When does a filament become a fuse?

cubby01

cubby01

aka Buck
I'm alloted a dumb question a day and I've been saving up. :D

Can one use a big 12v SLA battery to directly supply heaters of for example the 12xxx tubes in a preamp. Is it just a matter of tapping from the battery terminals to the correct pins in parallel to each tube (ie pins 4 and 5 of a 12AU7)? Or is something going to go poof if I do this?

Is there such a thing as too big a battery in terms of amp hrs current available. I've done some unintentional welding in the past with batteries. I don't quite understand how that filament won't just go poof if enough current is put to it.

Thx
 
Register to hide this ad
The resistance of the filament(s) and applied voltage determine how much current is drawn, the battery merely supplies what's asked of it.

The bigger the battery you use (at the correct voltage, of course) the longer it will supply power before needing a charge, but it does not supply more current/power than the filaments demand.

The reason for the unintentional welding is because the resistance was very, very low. At any given voltage a lower resistance means more current will flow. In the case of a short, the battery bascially attempts to dump all possible current though the short because that's what Ohm's Law dictates it will do.
 
Last edited:
Fuse; a device that will save itself even if it has to blow up 1000s of dollars worth of other components.:smoke:
 
cubby01 said:
Is there such a thing as too big a battery in terms of amp hrs current available.
Click to expand...

Like whoaru stated the battery size isn't important except for the capacity to run the filaments longer. I've used everything from a little 7ah backup battery to the 140 ah deep cycle battery at my cabin.

You can even run two 6.3v tubes in series off a battery as long as the heater current is the same between them.
 
What'cha planning? I've got a few screwy ideas floating around my noggin for battery based tube amps.
 
Scuzzer said:
What'cha planning? I've got a few screwy ideas floating around my noggin for battery based tube amps.
Click to expand...

Nothing special. Someone gave me some 12v SLAs (12 AH) that go into scooter chairs that I was trying to find a use for. Was thinking maybe heater supply for a preamp w/ phono. The batteries are 12 AH so 6 tubes drawing 150ma each should last several listening sessions. I'd still need to come up with a charging system and a way to monitor them, but I've got four so they could be rotated through duty.
 
cubby01 said:
Nothing special. Someone gave me some 12v SLAs (12 AH) that go into scooter chairs that I was trying to find a use for. Was thinking maybe heater supply for a preamp w/ phono. The batteries are 12 AH so 6 tubes drawing 150ma each should last several listening sessions. I'd still need to come up with a charging system and a way to monitor them, but I've got four so they could be rotated through duty.
Click to expand...

Those sound like the same type battery used in computer UPS (uninterruptible power supplies).
My UPS uses a single 12v 10 AH SLA battery. If you could find an old UPS laying around , you
could use that to charge the batteries.

Some of the high output UPS like 1000VA or more, use two of these batteries so if you find one
of those you could charge two batteries at the same time. Might have some luck finding one at
Goodwill/Craigslist or one of those electronic recycling events.
 
6.3- and 12.6- volt are the nominal voltages of fully charged 3- and 6-cell
lead-acid batteries, so actually most modern tubes were designed for
operating off a storage battery. The battery can be seen as a nearly ideal
voltage source, the filaments resistance determines the current.
 
Probably nothing to do with the topic, but take a look at the old 40's & 50's tubed car radios. I have one that uses two 12V6 tubes in push/pull for the amp section, a couple of 12BE6's 12BA6's and 12AV6 for the tuner. This is your 12 volt car radio. It uses a vibrator to supply the B+. you can hear the vibrator ticking when turned on and its supposed to be vibrating at 60 cycles-per-second to emulate the 60Hz sine wave.

I also have some SAMS photofacts of 12 volt tubed car radios W/schematics.
 
Last edited:
Safety first, some info.

mikey99 said:
Those sound like the same type battery used in computer UPS (uninterruptible power supplies).
My UPS uses a single 12v 10 AH SLA battery. If you could find an old UPS laying around , you
could use that to charge the batteries.

Some of the high output UPS like 1000VA or more, use two of these batteries so if you find one
of those you could charge two batteries at the same time. Might have some luck finding one at
Goodwill/Craigslist or one of those electronic recycling events.
Click to expand...

Charging the batteries using the UPS they came from at first seems a logical idea BUT a closer examination of the situation is needed.:scratch2:

Because the batteries are enclosed INSIDE the UPS unit, the battery terminals are not exposed to the user while in operation.

Because the manufacture knows the battery terminals are "protected" from contact in "normal" use, they are not expected to insure that these terminals are not "hot to the touch" when energized.

The UPS (enclosed battery types) may be wired to have the battery terminal(s) "hot to ground" in respect to the AC line (AC in or out of the UPS).

Assembling the circuits this way may simplify the UPS construction (Not worrying if the battery terminals are "hot" or not because they are protected from contact allows the manufacture to "leave out stuff").

NOW, if you have a UPS unit that ALLOWS the use of external batteries using EXPOSED battery terminals, these are likely NOT to have dangerously high AC line Voltages on them (designed just for that reason).

Summing up- UPS units having ENCLOSED batteries MAY have dangerous AC line Voltages on them.:sigh:

UPS units that have EXPOSED battery terminals for use of external batteries are LIKELY to be safe in regards to exposed AC line Voltages (check with the manufacture).

You are safer using a dedicated battery charger matched to the battery you are using rather than "cheaping out" and using the UPS to charge said battery and risking electric shock and equipment damage. :no:

We all here want to talk WITH you, not ABOUT you!:thmbsp:

Mark T.:music:
 
My 900VA "commercial grade" UPS uses four 7.2Ah 12V batteries, but they are wired in series to provide 48V at the inverter input, which means it also charges the series string at something like 55 volts, and is looking for 48V from an external battery pack if used. Point being, if the UPS uses multiple batteries could well be it's not 12V operation. Makes it much more complex to to develop a series/parallel switching arrangement to put the batteries in parallel for 12V supply but then into a series string for charging.

Simpler would be to buy a small 12V SLA-type charger say 2A, give or take, then wire that to a toggle switch setting up a "Charge" or "Listen" switching arrangement. I used this type of thing with a DAC I was running on two 12V SLAs.

All Electronics has some fairly inexpensive small chargers that work well for SLAs. They will charge at one voltage but then automatically switch to a float charge voltge so you don't have to worry about monitoring things.
 
Last edited:
I was going to build mine so that when I turned off the amp the battery would automatically start charging. I planned on using an old laptop power supply which is 16vdc/4A and the circuit on this page: http://www.myra-simon.com/bike/charger.html. The chargers max current is 2 amps so I should be able to top off anything less than 20ah overnight.
 
I bought a few of these for charging smaller SLAs. For $10 each I can't even justifiy a DIY build. This is the one I used to charge the two 7.2Ah batteries I was using for the DAC. Never had a problem keeping the batteries up. Used the charge/listen circuit I mentioned earlier. With switch in "listen" the DAC was powered from the batteries and the charger disconnected, in "charge" mode the DAC was off and the charger connected to the batteries.

http://www.allelectronics.com/make-a-store/item/BC-100/12V-1A-CHARGER-FOR-LEAD-ACID-BATTERIES/1.html
 
Last edited:
Get a portable "jump box" from a auto parts store or Harbor Freight, etc. Those have something that looks like a motorcycle battery in it and a wall wart charger. And they are cheap too!
 
You must log in or register to reply here.
Back
Top Bottom