Heater voltage.

[catch29]

Quick question with an easy answer (I think!).
I'm going to restore a Loewe- Opta Toccata 1745w, it's highest voltage is 220v, I read somewhere that with UK mains at 230v the heater voltage may be too high and damage the valves, the recommendation was to fix in series a 100ohm 5w resistor, presumably at the transformer (6.3v?) output. Any thoughts ??.

[ppppenguin]

That would be far too high a resistor to put in the 6.3V feed where you'll probably only need a few ohms. 100R in the mains feed feels about right. If you space to fit a buck transformer that's even better as there will be little or no heat.

Measure the heater voltage on your mains, which may be above or below 230V. If it's more than 5% above 6.3V then do something about it. Otherwise don't worry about the heaters.

There's a small possibility that the mains transformer will start to saturate and get too hot on a higher mains voltage. That's why dropping the mains voltage with a resistor or buck transformer is a good idea. The proper check involves measuring the current taken from the mains at both 220V and your mains voltage. If it rises more than proportionately at your mains then the transformer is saturating and you need to do something about it.

[Kalee20]

As per Jeffrey's advice!

Measure the heater voltage plugged into your mains... a few minutes of excessive voltage (if this is indeed the case) won't hurt.

If you do need to drop the voltage, other than rewinding the mains transformer, the most elegant way is to vet a little transformer with 12V 0.5A secondary. Connect the primary directly across the mains. Connect the secondary in series with the mains and your Loewe - if the voltage has gone up instead of down then just reverse the connections to either the secondary or the primary (but not both) of your auxiliary transformer.

[ppppenguin]

Kalee20's advice on a buck transformer is good. If your mains voltage is rather high then you might want a 24V secondary. Small toroidal mains transformer often have dual secondaries (12+12, 15+15 etc) that can be connected in series or parallel.

[Mike Watterson]

Your mains could be 230 to 250.

An issue with some sets is the transformer, not just the valves. A 15V to 20V transformer (check phase or you increase the voltage) for UK.. A resistor is cheaper but needs ventilation as it gets hot.

[BusyBee]

During initial restoration I would run it through an isolation transformer, if you have one with different voltage settings, to give the lower voltage. That way you test the original transformer before you start changing things.

[Radio Fixer]

The mains voltage here now is 242.4 V but often hits 245 V.

I have never bothered with a bucking transformer as in every radio I have worked on there wouldnt be room underneath and I woulndt want it on top. The easy way is to add a resistor in the primary circuit using one of the metal clad types. Bolted to chassis, with a little heat sink paste their heat easily disipates.

10% high MAX is sometimes quoted for running the valve heaters.

Gary

[ppppenguin]

Not sure why you would specifically want to use an isolating transformer with this set which has a double wound mains transformer and earthed chassis. An autotransformer or variac would serve equally well. As Kalee20 said, a few minutes plugged into a higher voltage won't hurt the set and will allow you to check the heater voltage.

[BusyBee]

Not sure why you would specifically want to use an isolating transformer with this set which has a double wound mains transformer and earthed chassis. An autotransformer or variac would serve equally well. As Kalee20 said, a few minutes plugged into a higher voltage won't hurt the set and will allow you to check the heater voltage.

You wouldn't, and you are right. It's just that I'm thinking of my setup in which I use one with which I also make use of it's settings to slightly under run my soldering iron so it comes readily to mind. After I wrote it I did think of an auto transformer as an option too, and obviously a variac would be ideal.

Tracy

[Kalee20]

Variac would be ideal to be able to twiddle it so that the heater voltage is exactly 6.3V - then you would know how much to reduce the mains by, with a little dedicated transformer (that needn't be large).

A Variac permanently connected is a tie-up of an expensive item!

There isn't of course a hard-and-fast rule as to what is acceptable and what isn't, the standard manufacturer's tolerances is, on the low side, what still gives acceptable performance; and on the high side, what doesn't excessively shorten life. If you don't need maximum emission from the cathode, slightly under-running is probably preferable.

Over-running goes along the lines of an hour at 10% above nominal reduces valve life the same amount as an hour and a half at nominal.