a cheaper alternative, you can use a Russian 6E5C, but you'll need to re wire the valve socket. If you want to know how let me know.

At a quarter of the cost it is well worth it

Michael
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Old 22nd Jan 2022, 8:12 pm   #13
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Default Re: Switching off EM34

I don't know much about valves, but I have a radio with a button to enable the magic eye when needed.

It looks like the circuit does not cut the heater, but does something with the cathode.

I'll let the valves experts explain, then I'll learn something as well !.

Cheers,

Buzby
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