amperage and watts

wg_ski

Member
2007-10-10 5:21 pm
rtill said:
Is it crucial that a transformer has a certain amount of amps to produce a certain amount of watts?

example.... a 5 amp transformer is needed to push out 100 watts, all I can find is a 2 amp transformer....will I get 100 watts?



If the supply voltage doesn't drop too much, yes. What you end up doing is running off a higher unloaded voltage so it droops down to what it would have off the higher-amp transformer. The catch is you can't play it as loud for as long before it runs hot.

This seems to be the basis for all these "smaller lighter cheaper" amplifiers out there, so you won't be doing anything unusual.
 
ok, so here's my next question.....
let's say the circuit needs 90 volts at 5 amps for 100 watts.

my choices for transformers are.....

a 60 Volt c.t. at 5 amps - after rectified I get 84 Volts
or
a 64 Volt c.t. at 2 amps - after rectified I get 90 Volts

Which would be a better choice?
Would the 6 Volts make a huge difference so I don't get 100 watts? or do I risk damaging the circuit by overheating with not enough amps.....
 

wg_ski

Member
2007-10-10 5:21 pm
rtill said:
a 60 Volt c.t. at 5 amps - after rectified I get 84 Volts
or
a 64 Volt c.t. at 2 amps - after rectified I get 90 Volts

Which would be a better choice?

You won't get 100 watts out of either one. But the 60V/5A one will probably be closer. Best guess without measurements puts the bigger one at 68 volts rectified under load and the smaller one at 63 volts.

You don't risk damaging the *circuit* with a transformer with too low a current - the transformer itself will run hot. If you wanted to bump up the unloaded voltage to make sure you get 100 watts, you need a 70-80 vct, perhaps 90 vct if it was really undersized. THAT may damage the circuit if it isn't meant to take it.
 
rtill said:
....or do I risk damaging the circuit by overheating with not enough amps.....

Maybe not damage the circuit, but by counting on the higher voltage transformer to droop in order to get the amps needed may cause the transformer core to saturate, that's why it gets hot. It is best to have a transformer that matches the impedance of the circuit, i.e. correct voltage and current. Use a larger transformer if needed. You can parallel transformers as long as the are exactly alike, observing polarity. Besides, you would want to have as little possible ripple on the filter caps anyway, eh? Sure not going to get this by an overdriven power transformer. core....:dodgy: :dodgy: :smash:
 

wg_ski

Member
2007-10-10 5:21 pm
CBS240 said:


Maybe not damage the circuit, but by counting on the higher voltage transformer to droop in order to get the amps needed may cause the transformer core to saturate, that's why it gets hot. It is best to have a transformer that matches the impedance of the circuit, i.e. correct voltage and current. Use a larger transformer if needed.

The transformer won't saturate by overcurrent. It gets hot because of too many amps per circular mil, plain and simple. Saturating happens if you overvoltage the primary or drop the frequency. That will cause it to get a LOT hotter than long term overcurrent. All hi-fi and most pro amps use undersizedtrafo to get to target price/weight points. Undersized meaning less than that required for full signal output, but appropriate for what the typical user considers "loud enough". Regulation on these power supplies tends to be poor and it's not unheard of for a 100Wx2/8 ohm amp to use +/-72 volt supplies because the trafo is dinky - maybe 150VA, IYL. 2.4kW pro amps routinely use 900VA cores but get about 15 minutes of 'blast' time at 4 ohms bridged before it takes its ball and goes home. The problem with guitar amps is that they may be called upon to deliver 40-110% of maximum power, continuously, and you can't go undersizing anything in the amp and expect not to "face the music" eventually.

CBS240 said:


You can parallel transformers as long as the are exactly alike, observing polarity. Besides, you would want to have as little possible ripple on the filter caps anyway, eh? Sure not going to get this by an overdriven power transformer. core....:dodgy: :dodgy: :smash:

Actually, if you pull enough current off a trafo, the leakage reactance will cause it to behave as a choke and ripple wil *decrease*. At infinite current, DC output will drop to 0.637 times the peak with zero ripple. With most conventional supplies, you have less ripple than simple C*dv/dt predicts because of this nonzero inductance that's not taken into account.


rtill said:
I'd be using this for a guitar amp put in place after a tube preamp.
So it wouldn't have to be "stereo" quality.


I can't get a hold of any of the shown transistors, however, I'm wondering if the PNP's could be replaced by TIP147's and the NPN's could be replaced by TIP142's.

Would that be ok?

DON'T go using TIP142/147. Recipe for trouble. If you must use TIP devices, use parallel pair of TIP35/36 with separate
2SC/2SA drivers. Any of the TO-220 Jap drivers may be used, as well as MJE1503XX's if you're not picky
about the exact sonic signature. For outputs, any of the old-school TO-3's that will take full rail
voltage will work.
 
Hi,
Onsemis' MJ15003/4 are readily available.
They are 250W, 200DegC, 20A devices that perform very well.
Any replacement should try to match their SOAR (temperature de-rated).
Do not use this with 6ohm or 4ohm speakers.

BTW,
a 90V supply will not get you your 100W of output power. 95V might get there, 100V will get there if the PSU and amplifier is properly designed.
If you want live music duty then the transformer should be at least VA=Woutput, but I would go VA>=1.5*Woutput.