220 / 230 / 240V conundrum

[dominicbeesley]

Evening all,

I have two German 220V set that were my first two vintage "buys" - I got them from a brilliant antiques shop in Munich where the chap let you stick the mains cord straight into the plug socket with match sticks to try before you buy! I was driving a huge bus and had plenty of space - I wish I'd bought more now, plus a few tellys and radiograms!

I bought two sets there that have seen pretty regular use direct on UK 240V mains without any bother since 2003. (Apart from a single replaced capacitor and a lot of drive cord untangling between the pair of them).

I am now about to get round to overhauling them and would like to know how important is it to fit a dropper for the 4.3% difference in voltage (now that we're supposedly on 230V)?

Did any of the manufacturers publish data on under / overrunning valves?

Cheers

Dom

[paulsherwin]

It depends on your actual mains voltage. The 230V thing was just a paper change in the specs for 'EU unification'. The voltage in a particular area is often consistently on the low or high side so measure it a few times. If it's consistently around 230V you don't have much to worry about, but if its 245V+ (which it often is) it's unwise to run a 220V continental radio without dumping a bit of voltage.

Postwar sets (particularly 1950s and 60s) seem to have tough valves which can stand overrunning, though they may have low emission when returned to the correct voltage. They're also generally easily replaced. I wouldn't run anything with rare and expensive valves on the wrong nominal voltage.

Paul

[Radio_Dave]

Hi Dom,

I'm faced with the same dilemma with my newly acquired valve amplifier.

It's supposed to be run on 220V but my mains supply measures 233V, which is 5% too high. I've always believed that valve heaters can tolerate +/- 5% of the quote voltage so I'm, probably, just on the outer limit.

However, I've decided that this is a bit too close to comfort and will fit a 13R 40W WW resistor in series with the primary of the transformer to knock, around, 13V off the mains voltage

David

[dominicbeesley]

Think I'll leave well alone with these late 50's German sets then. Last time I measured the voltage was about 232V but that was only during the early evening.

How much "regulation" will the transformer give?

Dom

[paulsherwin]

Originally Posted by dominicbeesley

How much "regulation" will the transformer give?

None Like you I would just use the sets though.

You could always build an external 'dropper' using, say, a 300W halogen bulb (the sort used in security lighting).

Paul

[Ray Cooper]

As already stated, most small-valve manufacturers seem to specify +/-5% limits for normal heater operation - when they state any at all, that is.

The normal 230V mains in this country is supposed to be maintained between limits of +6%/-10% of nominal. Outside these limits, you've the right to complain to the supply authority - inside them, you're on your own.

UK mains less 10% will give you 207V - your 220V set would be quite happy with this. UK + 6% gives 244V - this is more than a 10% overrun for a 220V set. You'll probably get away with it in the short-term. In the longer term, some sort of voltage reduction would be nice. People keep suggesting series resistors to do this - but you've got to be careful. Although the voltage applied from the mains will be sinusoidal, the current taken by the set won't be - rectifier current demand is quite peaky, and although you may end up with about the right heater volts, the HT volts may be a little low as a result.

I'm surprised that nobody has mentioned using a small autotransformer, I think that's what I'd do myself. Although they're fairly rare these days, you could easily simulate one using a 230v/12V transformer, and connect the secondary to the mains live in such a sense as to buck the mains volts by 12V. Make sure that the mains live is connected to the outer end of the primary winding, and you should have no insulation difficulties.

And of course a transformer wastes little power compared to a series resistor, so unless you feel you need a little extra space heating...

[ppppenguin]

Originally Posted by Ray Cooper

I'm surprised that nobody has mentioned using a small autotransformer, I think that's what I'd do myself. Although they're fairly rare these days, you could easily simulate one using a 230v/12V transformer, and connect the secondary to the mains live in such a sense as to buck the mains volts by 12V. Make sure that the mains live is connected to the outer end of the primary winding, and you should have no insulation difficulties.

Ray's suggestion is eminently sensible. I'm sure we've mentioned it in other threads but collective memory (including mine) can be a bit weak. For being really to the insulation put the low voltage secondary in series with the neutral thereby raising the neutral at the set to 12V. The transformer can be something very small since it's only supplying 12V x current drawn by the set. A tiny 10VA specimen should be ample for any radio.

[Ray Cooper]

For being really to the insulation put the low voltage secondary in series with the neutral thereby raising the neutral at the set to 12V.

A good wheeze - but be careful if you're using it to drive a 'universal' set with the chassis connected to mains neutral - if you connect a separate chassis earth, you could end up short-circuiting the transformer secondary...

The transformer can be something very small since it's only supplying 12V x current drawn by the set. A tiny 10VA specimen should be ample for any radio.

On the whole, I think I'd like to use a slightly more meaty transformer - the secondary does have to be able to handle the peaks of rectifier current: a really small transformer might saturate. It's possibly a good idea to have a separate mains fuse in the supply to the autotransformer input, just to be on the safe side and afford the transformer some protection. Even a 3amp plug-top fuse wouldn't be of much help here.

[Leon Crampin]

I would suggest that backing off (say) 20V of mains using a transformer is only advisable if the set has a full wave rectifier. If you do this with a universal set, the transformer core will saturate with the DC component from the HT, especially if it is a small one, which would be the kind most suitable for the exercise.

I would add resistive ballast somewhere for the heaters (not usually a problem with 0.1A valves unless the set is very small) and not worry about the HT, pentodes are constant current loads anyway.

Leon.

[ppppenguin]

Originally Posted by Ray Cooper

A good wheeze - but be careful if you're using it to drive a 'universal' set with the chassis connected to mains neutral - if you connect a separate chassis earth, you could end up short-circuiting the transformer secondary.

If you earth the chassis of an AC/DC set you're in for an even bigger surprise if it's live

I suppose you should use a bigger tranny to avoid saturation on peaks.

[Ed_Dinning]

Hi Gents, I would second Ray's comments ref the Autotransformer. They can be made quite small for the job to be done and if you have a few continental sets they will help to reduce the heating when the set is being used at its max voltage.
Something like the 20VA RS with 2, 6v windings or a 12v winding or similar should do nicely.

Ed

[Radio_Dave]

I think I'll have to make an isolating transformer for my amplifier

What size 12V transformer would I need? The amp draws 0.9A from the mains, when it's settled down.

David

[ppppenguin]

Originally Posted by Radio_Dave

I think I'll have to make an isolating transformer for my amplifier

What size 12V transformer would I need? The amp draws 0.9A from the mains, when it's settled down.

Sorry, I don't get the joke about an isolating transformer

If the amp draws 0.9A then you need 12V at 0.9A for the buck transformer which is 12x0.9=10.8VA. A 20VA transformer should be fine.

[Radio_Dave]

Originally Posted by Radio_Dave

I think I'll have to make an isolating transformer for my amplifier

Oops!, Just to clarify, I should have said "make an 220V autotransformer from a 12V isolating transformer"

David