20's valve ID ? - UK Vintage Radio Repair and Restoration Discussion Forum

greenstar · 12th Jul 2018, 9:07 pm

I would be grateful if anyone could help me pin these valves down. They are from a mid 20's home made set I was lucky to win at RWB. I am not sure at present whether or not they are dull emitters. My multimeters are not in agreement as to their filament resistances, but I seem to have about 7 Ohm for v1 and 2 for the others.
V1 RF, V2 Det, V3 AF, V4 AF.

On V1 the connecting wires are brought out to the pins externally. The anode is cylindrical and horizontal.
V2-4 are similar, with vertical rectangular anodes and connections to the base made inside the base.
All four are pip tops.

I realise I can measure filament resistance and also the rheostat resistance (four in this set) and guess at battery voltage, but I don't seem to be able to measure low resistances reliably.
Thanks,
Tony

Dave757 · 12th Jul 2018, 9:19 pm

Hi Tony,

I'm not sure if there is a reliable way of calculating the working voltage
of a filament when cold. I remember measuring the resistance of a 6.3V 0.3A
heater, and it was about a third of what it should have been hot.

Kind regards
Dave

GrimJosef · 12th Jul 2018, 9:23 pm

The easiest way to measure low resistances with good accuracy is to set up a constant current source (a 12V DC power supply or battery in series with a 470R resistor will give you a little over 25mA) and then feed that through the unknown resistance in question. You can then use the mV range on a DMM to measure the voltage across the resistance and Ohm's Law will do the rest.

Of course the resistance of a valve filament will increase as it gets hotter (EDIT as Dave has just pointed out) so to get the working resistance you will need to wind the current up to the working value.

Cheers,

GJ

greenstar · 12th Jul 2018, 9:47 pm

Ahh ... yes of course ... so, if I don't know the right working voltage and filament current I won't know what to run the valve at to measure the filament resistance ... so my strategy to guess from measuring resistance is not a sound one ... so really be good to identify the valves! They will almost certainly be 2v valves so I suppose it will be fairly obvious whether they are bright emitters when I do fire it up (be a while as lots to do on it). I will just be sure to start with the rheostats on low.

ms660 · 12th Jul 2018, 9:54 pm

http://g3ynh.info/valves/old/RM2/RM2.html

?

Lawrence.

greenstar · 13th Jul 2018, 8:28 am

Lawrence, thanks, that looks like V1. Definitely a tubular anode in my valve so if it's that, a bright emitter.
The tricky thing now, is, dare I use it? Given it's probably a rare survivor and not easy to replace, plus who knows how much it has been used.

12th Jul 2018, 9:07 pm	#1
greenstar Octode Join Date: Jul 2011 Location: Leicester, Leics. UK. Posts: 1,684	20's valve ID ? I would be grateful if anyone could help me pin these valves down. They are from a mid 20's home made set I was lucky to win at RWB. I am not sure at present whether or not they are dull emitters. My multimeters are not in agreement as to their filament resistances, but I seem to have about 7 Ohm for v1 and 2 for the others. V1 RF, V2 Det, V3 AF, V4 AF. On V1 the connecting wires are brought out to the pins externally. The anode is cylindrical and horizontal. V2-4 are similar, with vertical rectangular anodes and connections to the base made inside the base. All four are pip tops. I realise I can measure filament resistance and also the rheostat resistance (four in this set) and guess at battery voltage, but I don't seem to be able to measure low resistances reliably. Thanks, Tony Attached Thumbnails

12th Jul 2018, 9:19 pm	#2
Dave757 Heptode Join Date: Apr 2009 Location: Scratby, Norfolk, UK. Posts: 650	Re: 20's valve ID ? Hi Tony, I'm not sure if there is a reliable way of calculating the working voltage of a filament when cold. I remember measuring the resistance of a 6.3V 0.3A heater, and it was about a third of what it should have been hot. Kind regards Dave

12th Jul 2018, 9:23 pm	#3
GrimJosef Dekatron Join Date: Sep 2007 Location: Oxfordshire, UK. Posts: 4,311	Re: 20's valve ID ? The easiest way to measure low resistances with good accuracy is to set up a constant current source (a 12V DC power supply or battery in series with a 470R resistor will give you a little over 25mA) and then feed that through the unknown resistance in question. You can then use the mV range on a DMM to measure the voltage across the resistance and Ohm's Law will do the rest. Of course the resistance of a valve filament will increase as it gets hotter (EDIT as Dave has just pointed out) so to get the working resistance you will need to wind the current up to the working value. Cheers, GJ __________________ http://www.ampregen.com

12th Jul 2018, 9:47 pm	#4
greenstar Octode Join Date: Jul 2011 Location: Leicester, Leics. UK. Posts: 1,684	Re: 20's valve ID ? Ahh ... yes of course ... so, if I don't know the right working voltage and filament current I won't know what to run the valve at to measure the filament resistance ... so my strategy to guess from measuring resistance is not a sound one ... so really be good to identify the valves! They will almost certainly be 2v valves so I suppose it will be fairly obvious whether they are bright emitters when I do fire it up (be a while as lots to do on it). I will just be sure to start with the rheostats on low.

12th Jul 2018, 9:54 pm	#5
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: 20's valve ID ? http://g3ynh.info/valves/old/RM2/RM2.html ? Lawrence.

13th Jul 2018, 8:28 am	#6
greenstar Octode Join Date: Jul 2011 Location: Leicester, Leics. UK. Posts: 1,684	Re: 20's valve ID ? Lawrence, thanks, that looks like V1. Definitely a tubular anode in my valve so if it's that, a bright emitter. The tricky thing now, is, dare I use it? Given it's probably a rare survivor and not easy to replace, plus who knows how much it has been used.