A question on scale lamps.

[gary_crutchley]

Hi all.

I have a Ferranti 255 that I am currently working in. The manufacturer's service sheet calls for 6.3V 0.2A scale lamps. I have searched online and cannot find these bulbs, does anyone know a source or an alternative. It looks like the original type are no longer available.

Thanks in advance.

Gary

[stitch1]

I managed to find quite a few values NOS but as a modern equivalent I'd use something like these.
Not 0.2 but very near, I don't know if that would matter in your circuit.

https://www.amazon.co.uk/Ajax-Scient...s%2C312&sr=8-1

John

[cathoderay57]

Fitting a standard 6.3v 0.3A lamp would be perfectly OK because this is a set with E-type valves and a mains transformer and these bulbs are readily available from multiple sources. Jerry

[Andrew B]

I bought a packet of 5 from the local £1 emporium, they were labelled up as bicycle light bulbs, "bubble" head on MES thread 6v 0.25A.

Q. what is the difference between E10 and MES?

[Herald1360]

Originally Posted by cathoderay57

Fitting a standard 6.3v 0.3A lamp would be perfectly OK because this is a set with E-type valves and a mains transformer and these bulbs are readily available from multiple sources. Jerry

Probably, but they would take a 50% higher current than the original type.


The 0.15A type suggested by John (Stitch1) would perhaps be a safer bet.


It looks from the circuit that the bulbs are actually fed with less than 6.3V, though how much less isn't specified. An actual measurement of the dial lamp voltage might enable a slightly wider choice of possible lower voltage bulbs at the expense of possibly shorter lamp life.

[Herald1360]

Originally Posted by Andrew B

I bought a packet of 5 from the local £1 emporium, they were labelled up as bicycle light bulbs, "bubble" head on MES thread 6v 0.25A

Interesting! They would suit the front light for the older "Dynohub" system for which genuine OE bulbs have long been unavailable.


They would be less of an overload than the 0.3A common dial lamp bulb type.

[G6Tanuki]

Originally Posted by Andrew B

I bought a packet of 5 from the local £1 emporium, they were labelled up as bicycle light bulbs, "bubble" head on MES thread 6v 0.25A.

Be aware that bulbs intended for flashlights etc are engineered to run 'hot' in order to squeeze the maximum amount of light out of a battery or similar limited power-source. As a result they often have surprisingly-short lives - sometimes only a few hours.

"Panel bulbs" are less efficient in terms of converting volts/amps into photons but have longer lives - thousands of hours - because they're not driven so hard and run the filament cooler.

[broadgage]

Originally Posted by Andrew B

I bought a packet of 5 from the local £1 emporium, they were labelled up as bicycle light bulbs, "bubble" head on MES thread 6v 0.25A.

Q. what is the difference between E10 and MES?

Bicycle bulbs may have a rather short life in this application, not only are they designed for 6 volts which is 5% less than 6.3 volts, but more significantly they are designed to give good light from the limited power supply. This entails a higher filament temperature and shorter life.
Pilot lamps are intended to last many thousands of hours in applications where power is cheap and plentiful, efficiency is therefore unimportant.

In a parallel circuit it may be acceptable to replace 0.2 amp bulbs with the more common 0.3 amp types, but remember that this is increasing the load on the mains transformer which MIGHT lead to overheating if it was already marginal.

As regards MES/E10 they are for all practical purposes the same thing, there might be some subtle difference, but in any real world application they are interchangeable.

https://www.cp-lighting.co.uk/10X28-6V-1-2W-200ma-MES
Still available.

[cathoderay57]

I'm sticking to my guns on this one. The total heater load excluding the 2 scale lamps is 2.85A. The 2 scale lamps are fed at half the heater voltage so an extra 100mA (if using 0.3A lamps) against a total budget of 3A is not an issue. Cheers, Jerry

[gary_crutchley]

Hi all,

Many thanks for your advice, I appreciate it.

Regards,

Gary

[broadgage]

Originally Posted by cathoderay57

I'm sticking to my guns on this one. The total heater load excluding the 2 scale lamps is 2.85A. The 2 scale lamps are fed at half the heater voltage so an extra 100mA (if using 0.3A lamps) against a total budget of 3A is not an issue. Cheers, Jerry

I believe that you are PROBABLY correct, the extra loading is indeed very modest and PROBABLY fine.
I am just on general grounds, not keen on increasing the load beyond that originally intended. And of course 200ma lamps MIGHT have been specified rather than the much more common 300ma types due to concerns about transformer loading/temperature rise. Or it might have been a cynical effort to AVOID standard parts and to sell expensive replacements.

[Glowing Bits!]

I've never had a problem using 300ma bulbs in one of these sets, they work well but seem to have a short life, 8V bulbs would be better or wire them in series for longer life, albeit reduced brightness.
Just had a thought, does anyone know if 8V bulbs are available? If so, please let me know where from, it may solve the issue of premature bulb failure.

[cathoderay57]

Originally Posted by Glowing Bits!

I've never had a problem using 300ma bulbs in one of these sets, they work well but seem to have a short life

Must be something odd going on here because the 2 lamps are supposed to be connected between one side of the heater winding and the earthed centre tap and should therefore be running at only half their rated voltage. The only explanations for blowing filaments that I can think of are:
1. Somebody connected them across the full 6.3v winding to make them brighter.
2. One of the valves has a cathode heater leak that is putting DC into the heater circuit.
A check with the trusty multimeter should reveal all. Jerry

[Herald1360]

According to the circuit, they're connected between the earthed end of the heater winding and a tap part way up, which may or may not be a centre tap but should give <6.3V for the bulbs. If anything the underrun bulbs should last for "ever"!

[turretslug]

A bulb running off a lower-than-nominal voltage transformer winding will still feel a high cold inrush current though, even if reduced in peak amplitude.

[broadgage]

Originally Posted by Glowing Bits!

I've never had a problem using 300ma bulbs in one of these sets, they work well but seem to have a short life, 8V bulbs would be better or wire them in series for longer life, albeit reduced brightness.
Just had a thought, does anyone know if 8V bulbs are available? If so, please let me know where from, it may solve the issue of premature bulb failure.

8 volt bulbs are now rare but can still be found, CP lighting have them, link in earlier post.

They some times turn up on ebay, mainly from eastern European sellers.

[Herald1360]

Originally Posted by turretslug

A bulb running off a lower-than-nominal voltage transformer winding will still feel a high cold inrush current though, even if reduced in peak amplitude.

Yes. The peak inrush will reduce in proportion to the applied voltage since the cold resistance is the same. Running current will be slightly higher than the proportion of applied vs rated voltage since the filament resistance at reduced voltage will be lower than that at full voltage.

[Gabe001]

9v bulbs with an E10 fitting at 0.3a are also readily available if they are suitable

[Herald1360]

Those might be suitable if you reroute the dial lights to get full 6.3V. They'll probably be too dim on the standard tap.