Pilot Little Maestro Pilot Lamp Conversion

[kings]

Hello All,
I recently acquired a mint little bakelite Pilot Little Maestro, line cord removed new cable fitted with an American plug, (110 volts) also a resistor fitted to two points on valve five, radio is very responsive and works a treat with no pilot lamp, if one is fitted the heaters do not appear to work correctly and there is nothing coming out of the set until the bulb is used.
Can anyone please help with changing the wiring etc and perhaps fitting a power resistor to power the bulb instead of the heater chain bearing in mind it runs off a 110 volt transformer.
Many Thanks,
David.

[ValvoStef]

Is this the original lamp?Me thinks the lamp is incorrect rated. Is it wired parallel to the valve heaters? Too much current cause the heater voltage to drop, may overload the transformer.

[ValvoStef]

Sorry, if I read your post correctly, you add a lamp and the set stops working. As this is an AC set, are the heaters wired in series matching 110V or are they connected in parallel from a separate winding of the mains transformer. If the heaters wired in series and you add a lamp you mess up all the heater voltages. Have you any detailsofthelamp?voltage, wattage, current? This would help to find the problem.

[kings]

Hi There,
The lamp is 7.5 volts

[ValvoStef]

Kings,

Is the lamp connected in series with the heater chain?

[kings]

Yes it is through two connections of valve 5

[ValvoStef]

Then the lamp lowers the current through the heater chain and the heaters won’t work efficient enough.

[Herald1360]

Originally Posted by kings

- radio is very responsive and works a treat with no pilot lamp, if one is fitted the heaters do not appear to work correctly and there is nothing coming out of the set until the bulb is used.

You seem to be saying both that the radio doesn't and does work when a pilot lamp bulb is used?

Are you using "used" in an unusual sense- other than simply meaning "in use"?

Which version of the Little Maestro is this? Or at least what's the rectifier type?

An extra 7.5V drop even in a 110V heater chain shouldn't stop anything from working.

[Mike. Watterson]

"An extra 7.5V drop even in a 110V heater"
I agree, but it's ONLY an extra 7.5V if the current rating @ 7.5V is the same as the series heater chain.

For series it's the CURRENT rating, really resistance, that's important. The voltage rating of the bulb on its own is only useful information in a parallel circuit. The bulb current rating should be about the same as the series chain.

[Ed_Dinning]

Hi Kings, it sounds as if you have wired the bulb in parallel with the 2 pins on V5, this will reduce the heater current in V5 and if the bulb holder is a non-isolated type it may cause other problems in the heater chain.

What you need to do

Find a bulb with the same rating as the heater CURRENT of the valves in the set and a VOLTAGE rating of about 4 to 6 volts.

Use a lamp holder with connections isolated from its mounting clip

Remove ONE of the heater wires from, say V5.
Insert the 2 bulb wires in this "gap".
The set should now work.

In case of bulb failure it is common to fit a resistor across the bulb, say 120 to 180 ohms, depending on the bulb type.

Ed

[ms660]

What's the valve line up?

Lawrence.

[PJL]

My one uses 300mA series and the total heater chain requirement for the valves was 68.9V.

[kings]

Thanks all I bought the set from a retired radio restorer I will start with a smaller voltage bulb and work my way through all other ideas,
Many Thanks,
Dave.

[Cobaltblue]

I think the reason Lawrence is asking for the valve line up includes wanting to know what rectifier is used.

Many American designed sets used a rectifier with a tapped heater for example the 35Z5 http://r-type.org/exhib/aaq0191.htm

The tapped heater arrangement was specifically to run the dial lamp.

I don't recall seeing that on a Little Maestro but if it's been modded who knows.

Cheers

Mike T

[Sideband]

Hi Kings, Could you answer the specific questions asked....what is the valve line-up and what model number is it? Starting with 'a smaller voltage bulb' is not really the best way of doing things....

[kings]

Valve line up is 6A8G,6K7G,6Q7G,25A6G,25Z6G

[Sideband]

OK so the rectifier (assuming that is 'valve 5') has a standard 300mA heater, no taps. What 'two pins on valve 5' are you referring to? ....and we still don't know the model of your radio. If this is unknown perhaps you could provide a picture of the set so that we can hopefully identify it?

[Sideband]

OK I think we can safely say that yours is the 1939 model since looking through my circuits is the only one that uses that particular valve line-up. The original circuit has a line cord ballast with a couple of tappings to feed the rectifier anodes and the pilot lamp. The original heaters are all in series and it is a 300mA chain.

Personally I would have used a capacitive dropper with the value chosen to include the pilot lamp. The original pilot lamp is unusual since it's rated at 7.3v 250mA with a miniature bayonet fitting. Part of the original line cord is used as a shunt for the pilot lamp.

I think if it was me (using a capacitive dropper) I'd use Paul's calculator to get the value (it works out to around 4.4uF), fit a 6.3V 300mA lamp in series and experiment with some low value wirewound resistors to use as a shunt (guess about 50 ohms). At least that way it would run off normal 240V mains and save a lot of messing about ( and virtually no heat generation).

You would also need a small wirewound to feed the rectifier anode (start at about 100 ohms).

[PJL]

Mine has the same valves but the speaker is permanent magnet so I would not assume that the set exactly matches any published circuit.

Can you post some pictures.

[ms660]

According to a schematic I'm looking at the HT out of the rectifier is given as 202 volts,

110 volts in?

Lawrence.