Philips 0.1amp heater chain. - UK Vintage Radio Repair and Restoration Discussion Forum

johnn · 29th Mar 2007, 8:19 pm

My Philips B4G01U set has 6 valves & from valve data the heater voltage total 169v. The actual voltage is 179. the mains tapping is OK.
The UL84 should have a 45v & it's 50. I have used a DVM & my AVO 8.
I have read that it is better to measure each valve.
Will a blown dial lamp make any difference? I don't have a correct spare.
I want to fit a capacitor dropper to its little plastic cased brother the B3G99U & the Vintage Radio capacitor dropper calculator suggests 700ohm for the resistive dropper.
Philips have 375ohm (Mine check OK) + a thermistor type thing.
JOHN.

paulsherwin · 29th Mar 2007, 8:50 pm

Any radio of this type has just the heater string and a dropper resistor, which may be made up of a combinatiion of resistors, thermistors and panel bulbs. The first thing you should do is check the voltage being dropped across the 'dropper resistor'. This may have drifted out of spec and you may need to add a bit more resistance.

A blown panel bulb will usually increase the 'dropper' value and reduce the voltage reaching the valves, though it does vary with different designs.

If a dropper is supposed to be 700 ohms, 375 ohms plus a thermistor is unlikely to be correct. The thermistors used to limit switchon surge only measure a few tens of ohms once warmed up.

Paul

yestertech · 29th Mar 2007, 11:40 pm

A good one this ! It is hard to see how the original design could have avoided overruniing the heaters on anything above 220v !
I get the same number of 710 from the calculator, but the actual dropper as given in the s/data is only 235 ohms !
The overrun is not as bad as it would first appear tho' as the hot resistance of the "varite" thermistors is appreciable.
Whilst the Mullard book lists the figs, neither of the two service data actually states which "varite" is fitted, but if the pilot lamp is 19v .097 amps. then there must be around 15 volts across the thermistor for a reasonable brightness. This infers their hot resitance to be around 150 ohms ??
There are two such thermistors, so the total resistance would be 235 + 300 which gives 535 ohms. If the heater current is 100mA this would drop 53.5 volts, which added to the 169 reqd. by the valves, gets us to 222.5 V.
Given that the tappping is marked "220-250" then one must assume the rest is overrun on the valves, as designed, which I assume is what you can now see. ( +/- some for tolerances ). It woudl be interesting to measure the volts drop across the two thermistors when operating temp is reached, to see how close the measured numbers are to my supposition !
Andy

johnn · 30th Mar 2007, 10:07 am

Thanks Paul & Andy.
I had concluded that the valves were being overrun not that there was a fault. The sections of the dropper resistor are as per the service data.
Andy... I will attempt to do some voltage measuring around the thermistors on both B3G99U & B4G01U sets. Both are identical except for tone control parts...
JOHN

johnn · 31st Mar 2007, 11:39 am

I found both my Philips sets to have the same voltages. I got "cold" resistance readings from the thermistors, but not when in operation!
As Paul said, The voltage dropped when the dial bulb was removed & apparently went too high with it in. Learn't something there!
I only have 6v. & 12v. 0.1amp bulbs, not the correct 17v.
I will try out a capacitor dropper/thermistors to see what I get......

johnn · 10th Apr 2007, 5:45 pm

the B3G99U now has a capacitive dropper + original thermistors with an easily obtainable 12v. 100mA dial bulb & works well. I used Andies estimate of the "hot" thermistor value to help work out where to start.
Many thanks...John.

29th Mar 2007, 8:19 pm	#1
johnn Retired Dormant Member Join Date: Jun 2003 Location: Scunthorpe, UK Posts: 154	Philips 0.1amp heater chain. My Philips B4G01U set has 6 valves & from valve data the heater voltage total 169v. The actual voltage is 179. the mains tapping is OK. The UL84 should have a 45v & it's 50. I have used a DVM & my AVO 8. I have read that it is better to measure each valve. Will a blown dial lamp make any difference? I don't have a correct spare. I want to fit a capacitor dropper to its little plastic cased brother the B3G99U & the Vintage Radio capacitor dropper calculator suggests 700ohm for the resistive dropper. Philips have 375ohm (Mine check OK) + a thermistor type thing. JOHN.

29th Mar 2007, 8:50 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,947	Re: Philips 0.1amp heater chain. Any radio of this type has just the heater string and a dropper resistor, which may be made up of a combinatiion of resistors, thermistors and panel bulbs. The first thing you should do is check the voltage being dropped across the 'dropper resistor'. This may have drifted out of spec and you may need to add a bit more resistance. A blown panel bulb will usually increase the 'dropper' value and reduce the voltage reaching the valves, though it does vary with different designs. If a dropper is supposed to be 700 ohms, 375 ohms plus a thermistor is unlikely to be correct. The thermistors used to limit switchon surge only measure a few tens of ohms once warmed up. Paul

29th Mar 2007, 11:40 pm	#3
yestertech Nonode Join Date: Dec 2002 Location: Coulsdon, Surrey, UK. Posts: 2,130	Re: Philips 0.1amp heater chain. A good one this ! It is hard to see how the original design could have avoided overruniing the heaters on anything above 220v ! I get the same number of 710 from the calculator, but the actual dropper as given in the s/data is only 235 ohms ! The overrun is not as bad as it would first appear tho' as the hot resistance of the "varite" thermistors is appreciable. Whilst the Mullard book lists the figs, neither of the two service data actually states which "varite" is fitted, but if the pilot lamp is 19v .097 amps. then there must be around 15 volts across the thermistor for a reasonable brightness. This infers their hot resitance to be around 150 ohms ?? There are two such thermistors, so the total resistance would be 235 + 300 which gives 535 ohms. If the heater current is 100mA this would drop 53.5 volts, which added to the 169 reqd. by the valves, gets us to 222.5 V. Given that the tappping is marked "220-250" then one must assume the rest is overrun on the valves, as designed, which I assume is what you can now see. ( +/- some for tolerances ). It woudl be interesting to measure the volts drop across the two thermistors when operating temp is reached, to see how close the measured numbers are to my supposition ! Andy

30th Mar 2007, 10:07 am	#4
johnn Retired Dormant Member Join Date: Jun 2003 Location: Scunthorpe, UK Posts: 154	Re: Philips 0.1amp heater chain. Thanks Paul & Andy. I had concluded that the valves were being overrun not that there was a fault. The sections of the dropper resistor are as per the service data. Andy... I will attempt to do some voltage measuring around the thermistors on both B3G99U & B4G01U sets. Both are identical except for tone control parts... JOHN

31st Mar 2007, 11:39 am	#5
johnn Retired Dormant Member Join Date: Jun 2003 Location: Scunthorpe, UK Posts: 154	Re: Philips 0.1amp heater chain. I found both my Philips sets to have the same voltages. I got "cold" resistance readings from the thermistors, but not when in operation! As Paul said, The voltage dropped when the dial bulb was removed & apparently went too high with it in. Learn't something there! I only have 6v. & 12v. 0.1amp bulbs, not the correct 17v. I will try out a capacitor dropper/thermistors to see what I get......

10th Apr 2007, 5:45 pm	#6
johnn Retired Dormant Member Join Date: Jun 2003 Location: Scunthorpe, UK Posts: 154	Re: Philips 0.1amp heater chain. the B3G99U now has a capacitive dropper + original thermistors with an easily obtainable 12v. 100mA dial bulb & works well. I used Andies estimate of the "hot" thermistor value to help work out where to start. Many thanks...John.