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Regulary AC- mains voltage, a letter from the UK

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Forum » Technique, Repair, Restoration, Home construction ** » Repair and restoration: Tips and Tricks » Regulary AC- mains voltage, a letter from the UK
           
Eilert Menke
 
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03.Jun.04 21:26

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"In Great Britain, the EU-harmonised voltage is now 230V +10%, -6% (was 240V +-6%, and is still usually 235-245V).
 
When using Tesla Talisman (308U) 220V sets here, this often causes the UY1 rectifier to arc over and heater-cathode leakage in the UBL21 which goes away if the supply is reduced to 220V.
 
Returning to the original question, I have heard that under-running heaters, especially rectifiers, can be more harmful than over-running, leading to cathode-stripping. I wonder if this is true (Scale lamps would last longer under-run, but they are much cheaper than rectifier valves!)?
 
In the Brimar valve data book, it states that all should be O.K. in a transformer-operated set if the heater supply was within +-5% at the rated input voltage, and the mains varied by under +-10%, which seems a very wide tolerance, but probably typical of conditions here from 1947- early 1950s.
 
American sets seem to be commonly rated 105-125V, again a very wide range.
 
Yours,
 
Martin (G8UWM)"
William J Blanchflower
William J Blanchflower
 
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30.Jun.04 20:42

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Martin,

When using 220V AC/DC continental sets such as the Tesla Talisman in the UK, I normally add a 150 ohm, 10W resistor in series with the mains input. For most sets, this will drop about 20V. The resistor will become quite hot, dissapating about 7W of heat, so care needs to be taken with it's positioning inside the set. If possible, I use an aluminium clad wirewound resistor and try to mount it somewhere on the chassis near the power supply components. The heat will then be dissipated into the chassis and not cause any hot spots on the case.

 

Konrad Birkner † 12.08.2014
Konrad Birkner † 12.08.2014
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01.Jul.04 10:00

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Hello friends,

May I draw Your attention to another way to get rid of "surplus" AC mains voltage for AC/DC sets: use of a series capacitor.

  For a Talisman, which has a consumption of ca .22amp, the correct value is around 8.5µF. But anything between 7µF and 10µF will do the job as well.

The capacitor must not be an electrolytic one! An MP is ideal for the purpose. e.g. for phase compensation in fluorescent lamps You can find values of 3.4µF, 4.5µF, and others. (One each in parallel is ideal with 7.9µF).

The voltage rating has to be sufficient for the full mains value,i.e. min~250volt (AC; which means min 350vDC).

For those of our friends which are not familiar with AC calculus a WARNING:

This is not a simple series resistor, as it drops at 50Hertz and .22amps not less than 82volt!

But this does not add simply add up to 220+82=302 ! NO! it makes exactly 235v.

Of course : such caps are bigger than a resistor: one of the a.m.values of 3.4-4.5microfarad has 1"diameter and a length of 3-4". But there is no heat to dissipate, and no extra power to waste and pay for (that might be a point for our scottish friends).

Eventually another WARNING: Never try this in series with a transformer! It may come to resonance effects with subsequent harmful consequences! It only works with "ohmic" load!

regards KoBi

This article was edited 01.Jul.04 10:01 by Konrad Birkner † 12.08.2014 .

Konrad Birkner † 12.08.2014
Konrad Birkner † 12.08.2014
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01.Jul.04 10:12

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As far as I know the detrimental effect of underrunning is as follows:

The cathode surface temperature goes down and so the electron emission. The hotter areas will have to take the full load which leads on the long run to emission fatigue at these spots. This is especially true for high emission(= power)tubes, i.e. output and rectifier valves.

 

With incandescent lamps it is different:

Overheating the filament leads to excessive vaporisation of the hottest part and premature break. There is no forced electron emission to wear out an underrun lamp.

Regards

KoBi

This article was edited 01.Jul.04 10:19 by Konrad Birkner † 12.08.2014 .

Hans Kamann
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01.Jul.04 12:53

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Dear Konrad Birkner,

referring to your "low cost" voltage decline I´d like to comment that the Scots are not happy at all to be regarded penurious. There was an article in "Kölner Stadanzeiger" yesterday which leads to the conviction  that only Germans consider the Scots like this, ads like "Schottenpreise" won´t be understood elsewhere in the world.

Nevertheless, your technical know-how is ( as usual ) absolutely correct.

 

This article was edited 01.Jul.04 13:25 by Hans Kamann .

Robert Sarbell
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01.Jul.04 15:58

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Dear Hans Kamann,
This reply is also addressed to Herr Konrad Birkner, and a recent response from Herr Eilert Menke. . .

During a lengthy email to Mr Ernest Erb quite some months ago (and recent telephone conversation) I expressed my admiration and appreciation for an elderly German high school teacher - Mr Leland Kirst - of algebra, geometry, and physics I had while in high school from 1955-1958. He instilled a love of technical learning in me, and I even enrolled in several courses of training in basic electricity and electronics fundamentals offered by the US Navy to military veterans - needless to say at the age of 16 I was NOT a veteran of any sort. A recommendation by Mr Kirst was proffered and I was allowed to enroll in the courses.

Ever since those years I have been able to perform most repairs on many of the tube-type radios and TVs I have owned over the years. However, I never developed fully the indepth knowledge that you gentleman exhibit in order to effect design changes or "workarounds" when specific circuit components were not available. My professional career within the military led me, however, into astronomy and I became an aeronautical navigator.

Ernest specifically mentioned your names - and he indicated there were several other members who were incredibly gifted. Reading these forum entries instill a sense of awe in me.

My profound compliments to you and to all the members of Radiomuseum who offer such valuable insights.

This article was edited 01.Jul.04 16:00 by Robert Sarbell .

Gabriel Toth
Gabriel Toth
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02.Jul.04 21:25

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Dear Martin

the Talisman and similar models were specified by the manufacturer for 220V only. I compared the schematics with similar (tubes) models, but specified for 240V mains. It seems, the problem was solved with additional 130Ohm/6W (Pionyr) or 170Ohm/7W (Accord) in series with the heater circuit.

BR

Gabriel

Martin Crossley
 
 
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08.Jul.04 01:45

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DR OM Hans & everyone else who replied on this thread,

Many thanks.

This was my first post to the Forum , and was originally to a thread in German, (by people in Germany)  about setting the mains adjustment to 240/245V instead of 220V because their supply was now nearer to 230V than 220V (as I understood it.I'm very sorry that my German reading is fairly poor, but much better than my writing.)

Of course, I do not regularly run 220-V sets on 240V.

I have a 9-0-9V transformer connected up to "buck" the voltage.(i.e. in antiphase)

Actually my mains is usualy 235V here, but 245V at my friend's & my parents.

I appreciate the comments about life on cathodes & lamps and am glad that what I believed is correct.

Because I am English,(and therefore not Scottish...) I do know the phrase "A canny Scot"

and they do have a reputation for "financial prudence" as our Scottish chancellor of the exchequer ,Gordon Brown, would say!

73,

Martin.    

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