Dropping filament voltage for a valve - UK Vintage Radio Repair and Restoration Discussion Forum

JayBee66 · 13th Jan 2017, 3:11 pm

The repair of my recent Hunts CRB3 acquisition has gone better than expected. I only needed to replace the electrolytic filter capacitors and get an EM34 magic eye (a resonably priced and tested one from Mullard Magic - thanks!)

The capacitor and resistor measurements are all good on the CRB3. I haven't tried any of the other functions so maybe there will be a fault there somewhere.

One thing I have noticed is that the filament on the EM34 has 6.95V across it. I would like to drop that down to 6.3V. Not having done this before, do I want a resistor in series or should I put one in parallel with the filament? Initial thoughts suggest series but I want to check with wiser heads.

Thank you.

Biggles · 13th Jan 2017, 3:22 pm

Assuming that the set uses parallel transformer fed 6.3v valves, you would use a resistor in series with the magic eye heater. You need to know the current taken by the valve heater, and you can work out the voltage drop required, so by simply using ohm's law you can calculate the resistor value required. One thing to bear in mind is that your meter may not be totally accurate on AC, and also I'm not sure how tolerant valves are to a bit of variation in heater supply voltage, hence it may not be worth adding the resistor. Mains supply voltage variations may also come into play.

Laurdys · 13th Jan 2017, 3:32 pm

assuming the values you have given are correct:

heater current = 0.2A
voltage present = 6.95v
required voltage = 6.3v
voltage drop required = 6.95-6.3 = 0.65v

r = v/i

r = 0.65/0.2

r = 3.25 ohms

closest preferred value = 3.3 ohm

p = i*i*r

p = 0.132 watts or greater

and yes the resistor should be in series with the heater.

mole42uk · 13th Jan 2017, 3:40 pm

This is the only valve in the equipment, I think?

Is the mains transformer on the correct voltage tapping, if so provided?

JayBee66 · 13th Jan 2017, 4:30 pm

Yes, only the one valve.

There is no mains selector.

It was built just after 1959, judging by the dates on the two pots, so the mains voltage has probably increased since then.

13th Jan 2017, 7:37 pm

I wouldn't worry, the screen will degrade much faster than the cathode.

julie_m · 13th Jan 2017, 9:49 pm

It's only 10% out. That's within tolerance; valves were rated ±20%.

Maarten · 15th Jan 2017, 12:52 am

I think valves were often rated 10% but with 5% recommended. For optimal life, the voltage should be spot on or (for tubes that don't easily suffer from cathode poisoning) slightly lowish. I tend to agree however, that the target is likely to degrade faster, if used full on for most of the time.

13th Jan 2017, 3:11 pm	#1
JayBee66 No Longer a Member Join Date: Nov 2015 Location: Kilkenny, Ireland Posts: 138	Dropping filament voltage for a valve The repair of my recent Hunts CRB3 acquisition has gone better than expected. I only needed to replace the electrolytic filter capacitors and get an EM34 magic eye (a resonably priced and tested one from Mullard Magic - thanks!) The capacitor and resistor measurements are all good on the CRB3. I haven't tried any of the other functions so maybe there will be a fault there somewhere. One thing I have noticed is that the filament on the EM34 has 6.95V across it. I would like to drop that down to 6.3V. Not having done this before, do I want a resistor in series or should I put one in parallel with the filament? Initial thoughts suggest series but I want to check with wiser heads. Thank you.

13th Jan 2017, 3:22 pm	#2
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	Re: Dropping filament voltage for a valve Assuming that the set uses parallel transformer fed 6.3v valves, you would use a resistor in series with the magic eye heater. You need to know the current taken by the valve heater, and you can work out the voltage drop required, so by simply using ohm's law you can calculate the resistor value required. One thing to bear in mind is that your meter may not be totally accurate on AC, and also I'm not sure how tolerant valves are to a bit of variation in heater supply voltage, hence it may not be worth adding the resistor. Mains supply voltage variations may also come into play.

13th Jan 2017, 3:32 pm	#3
Laurdys Pentode Join Date: May 2009 Location: Stockton-on-Tees, County Durham, UK. Posts: 154	Re: Dropping filament voltage for a valve assuming the values you have given are correct: heater current = 0.2A voltage present = 6.95v required voltage = 6.3v voltage drop required = 6.95-6.3 = 0.65v r = v/i r = 0.65/0.2 r = 3.25 ohms closest preferred value = 3.3 ohm p = iir p = 0.132 watts or greater and yes the resistor should be in series with the heater. __________________ If it can be built, it can also be fixed.

13th Jan 2017, 3:40 pm	#4
mole42uk Nonode Join Date: Aug 2010 Location: Resolven, Wales; and Bristol, England Posts: 2,608	Re: Dropping filament voltage for a valve This is the only valve in the equipment, I think? Is the mains transformer on the correct voltage tapping, if so provided? __________________ Richard Index: recursive loop: see recursive loop

13th Jan 2017, 4:30 pm	#5
JayBee66 No Longer a Member Join Date: Nov 2015 Location: Kilkenny, Ireland Posts: 138	Re: Dropping filament voltage for a valve Yes, only the one valve. There is no mains selector. It was built just after 1959, judging by the dates on the two pots, so the mains voltage has probably increased since then.

13th Jan 2017, 7:37 pm	#6
Guest Guest Posts: n/a	Re: Dropping filament voltage for a valve I wouldn't worry, the screen will degrade much faster than the cathode.

13th Jan 2017, 9:49 pm	#7
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Dropping filament voltage for a valve It's only 10% out. That's within tolerance; valves were rated ±20%. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

15th Jan 2017, 12:52 am	#8
Maarten Dekatron Join Date: Dec 2007 Location: Haarlem, Netherlands Posts: 4,199	Re: Dropping filament voltage for a valve I think valves were often rated 10% but with 5% recommended. For optimal life, the voltage should be spot on or (for tubes that don't easily suffer from cathode poisoning) slightly lowish. I tend to agree however, that the target is likely to degrade faster, if used full on for most of the time.