Lamp Limiter, understanding the bulb behaviour

[oldticktock]

Hi all,

Thought it was about time I built a series lamp limter.

It's done and I have tested it with a table lamp connected as per this sites testing instruction and it worked as described.

100w bulb fitted.
lamp limiter fed via variac

I then tested it with the Derwent I've been working on, a working set. Connected the Derwent and had the lamp switched in with variac at 240v. The lamp lit dim then faded the came back up and stayed at a steady low glow. The derwents dial bulbs were lit and the valves were glowing. Switched lamp off and set became brighter and stations heard.

Then connected a Philips 206A, i've just picked up an unknown, visual inspection looked ok and smoothers (radio spares) look newer than the 1940's so must have been replaced at some point maybe the 60's?. Connected to limiter with lamp switched in. Dial bulbs lit but the limiter lamp does nothing, just stay off.

So after this long description, what I'm asking is I don't have any experience using the limiter and was wondering what this scenario means.

Cheers
OTT

P.S. Is the 100w ok I have seen elsewhere 60w suggested

[paulsherwin]

100W is right for typical mains radios. 60W is better for mains/battery sets.

What typically happens is that the bulb will glow quite brightly for a second or two as the cold heaters cause a switchon surge. The bulb will then become dimmer until the rectifier and output valve warm up, at which stage it will glow a bit brighter again. Bad smoothers will cause the second brightening to be much more noticeable.

If the bulb isn't glowing at all, the radio is taking little or no current. There are many possible reasons for this but the first thing to check is that the valve heaters are glowing.

Paul

[oldticktock]

Opened the case up and I had left the rectifier out..... My stupidity knows no bounds. I remember taking out the EBL31 to solder the top cap back which had come adrift and had removed the AZ31..... nuff said

The lamp limiter now shows dim light, radio starts to come to life all valves glow but its going phut! phut! phut! phut! like an outboard and increases in speed as you wind up the volume.

I wonder if what i'm describing is motor boating and is due to one of the valves (EF9) red coating looking in very poor condition.

OTT

P.s. can I take it that as the limiter does not show bright that i can assume the smoothers are indeed ok, is this a trueisum

[julie_m]

The "motorboating" could be because the lamp is not letting quite enough current through, and the smoothing capacitor is discharging too deeply for the set to continue to work. The output valve stops conducting, the capacitor recharges and eventually the output valve comes back to life. The louder the volume is turned up, the quicker the energy in the capacitor will be spent in the loudspeaker.

If this is occurring at 100% voltage with a 100W bulb, then suspect the smoothing can, the rectifier and all in between. But a dim bulb is a good sign, so I'd try the set straight on the mains.

[oldticktock]

Interesting, Trader sheet 478 states the if amp valve V2 as a EF9, on the back of the fibreboard the sticker states EF39. Just removed the valve and it is an EF39.

The rectifier on sheet 478 states AZ1 I have an AZ31 also V1 is stated as ECH3 and mine is ECH35. Bit confused maybe this set is a later model...........


The metal coating is pretty poor, shame I have an EF9 but its side contact,ah maybe thats the clue searching this forum reveals this set was produced with side contacts, I digress.

So shall I let it go full wack on the mains as AJS says.

OTT

[Sideband]

Hi OTT. The valve types are easy to explain. Your set obviously has later types fitted. The early circuit shown on the Trader sheet is giving the numbers of side contact valves. Most of these valves where replaced with Octal types in later production. For example EF9 (side contact) with EF39 (octal). The valve is identical, only the base is different. Same with EM4 (side contact tuning indicator) replaced with later EM34 (octal) and EBL1 with EBL31.

Hope that clears the confusion up.

Now back to the lamp limiter. Remember that a filament lamp has a low resistance when cold and a high resistance when hot. This is the essence of how the lamp works as a limiter. A radio drawing normal current (quite low) will only make the lamp glow dimly so the lamp will only insert a low resistance and the radio will operate reasonably well on only slightly reduced mains. However if there is a major fault and the radio draws heavy current, the lamp glows brightly, inserts maximum resistance cutting the mains down to a much lower level and helps prevent further damage or at least limits the damage whilst also providing a visual warning that something is very wrong.


Hope this helps.

[oldticktock]

Hi Richard,

Thanks for the clarification and thanks for bringing the thread back on track I was in danger of drifting it.
So that would suggest that as this set is running via the limiter and the bulb glows low, then I should be able to switch the limter out and go full mains?

All the best.

OTT

[paulsherwin]

It's quite normal for a radio to work poorly when run through a lamp limiter. The purpose of the LL is to indicate major faults which would otherwise cause serious problems like blown fuses, component failures or even fires. If the LL has a dull glow and the HT looks reasonable the next stage is to switch out the LL. You can't really do fault finding with the LL in place once the radio is working at a basic level.

Paul

[oldticktock]

Thanks Paul, Just switched out the LL. The set is still motor boating guess this is down to the EF39 condition, but that's another issue. Thanks Paul, AJS and Richard for guidance, this thread has served it purpose.

[Ed_Dinning]

Hi OTT, try cooking foil wrapped round the valve , a few turns of TC wire at the junction of bulb and base for a contact, then carried down the side of the base and wrapped round the pin that is earthed. This often cures that type of instability.
If it is a problem in the HT side a 0.1uF from the HT rail to the FC and IF valves often helps as the impedance of the E-caps rises with age to high frequencies.

Ed

[oldticktock]

Hi Ed,
Thanks for the tips
Regards
OTT

[jay_oldstuff]

just a little tip as you are useing your lamp limiter in conjunction with a veriac you can crank the veriac up to compensate for the small voltage drop in the lamp the lanp limiter will still work as intended and limit currient to the radio on test but the radio will be receveing the correct voltage, i do this my putting a volt meater on the load side of the lamp and with the lamp in circuit crank the veriac up till the meater reads the input voltage of the radio.

Jay

[Adjuster]

Don't forget that ordinary GLS tungsten bulbs are becoming unavailable, so put some aside for this job. The new compact fluorescent energy saving bulbs won't do, as they have totally different characteristics and can't be used on reduced power.

[richrussell]

There's no restriction on the sale of filament 'rough service bulbs' - ie for garage inspection lamps and outdoor lighting. They're readily available in 100W and 60W with LES or standard bayonet fittings.