Amps and car battery?

Valvehog

Member
Messages
31
I have seen this discussed on occasion but didn't see anything conclusive on it.

If you need to test gear for sale, and have to meet someone half way where there just isn't
a power outlet, is this a viable/smart option? Sounds like it would work in principle,
but maybe only for small low watt boxes ?

If you know it's dangerous or not, or have ever tried it give me a heads up please.
 

jimk4003

Member
Messages
1,058
Car batteries are usually between 12-14 volts DC, with a current capacity in the region of 50Ah (meaning it can supply 50A for one hour, 25A for two hours, etc.). You get bigger ones and smaller ones, but those values are fairly typical.

If your amp is designed for mains use, you'll need between 110-240V AC depending on region, and mains supplies are usually on breakers rated between 13-15 amps.

For it to work, you'd need to use an inverter to convert the DC to AC, and then pump that AC through a step-up converter to get a suitable working voltage. Stepping up 12V to 110V will reduce the current capacity almost tenfold; assuming that the inverter and step up transformer are 100% efficient, (which they won't be). In reality, the available current will be lower.

Effectively, you'd need to carry a battery, an inverter, and a step-up transformer. What you'd be left with would be a 'mains voltage' of dubious current capacity, liable to fluctuating voltage, and which would only last for a short while before the battery died.

Basically, it wouldn't work.
 
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fusionbear

exquirentibus veritatem
Platinum Supporting Member
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10,870
The inverter needs time be a true sine wave type, otherwise it can induce noise and overheat the PT.
 

Motherfuzzer

Silver Supporting Member
Messages
1,775
I have a Fender Sidekick 10. It has an outlet on the back to connect to a car lighter. I have never tried it. I don’t have the proper cable. It also has a battery option I believe.

I put a Jupiter
Speaker in it and it sounds really good for a little SS amp.
 
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jdh

Member
Messages
201
Yes, inverters are for sale that convert 12V DC into 120V AC. They have been around for many many years, and are designed for your application: run a home appliance from a car battery. A Princeton Reverb is rated at 125W power draw from the wall. 125 Watts divided by 12 volts is 10.5Amps. The inverter will dissipate power as well, so the battery will probably have to deliver about 13 Amps. Note: the extra power for the Princeton is because amps are only about 50% efficient (15 watts out requires 30 watts in), and the tube heaters require considerable power to operate as well. The inverter may add some noise depending on it's design as would the car's charging system if it's running. Test the inverter with your car at home with your own amp to get a baseline for any noise the system might add.
 
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Valvehog

Member
Messages
31
Basically, it wouldn't work.

thanks for the explanation, I sort of expected that would be the conclusion, so really not much substitute for finding a Starbucks with a free outlet
that would me let me try out an 800 head... with my cab :D
 

Valvehog

Member
Messages
31
Pedals should be fine. Amps? You just need to keep a CLOSE eye on the draw.
I have a Roland micro cube to test out pedals if need be... it's perfect in that capacity..
throw in 6AA and a 9 volt and you're good to go.

Good point.. if it only runs off 9 Volt adapter..yes.. I have the car inverter for that too
.
 

Jon C

Gold Supporting Member
Messages
17,322
Had a guy use a UPS to test my SWR California Blonde when I sold it, that worked on that.

I don’t know that I’d want to use it for a tube amp. At best you’ll cnfirm it works but not really the steady power imo you want to tell how a tube amp sounds IRL.
 
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jdh

Member
Messages
201
Had a guy use a UPS to test my SWR California Blonde when I sold it, that worked on.

I don’t know that I’d want to use it for a tube amp. At best you’ll cnfirm it works but not really the steady power imo you want to tell how a tube amp sounds IRL.
A UPS is the exact system described above (battery + inverter). In fact UPS systems are well regulated as they feed Data Processing systems for banks and financial institutions. I might add: how did 6 Volt batteries run vacuum tube radios in cars (granted, an "inverter" in a 1934 Buick was far different techology than today)?
 
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Jon C

Gold Supporting Member
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17,322
A UPS is the exact system described above. In fact UPS systems are well regulated as they feed Data Processing systems for banks and financial institutions.
What’s the draw of, eg, a 35-40 watt tube amp? Will a typical home PC UPS have the power to handle it with proper voltage range (120 +/- ?%?) , ie not “browning” out?
 

Zonk

Member
Messages
736
The inverter needs time be a true sine wave type, otherwise it can induce noise and overheat the PT.
Right, I have a inexpensive Harbor Freight 1000w modified sinewave inverter and have ran 50 watt solid state amp off it no problem, I also have a 2000w pure sinewave inverter that would probably run a tube amp although I've been leery of trying it, might try it with my 5w champ style just to see how it works.
 

jdh

Member
Messages
201
What’s the draw of, eg, a 35-40 watt tube amp? Will a typical home PC UPS have the power to handle it with proper voltage range (120 +/- ?%?) , ie not “browning” out?
A 45 Watt Bassman has a 3 Amp fuse, so this is 120 X 3 = 360 Watts maximum input. The 12 Volt battery must deliver 360/12 = 30 Amps. The size of the battery (amp hour rating) inside the UPS unit will determine how long the UPS can deliver those output watts.
 

zenas

Member
Messages
8,701
I did over the road trucking for a few years. Seemed like a good way to see the country and get away from my wife at the time.
So I had a Vibro Champ and picked me up one of those inverters that plugs into the cigarette lighter. As soon as I plugged it in the inverter shut itself down. Thing wouldn't run a Mr Coffee either.
I soon found out you need a really big inverter do do anything! Like one rated at 4,000 watts that has cables that hook to the battery. It'll suck that battery deader than dead in no time flat so don't shut your car off! Hell it might fry the alternator in most cars too.
If someone showed up to try an amp I was selling I wouldn't let them plug into an inverter! But I'm in a rural area where you call a dude off Craigslist and just go to his house and try it.
 

Jon C

Gold Supporting Member
Messages
17,322
A 45 Watt Bassman has a 3 Amp fuse, so this is 120 X 3 = 360 Watts maximum input. The 12 Volt battery must deliver 360/12 = 30 Amps. The size of the battery (amp hour rating) inside the UPS unit will determine how long the UPS can deliver those output watts.

So you need a decent sized UPS....

Eg, insufficient (255w):

https://www.staples.com/APC-Back-UP...b-lUWtxPhRaSLIiwFkUMxM-B83r8QLA0aAvcTEALw_wcB


Sufficient (?) - 600w

https://www.staples.com/american-po...yvo7q5Fl5kJIXrldF5KjYXVbMQz1xJ-QaAhZqEALw_wcB
 

zenas

Member
Messages
8,701
A UPS is the exact system described above (battery + inverter). In fact UPS systems are well regulated as they feed Data Processing systems for banks and financial institutions. I might add: how did 6 Volt batteries run vacuum tube radios in cars (granted, an "inverter" in a 1934 Buick was far different techology than today)?
The old tube radios used a "vibrator" an electro mechanical thing that pulsed the DC to turn it in AC. Transformers don't work with DC current. Those old radios drew a lot of current. My dad said when he worked in service stations in the '50s and '60s the vibrator was the most problematic part. Today they make solid state replacements.
6v6 tubes were actually fairly common in car radios. You'll see them stamped with names normally associated with auto parts from time to time.
 

Don A

Gold Supporting Member
Messages
5,935
I've run a Marshall Class 5 on a power inverter plugged into my car's accessory outlet and it worked well. I doubt I'd use it for an amp with any more power than that and only for a quick demo.
 




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