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basic battery valve filament supply.

 
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basic battery valve filament supply.

Post by Rebel Rafter » Tue Jan 12, 2016 8:43 pm

Hi, form RR. How's this for a suggestion. Why not get an old small mains transformer that can easily be dismantled, i.e. one that's not varnished, preferably one with separate bobbins for primary and secondary windings like one of those used in old 80's or 90's portable radio/cassette/CD player etc. and remove some of the secondary winding until it gives only one volt AC, actually ISTR the older Nokia phone chargers have a suitable transformer, I've already modified one of these to make a replacement o/p TX for a battery valve radio, see my other post. And when full wave rectified and smoothed this will give 1.4 volts of smooth DC without having to worry about a shorting regulator blowing all your precious filaments. It would have to be very well smoothed as the filaments are a bit delicate. Is this a good suggestion or does a filament supply really need to be regulated? if so it would need a good OVP circuit of some kind, not easy to make for such a low voltage. RR.

 
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Re: basic battery valve filament supply.

Post by Michael Watterson » Wed Jan 13, 2016 12:14 pm

Or you can buy a transformer with dual 110/240 primary and 1.5V and 7.5V AC outputs at enough current for even "big" battery valve sets (like Hallicrafters S72). Readily available.

Also you need a regulator! Even the contemporary unregulated ones used a choke to improve the smoothing, as the capacitors get infeasibly big and peak current in rectifiers gets high and generates RF! The resistance of choke and voltage was chosen to account for drop in selenium rectifiers and give 1.35V. A parallel load to use it on 125mA nominal sets instead of 250mA. Such PSUs designed for the number/type of valves. They would over run the remaining valves if one failed. This is why from 1938, (Octal Sylvania 1.4V tubes) in USA, and 1939 in UK that any sets for mains used series filaments and higher voltage supply, the dropper provided more smoothing and regulation.
German sets used Y base metal can tubes optimised for 1.25V so that an NiCd (DEAC) could act as parallel regulator. Even up to 1959 and Dx96 tubes that could be serial, the German models used this scheme.

So a simple solution is 5V phone PSU, resistor and a 10,000mAH NiMH D cell as regulator. Select resistor for what ever current the radio takes to drop 5V to 1.35V. This will be less than 1/10th C. Don't leave it plugged in too long if battery is charged. (i.e. a box with USB socket like on any gadget, RF filter, wire wound resistor and NiMH cell is your battery eliminator. For a 125mA set a 3,500mAH C cell will do)

Note that recommended supply is 1.35V, +/- 5% by valve makers. They are nominal 1.4V tubes and the battery is 1.62V fresh to 0.9V at end point. They are not 1.5V filaments.
A 10% increase from 1.35V will more that HALF life of filament.

 
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Re: basic battery valve filament supply.

Post by Rebel Rafter » Wed Jan 13, 2016 12:58 pm

Hi, from RR. I remember now, only after I posted the message of course! Those little transformers have dreadfully poor regulation. I think I'll have to use an LM317 and experiment with a potential divider for a crowbar trip, since a zener is no use at such low voltage. 2.2 volt is the lowest I've ever seen and that was an uncommon one. Or maybe rig an op-amp as a comparator to trip it. RR.

 
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Re: basic battery valve filament supply.

Post by Michael Watterson » Wed Jan 13, 2016 5:20 pm

1.25V is the lower limit for an LM317 and you'll struggle to get better than 5% accuracy at 1.35V with one.

Zeners below 3.6V are not very accurate, they also have soft knee. The Absolute max is 1.6V for the filaments of 1.4V valves. A Zener trip won't work.
Fuse and 2 x 1N4007 in series will work better!

After several years of thought and experiment, I've concluded that the 18 years of German usage is a nice simple route, the rechargeable Cell as a regulator with simple series resistor from 5V supply that will only give 1.35V volts if the battery is out of circuit and the correct number of filaments connected.

I file and solder the battery at corners to avoid accidental disconnection.

The NiMH is bit higher voltage than NiCd and can't cope with continuous trickle charge at as high a current once charged.

Other than that a LM317 with 1% resistors, 5V in and also fuse and 2 x 1N4007 series parallel (the IN4006 and lesser have a lower voltage drop, as does the 1N540x series).

 
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Re: basic battery valve filament supply.

Post by Rebel Rafter » Wed Jan 13, 2016 8:09 pm

Hi, from RR. How do you get 1.6 volts drop across two 1N4007's? I thought silicon diodes drop 0.6-0.7 volts, so two would make 1.2-1.4 volts, wouldn't they? or do the 1000 volt ones have a higher volt drop? RR.

 
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Re: basic battery valve filament supply.

Post by Michael Watterson » Wed Jan 13, 2016 11:16 pm

a) the voltage drop depends on current
b) the 1N4007 x 2 gives 1.3V to 1.6V depending on current through them
c) Yes the 1N4007 are somewhat different to 1N4006, not just voltage. You can ALSO use them as varicap diodes, and forward biased they are so slow you can vary DC current to vary RF, they won't rectify the RF! A cheap PIN diode for DC control of RF gain.

Silicon diodes can be 0.2V to 1V depending on construction and current. 0.65V is a typical Vbe or ordinary small g.p. silicon diode, but can be 0.58 to 0.72 depending on current.


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