I have a PT from a Ampeg B-25. I know how much current it was supplying. But is there a way I can measure the current it is CAPABLE of supplying.? Can I connect it to the 120V wall source and then measure the current from the secondary.?
Reason I ask is because I want to build an amp that requires 3 Amps for the rectifier tube.
Thank You
PT Current Availability
Moderators: pompeiisneaks, Colossal
Re: PT Current Availability
Run it with increased current draw intil it fails, and then assume that the limit was just a little bit lower.
Just kidding of course, but not really. You cannot "measure" current capacity in a direct way. You might be able to guess at the current capacity by monitoring the voltage as you increase the current draw. If voltage starts to drop, you have surpassed the current limits. But heat build up is another factor, and I am not sure how to deermine how much heat is too much heat.
If it is a name brand PT, you should be able to just ask the manufacturer.
Just kidding of course, but not really. You cannot "measure" current capacity in a direct way. You might be able to guess at the current capacity by monitoring the voltage as you increase the current draw. If voltage starts to drop, you have surpassed the current limits. But heat build up is another factor, and I am not sure how to deermine how much heat is too much heat.
If it is a name brand PT, you should be able to just ask the manufacturer.
Re: PT Current Availability
The B25 uses a 5AR4 rectifier. The filament requirement is 1.9A. Moving up to 3.0A is probably going to be a real stretch and I suggest it's a bad idea. It might be in there, but as Steve says, do you really want to run it 'til it fails? Once you let the smoke out, it's tough to put it back.
If you really need 3.0A for the rectifier, find a separate 5V filament transformer. This sort of thing can often be had on eBay for $10-15 shipped. Do that or get the correct PT.
If you really need 3.0A for the rectifier, find a separate 5V filament transformer. This sort of thing can often be had on eBay for $10-15 shipped. Do that or get the correct PT.
Re: PT Current Availability
sliberty, Phil....Thank You
And my real question was a general one. So regardless of what guitar amp PT I might have, there is no safe and conclusive way to measure current capacity with a meter.
Thanks Again
And my real question was a general one. So regardless of what guitar amp PT I might have, there is no safe and conclusive way to measure current capacity with a meter.
Thanks Again
Re: PT Current Availability
You test for the load you want to support with the PT. There are a number of different ways to estimate the capacity, and the more toys you have to do it with, the better. Big factors are the size of the core (the middle thing on the "E", the overall size of the laminations, the resistance of the windings, etc. If you know what it came from, as in this case, it is pretty easy to determine it's minimums. Unfortunately, those might also be the maxiumums.
On occasion, I've built out a power supply and hooked a big honking resistor or 3 to the DC out. Applying Ohms Law, measure the voltage drop and use the R to calculate current: simple V=I*R math. The lower the resistor value, the higher the current draw. Careful, as the resistors get very hot. You need a load rated for at least 25W and probably 50W or 75W. Once you get the number of mA you need and it appears to hold that load without letting the smoke out, there's no reason in the world to continue to reduce the load because you will eventually let the smoke out.
Here's a nifty chart I got from someone that isn't too bad at getting you in the ball park:
For a 120 volt AC supply the VA rating and primary resistance is as
folows.
30 VA = 30 to 40 ohms
50 VA = 13 to 16 ohms
80 VA = 7 to 9 ohms
120 VA = 5 to 6 ohms
160 VA = 2.5 to 3.5 ohms
225 VA = 1.8 to 2.2 ohms
300 VA = 1.0 to 1.3 ohms
500 VA = 0.45 to 0.55 ohms
Simply multiply all ohmage values by four (4) for a 230 / 240 volt supply. Derate to 65%, which is probably reasonable and to allow 15VA for the filament windings.
Then, there is this bit of advice I picked up somewhere, working with the secondary windings:
One very general way, based on a copper loss of say 4%. The ht winding rating is probably the most important. Find the secondary voltage, e.g. 300Vac. Take 4%, giving 12V. Measure the winding resistance; one half if 300-0-300V (i.e. a 300V winding), if bridge then the whole winding. The current will now be that which causes a 12V drop across the winding d.c. resistance. Thus dividing 300V by the resistance would give a ball-park figure for current (Ohms Law). Heaters more difficult; same method, but low voltage winding resistance is usually impossible to measure accurately. The heater current could be expected to be in line for the output stage that would require the previously calculated anode current.
Naturally, trying to size the 5V winding by this method would be difficult. You might be able to do it with precision equipment. It would be cheaper to buy a new tranny. IOW, you want 3A @5V. This means the resistance winding will be maybe 0.1 ohms or less. Wait a minute, you think this sounds crazy? I just metered the secondary on a 6.3V filament tranny (I think it is 3A or 5A, can't remember) and it shows 0.22 Ohms on an old Fluke. You get the point here...it will be tough to tease out a rectifier winding and discern the differnce between 2A and 3A capacity. The high voltage windings are easier.
One last thought. The 5V winding is often the outermost secondary winding on a PT. It typically needs less than 20 turns. You could breadboard a socket, put the 3A rectifier in it (5U4?), let it run for an hour, and see if it smokes or not. I'd say, after an hour, you are in the clear. The risk is that you will also smoke the primary and possibly arc over to one of the other secondary windings. I'm just saying, you could roll the dice if you think the PT might be up to it. I don't think I would with the B25 listing a 1.9A tube for the 5V.
On occasion, I've built out a power supply and hooked a big honking resistor or 3 to the DC out. Applying Ohms Law, measure the voltage drop and use the R to calculate current: simple V=I*R math. The lower the resistor value, the higher the current draw. Careful, as the resistors get very hot. You need a load rated for at least 25W and probably 50W or 75W. Once you get the number of mA you need and it appears to hold that load without letting the smoke out, there's no reason in the world to continue to reduce the load because you will eventually let the smoke out.
Here's a nifty chart I got from someone that isn't too bad at getting you in the ball park:
For a 120 volt AC supply the VA rating and primary resistance is as
folows.
30 VA = 30 to 40 ohms
50 VA = 13 to 16 ohms
80 VA = 7 to 9 ohms
120 VA = 5 to 6 ohms
160 VA = 2.5 to 3.5 ohms
225 VA = 1.8 to 2.2 ohms
300 VA = 1.0 to 1.3 ohms
500 VA = 0.45 to 0.55 ohms
Simply multiply all ohmage values by four (4) for a 230 / 240 volt supply. Derate to 65%, which is probably reasonable and to allow 15VA for the filament windings.
Then, there is this bit of advice I picked up somewhere, working with the secondary windings:
One very general way, based on a copper loss of say 4%. The ht winding rating is probably the most important. Find the secondary voltage, e.g. 300Vac. Take 4%, giving 12V. Measure the winding resistance; one half if 300-0-300V (i.e. a 300V winding), if bridge then the whole winding. The current will now be that which causes a 12V drop across the winding d.c. resistance. Thus dividing 300V by the resistance would give a ball-park figure for current (Ohms Law). Heaters more difficult; same method, but low voltage winding resistance is usually impossible to measure accurately. The heater current could be expected to be in line for the output stage that would require the previously calculated anode current.
Naturally, trying to size the 5V winding by this method would be difficult. You might be able to do it with precision equipment. It would be cheaper to buy a new tranny. IOW, you want 3A @5V. This means the resistance winding will be maybe 0.1 ohms or less. Wait a minute, you think this sounds crazy? I just metered the secondary on a 6.3V filament tranny (I think it is 3A or 5A, can't remember) and it shows 0.22 Ohms on an old Fluke. You get the point here...it will be tough to tease out a rectifier winding and discern the differnce between 2A and 3A capacity. The high voltage windings are easier.
One last thought. The 5V winding is often the outermost secondary winding on a PT. It typically needs less than 20 turns. You could breadboard a socket, put the 3A rectifier in it (5U4?), let it run for an hour, and see if it smokes or not. I'd say, after an hour, you are in the clear. The risk is that you will also smoke the primary and possibly arc over to one of the other secondary windings. I'm just saying, you could roll the dice if you think the PT might be up to it. I don't think I would with the B25 listing a 1.9A tube for the 5V.