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Old 1st Apr 2010, 12:51 pm   #1
phil.turrell
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Default Are NiMh batteries ok for '96 valve filament current?

Hi,

I'm currently restoring an Every Ready Sky King that uses DK96, DF96, DAF96 and DL96 valves. The manufacturer's information says the filament voltage is 1.4 volts although the original B136 battery would have supplied 1.5v.

Having recapped the radio it appears to work fine with the filament current supplied by 4 NiMh AA cells connected in parallel. Fully charged NiMh cells give around 1.3 to 1.4 volts. However, a recent article I read suggested that '96 series valves could be harmed by a filament voltage that is too low. Is this true?

I'm aware that over running the filaments will reduce valve life but I hadn't thought that there was any problem in under-running valves provided the current was high enough to produce enough emission for the valve to operate.

Thanks for you help.

Phil.
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Old 1st Apr 2010, 1:17 pm   #2
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Default Re: Are NiMh batteries ok for '96 valve filament current?

I'm a bit concerned about putting NiMH cells in parallel. There are likely to be some rather high circulating currents as the voltages equalise. Also you can't really charge them like that. Surely better to use a single larger cell.

D series valves were desiged to run from a 1.5V cell. This would have given around 1.6V when new, falling to somewhere between 1V and 1.2V when exhausted. Obviously you would change it when the performance of the set dropped off. NiMH will maintain 1.2V over most of its life, then it drops rapidly.
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Old 1st Apr 2010, 5:57 pm   #3
kalee20
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Default Re: Are NiMh batteries ok for '96 valve filament current?

I echo ppppenguin's comments. But if the AA cells are all from the same batch, then they should be reasonably well matched. However, not as elegant as a single large cell.

I do regard D96 series valves as rather delicate. Whether anyone has damaged them by cathode poisoning by prolonged under-running I don't know, I just don't want to find out the hard way. So I would err on the side of caution. If you have a weak set of valves (the DK96 gets reluctant to oscillate at the end of its life), it may be worth comparing poerformance at 1.2V and 1.4V.
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Old 2nd Apr 2010, 12:12 am   #4
phil.turrell
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Default Re: Are NiMh batteries ok for '96 valve filament current?

Thanks for your replies.

I do appreciate your warnings about charging NiMH cells in parellel. I know that charging in parellel is both dangerous and ineffectual, and you'll be glad to know that I won't be doing this!

I'm using brand new cells that will only ever be used for this application, and they'll always be charged together in a standard "off-the-shelf" charger. Incidentally, the reason I'm using NiMH AA cells in parallel rather than a single NiMH D cell is that 2500 mAh AA cells are now very cheap. For instance, Maplin currently offer 12 for £9.99.

I think that most NiMH D cells are just single AA cores packaged in a D case (look at the capacity rating and feel how light they are), so offer little or no more capacity than a standard AA cell. From what I can see you have to pay well in excess of £10 for a proper NiMh D cell, and they're not freely available on the high street. Also, AA cells quite often come with a charger thrown in, whereas I'd have to buy a D cell charge separately.

Actually the whole point of this is that I'm trying to maintain the Sky King's portable status by running the filaments directly from NiMh cells and the HT via an inverter, also powered from AA cells - hence the interest in cheap AA cells!

It seemed to me that using rechargeable cells for the 1.4v filaments was a good plan since the battery voltage is naturally close to 1.4 volts, there's no running cost and (I thought) the filaments would last longer. I'd read many times that filaments should be de-rated for maximum valve life. I had assumed that the only penalty in doing this was that the valve might not operate if the de-rating was overdone.

However, (as I say) a recent article suggested, without explanation, that too low a filament voltage also reduces valve life. Can anyone tell me why this is? I thought cathode poisoning was the result of running valves with the LT connected and the HT disconnected? i.e. electrons being liberated with no where to go? Sorry if this is way off the mark but that is my non technical understanding!
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Old 2nd Apr 2010, 12:25 am   #5
AlanBeckett
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Default Re: Are NiMh batteries ok for '96 valve filament current?

Quote:
Originally Posted by phil.turrell View Post
I thought cathode poisoning was the result of running valves with the LT connected and the HT disconnected? i.e. electrons being liberated with no where to go? Sorry if this is way off the mark but that is my non technical understanding!
You are correct. The P.O. always under ran the valves in the fdm repeaters, after ageing them to flush out 'infant mortalities'. Provided HT is there you won't have a problem.
Alan
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Old 2nd Apr 2010, 8:53 am   #6
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Default Re: Are NiMh batteries ok for '96 valve filament current?

At 2500mAH you should get in excess of 15 hours play with a single AA cell. Why bother with 4 in parallel.
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Old 4th Apr 2010, 10:51 pm   #7
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Default Re: Are NiMh batteries ok for '96 valve filament current?

Hi !
I run a cuple of Telefunken Bajazzo from 55 and 5- with NiMh driving the heaters.
I use either a NicadD cell of 4.5 Ah or, when the Nicad is in the charger A "Ansmann" "MaxPlus" D cell NiMh of 8500 mAh.
These sets are in regular use for month on batteries (I use a DC/Dc converter powered by 5 NiMh cells) and I've not seen any degradation in the performance of the set.
Power output goes a little low and distorted when heaters are run at near a volt, and the sets dies when voltages goes under 1.0 V but this indicate a very discharged cell !
Bear in mind that recharging a 8500 mAh takes time .....
Hope this helps.
P.S. as a substitute for the DK96 or DK92 I use an "solid state" 1L6 bought from an American uy. The sets seems to be more sensitive and selective with them compared to the original tubes.... I say seems because I've not made any measurements and one is always willing to see what he paid for be better ;-)
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Old 6th Apr 2010, 2:31 pm   #8
cheerfulcharlie
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Default Re: Are NiMh batteries ok for '96 valve filament current?

Might I also ask you to give good old lead acid batteries a consideration?

http://www.budgetbatteries.co.uk/208...-enersys-cell/

(scroll to bottom)



To get your voltage you could put a wirewound dropper in the heater chain and this would give you a nice gentle power up .

..and of course lead acid batteries just love being put in parallel.

I replaced a camcorder powering, with a small lead acid battery system and regulator, the battery lasted 10 years and much better than the mickey mouse lithium batteries supplied which use to last about 8 months and cost around £50 a shot.

Last edited by cheerfulcharlie; 6th Apr 2010 at 2:43 pm.
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Old 6th Apr 2010, 4:57 pm   #9
G8HQP Dave
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Default Re: Are NiMh batteries ok for '96 valve filament current?

Quote:
However, (as I say) a recent article suggested, without explanation, that too low a filament voltage also reduces valve life. Can anyone tell me why this is? I thought cathode poisoning was the result of running valves with the LT connected and the HT disconnected? i.e. electrons being liberated with no where to go? Sorry if this is way off the mark but that is my non technical understanding!
If you run with too low a filament voltage and HT connected (i.e. the valve is in use) then there is not a protective space charge of electrons around the filament, as any electron emitted immediately gets pulled off to the anode. There will always be a small amount of gas in a valve, which can get ionised when hit by electrons. Positive ions get attracted to the filament and hit it, damaging the emissive surface. The space charge, present when the filament is hot enough, neutralises these ions and so protects the filament. The same thing is true for indirectly heated cathodes if the heater voltage is too low. This is cathode poisoning.

On the other hand, running with the heater at full whack with no HT (and so no cathode current) causes an insulating layer to build up between the cathode base metal and the oxide coating. This is called sleeping sickness. Some valves (e.g. some military versions and those meant for computers) are specially designed to prevent this happening. I'm not sure if the same effect happens with filament valves.

Running a valve with low heater/filament voltage and no HT should be safe, as in this case neither of the above effects can occur.
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Old 6th Apr 2010, 5:47 pm   #10
kalee20
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Default Re: Are NiMh batteries ok for '96 valve filament current?

Quote:
Originally Posted by cheerfulcharlie View Post
Might I also ask you to give good old lead acid batteries a consideration?...To get your voltage you could put a wirewound dropper in the heater chain and this would give you a nice gentle power up
Personally, I would avoid a higher supply voltage and a dropper - unless you have one of those nasty series filament chains.

Sure, you can size the dropper to give you correct voltage on load. But if one of your valves fails, has a dodgy valveholder contact (quite common in battery sets due to yesteryear's chemical fumes from leaking batteries), or you just pull one out, then the LT current falls, the voltage across the dropper also falls, and the remaining valves are over-volted.

However, I do agree that lead-acid is a good technology, and it's cheap in terms of watt-hours per unit cost!
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Old 7th Apr 2010, 9:11 am   #11
phil.turrell
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Thumbs down Re: Are NiMh batteries ok for '96 valve filament current?

Quote:
Originally Posted by G8HQP Dave View Post
If you run with too low a filament voltage and HT connected (i.e. the valve is in use) then there is not a protective space charge of electrons around the filament, as any electron emitted immediately gets pulled off to the anode. There will always be a small amount of gas in a valve, which can get ionised when hit by electrons. Positive ions get attracted to the filament and hit it, damaging the emissive surface. The space charge, present when the filament is hot enough, neutralises these ions and so protects the filament. The same thing is true for indirectly heated cathodes if the heater voltage is too low. This is cathode poisoning.
Thanks. That's a very clear explanation Dave.

I think I will stick with the NiMh batteries. I think it's likely that damage would only occur after performance has started to drop off which is when I would check the state of the batteries.

The reason I didn't want to use higher voltage batteries was simply efficiency.

Last edited by Dave Moll; 7th Apr 2010 at 10:05 am. Reason: quote fixed
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Old 7th Apr 2010, 3:47 pm   #12
cheerfulcharlie
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Default Re: Are NiMh batteries ok for '96 valve filament current?

Quote:
Originally Posted by kalee20 View Post
Quote:
Originally Posted by cheerfulcharlie View Post
Might I also ask you to give good old lead acid batteries a consideration?...To get your voltage you could put a wirewound dropper in the heater chain and this would give you a nice gentle power up
Personally, I would avoid a higher supply voltage and a dropper - unless you have one of those nasty series filament chains.

Sure, you can size the dropper to give you correct voltage on load. But if one of your valves fails, has a dodgy valveholder contact (quite common in battery sets due to yesteryear's chemical fumes from leaking batteries), or you just pull one out, then the LT current falls, the voltage across the dropper also falls, and the remaining valves are over-volted.

However, I do agree that lead-acid is a good technology, and it's cheap in terms of watt-hours per unit cost!

My mistake Kalee20 . I thought it was a series heater chain for some reason
only known to me


anyway this is another thought, a 1.4 regulator circuit which from what I can make out can handle a rather juicy 5 amps


http://www.datasheetarchive.com/data...A00303146.html
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Old 7th Apr 2010, 4:00 pm   #13
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Default Re: Are NiMh batteries ok for '96 valve filament current?

Would a silicon rectifier diode do a decent job of dropping 2V from a lead-acid down to 1.4V? Avoids most of the problem with a resistor.

That chip won't work as it needs more than 2V input.
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Old 7th Apr 2010, 5:26 pm   #14
kalee20
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Default Re: Are NiMh batteries ok for '96 valve filament current?

Yes it would, possibly a slight temperature sensitivity, but won't be far out.

Also occurring to me, is two silicon diodes (1N4003 sort of thing) in series, across the heater chain. These should be on the verge on turning on, with 1.4V. Any higher, and they'll clamp reasonably well, so avoiding overvolting the filaments.

Only down side is the purists might object...
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Old 7th Apr 2010, 9:27 pm   #15
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Default Re: Are NiMh batteries ok for '96 valve filament current?

Hi Gents Jeffrey's diode trick will also work on the output of a chip regulator as it then regulates down to 2.8 V and 3 diode drops will bring you down to 1.4V.
Fine tune filament voltage with diodes fitted and adjustment of chip regulator.

Important note: If you have a high value of cap on the chip reg output, fit a diode "backwards across the reg to prevent damange on power down by the output being above the input.

Ed
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