Pilot Little Maestro Pilot Lamp Conversion

[kings]

Revised valve line up (with readers on) 6Q7G, 678G,7K7G,25A6G,25Z4G

[kings]

Line cord removed

[Sideband]

Originally Posted by kings

Revised valve line up (with readers on) 6Q7G, 678G,7K7G,25A6G,25Z4G

Those numbers aren't quite right. The original numbers you gave were fine There is no such valve as a 678G or a 7K7G. More likely a 6K8G and a 6K7G. Can we get this correct please otherwise we are not likely to get anywhere.

Good point made by Lawrence....with only 110 volts in, the rectifier will never give the correct 202 volts output so nothing will work properly.

[stitch1]

Originally Posted by 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.

I think there's an ongoing debate as to which version this is but I have the 1939 version in a wooden cabinet and its 200-240V AC/DC and the line cord ballast resistance is for the series heaters. So as Lawrence says it work on 110V. I've not restored it as I intend to use a capacitor dropper to replace the line cord.

John

[ms660]

Just to be clear, I did not say it wouldn't work ok or it would work ok by being powered from 110v, I was just highlighting the fact that the HT will be reduced, my gut feeling is that it will work ok with the reduced HT but the power to the loudspeaker will be reduced compared to 240v operation.

According to a schematic I'm looking at the series scale bulb (7.5v @ 250mA) is shunted by a 15 ohm resistance, a quick calc for 110v to the series heater/bulb string suggests a 130 ohm (or there about) dropper.

Lawrence.

[PJL]

We need a picture of the chassis out of the case that includes the speaker from the top. Without knowing if it is a field coil speaker it will be difficult for us to tell if it will work on 120V. I can also see a green dropper resistor hidden in there!

[Sideband]

Without wishing to put a dampener on things, it seems this set has been 'bodged up'. At least there are better ways of getting the set working and much nearer to the way it was designed rather than running it from a 110V transformer which still, after all, requires some form of dropper to get the valve heaters running properly. Plus, at the moment, it involves running it from an external transformer.

Personally I'd start again with a capacitive dropper (possibly mounted where the green dropper is now). Some slight adjustment of the value would be required to cater for the dial lamp and a small wirewound resistor would be needed to feed the anode of the rectifier from the switched side of the mains. No major wiring changes and the radio would operated much closer to it's original design.

[Nuvistor]

Is it possible that the HT is from a voltage doubler circuit?

[Silicon]

If the original flex had 3 conductors such as neutral, live and a dropper resistance, the rectifier anode could be fed directly from 240v AC and the heaters via the dropper resistance.

The bulb and its parallel resistor could be part of the dropper resistance.

[Sideband]

If the original valve line-up is correct and includes a 25Z6 then yes it could use a voltage doubler (with some rewiring and additional components). However there is some uncertainty since the 'revised' valve line-up states a 25Z4 so a voltage doubler wouldn't be possible. Until we know for certain what has been done to the radio we can't tell.


The original 1939 circuit shows a 25Z6. This is two separate rectifiers that can be wired as a voltage doubler, or a half-wave (both sections in parallel) as required. However if a 25Z4 is fitted, only half-wave is possible. The original circuit shows the 25Z6 wired in parallel so normal half-wave.

[PJL]

Plenty of 120V US sets use this line-up without voltage doubling but they use a baretter for droppers. The field coil is then in parallel and not series with the HT.

We need pictures of the top and under the chassis to see what changes have been made.

[Herald1360]

A voltage doubler can always be done using the original half wave rectifier plus a silicon rectifier along with rearrangement of wiring and an extra capacitor. This may well be outside of the scope of this thread, though......

I would go with 240V into the set and a capacitor dropper for the heater chain- I think someone already mentioned this approach.

[Silicon]

The maximum anode (plate) voltage of the 25A6 audio output valve is 160V.
Maximum G2 voltage is 120V.

A dropper resistor (or something similar) would be required in the HT supply if you want to use it on 240V AC mains.

[Ambientnoise]

This link shows a capacitive dropper in a little maestro, maybe relevant.

http://www.pasttimesradio.co.uk/arti..._projects.html

Scroll down the page.

Ken

[Herald1360]

That looks very relevant, it also explains the ht voltage problem with just a straight heater capacitor dropper.

I would take issue with the X-type capacitor recommendation, though- it's ultra safe, but not very reliable long term, as owners of many modern gismos with these capacitors in their PSUs have found. Motor run type capacitors are better- just add a suitable fuse as well in case it decides to go short. There's no worse a shock hazard than original and somewhat less fire hazard!

[julie_m]

X-rated capacitors are unsuitable for use as voltage droppers, by dint of their self-healing property. A capacitor used as a voltage dropper should not be a self-healing type. Motor running capacitors are ideal. Motor starting capacitors often are not rated for continuous operation, and should be avoided unless you know exactly what you are doing.

To mitigate against the capacitor going short-circuit you can connect a metal film resistor in series with the capacitor; its value being carefully chosen so its power dissipation will be right on the limit in normal operation.

[Hartley118]

That's a nice little set. Quite unusual to see that cabinet used in its original Pilot application rather than the many post-war kits produced by Premier Radio, Barton etc.

The 'death by pilot bulb' is unusual. When you refer to V5, I'm not clear just which valve that is. Is it by any chance the 6K8G frequency changer? If so, the pilot bulb could be robbing the local oscillator of heater current. If the local oscillator stops, the set stops being a radio.

The cure would be Ed Dinning's minor wiring change in post 10.

Martin

[usradcoll1]

Originally Posted by PJL

Plenty of 120V US sets use this line-up without voltage doubling but they use a baretter for droppers. The field coil is then in parallel and not series with the HT.

We need pictures of the top and under the chassis to see what changes have been made.

The practice of using the speaker Field coil in parallel with the HT was discontinued in the earlier 30's in the US. Most of the sets used a 450 ohm field as a choke located between the smoothing and storage capacitors in the HT supply.
Not seeing the original schematic, I was wondering what HT voltage was originally used in that set. The original poster was planning to run the set on reduced HT.
Looking at the schematic posted by the restorer shows a resistor in series with the rectifier anodes to reduce the HT voltage to a slightly lower value, but not as low as 120 volts. I am a great user of cap heater droppers, as I run into several sets with a bad resistive dropper.
I always use a shunt resistor at least 20 ohms in parallel with the scale lamp.
Dave, Midwest USA.

[Sideband]

Motor run capacitors are cheap and plentiful. A wide range of values are available. Agree that X caps may not be the best types to use but they are useful for adjusting the value of the main cap. I needed a 2.5 uF and at the time I could only find a 2uF (motor run cap). I added a .47uF X cap in parallel and it's been fine for years.

As stated previously, make sure you use a motor run cap. A motor start cap probably won't last long.

[Ed_Dinning]

Hi Julie, what is the reason for not liking X caps as droppers?
The mode of failure is to reduce in capacitance and pass less current, they are also rated to pass whatever current would be supplied by 240v mains. Current is likely to be less when used as a dropper with other resistance in the circuit.

Ed