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Power Transformer replacement

Started by MonkeyBoi, September 18, 2008, 09:19:43 AM

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MonkeyBoi

I am in need of a replacement power transformer for a Marshall Lead 12 3005 amp. The current one looks like it was leaking something or had something spilled on it and looks like it is corroded. It in the schematic here http://www.drtube.com/schematics/marshall/3005.gif and Marshall has told me the direct replacement is a Dagnall B1PTB63015 which can be found here: http://www.gretschparts.com/products/1724/Marshall-Transformer-Power-50-watt.htm

I would prefer something that is well made like Mercury Magnetics, but they say they do not have anything that will work. I am not even sure what specs I need to give to a manufacturer when querying them about transformers. Any help would be appreciated.

The filter caps on this thing are also shot and am looking for good replacements there as well.

Thanks

teemuk

Model 1987, where the replacement transformer suggested by Marshall was used, is a 50W tube amp. For some reason I think that the particular transformer is not the right one for the Lead 12... Seriously... What have they been thinking...

What you need is a generic power transformer with following specs:

Center-tapped secondary
Secondary voltage: 2 x 14.3V
40VA or higher

That kind of part should be rather inexpensive in most stores that sell electronic components. Even generic stuff is usually very well made.

Filter caps can be replaced with any part that has the same (or a bit higher) voltage rating and capacitance. I would worry more about physical dimensions of those parts (lead distance, lead length, diameter, axial/radial etc.).

MonkeyBoi

I was kind of leery about the 50W transformer they were suggesting.

One last question. How does one determine the VA rating of a power transformer when replacing one?

Thanks for the help

teemuk

It's basically a power rating the transformer can produce in maximum: total secondary voltage (V) times total secondary current (A), hence VA. If the circuit tries to draw more power than what the VA rating allows the transformer can't supply it and basically the secondary voltage begins to drop etc. In other words, the VA rating pretty much defines the output power limit of an amp – or any circuit.

The rating is not so easy to determine by means of measuring something but you can approximate it when you know the output power and efficiency of the amp. For SS amps I often use a rule of thumb that approximates VA as 2x or 3x the output power. Some people use a rule of thumb that approximates only 1.5x the output power. Yet, schematic of 3005 says: Power input 40W, which is somewhat higher than the aforementioned approximations. Since it actually designates this information I would be tempted to follow that rating. I don't know why the chose to use transformer with that VA rating (a 12W amp could do well with 30VA as well) but having higher VA rating than what you really need basically does not harm anything unless we begin to talk about ratings that are higher than about 200VA. In these cases the current peak drawn by the transformer when it magnetizes begins to have serious effects that typically burn out fuses when you flick the power switch. With small transformers, such as those used in <30W SS amps, the VA ratings are so small that you don't have to care about this issue too much.

In fact, higher VA means the power supply will be regulated much better since the transformer is not working near its limits (getting hot and all) and it has no issues supplying the current drawn by the amp. It also means the amount of sag (and hence output power reduction and increased ripple) is reduced, which also means the power supply can cope with smaller capacitance to produce the same level of regulation as with transformers that have smaller VA rating. Naturally, whatever the VA rating is you still need to match secondary voltage to that required by the circuit.

darwindeathcat

Well, the simple answer is: figure out the voltage of the secondaries given the primary voltages, then figure out the amp rating of the transformer, and multiply them to get the VA (VoltAmp) rating of the tranny. This may not be as easy as it sounds. Best would be to find some sort of indication written on the xformer itself. Barring that, you can use any standard multimeter to get the AC voltage of the secondairies (make sure you read the instructions for your multimeter thouroughly first! and just be sure to take all the necessary precautions when dealing with mains (potentially lethal) voltages...) You just hook up the multimeter across the secondaries (no load) and set the multimeter to the proper range of AC volts. Getting the current rating with a multimeter is slightly more problematic. First you must have a multimeter that can read AC current (mine does only DC current), then I believe if you measure the current on the xformer secondaries in a dead short (essentially connecting the two leads of the secondaries through the multimeter), you should get the max current your tranny is rated for (someone correct me if I'm wrong here).
Cheers,
DDC
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MonkeyBoi

No load would mean nothing connected to the output of the secondaries? I do have a Fluke that can read AC, DC and Amperage

Jack1962

Check out tubesandmore.com , I think they have 1  relatively cheap(as far as iron goes), if that's to high priced , what I used to do back when I started messing around with these things(we won't talk about how long ago that was) find a 20Watt home stereo at a garage sale or thift store , and gut the Iron out of it.

                                                                Rock On

darwindeathcat

Quote from: MonkeyBoi on September 18, 2008, 05:10:32 PM
No load would mean nothing connected to the output of the secondaries? I do have a Fluke that can read AC, DC and Amperage
With no load means with nothing else connected to the output of the secondaries. Of course you have to connect your multimeter, or you ain't measuring squat!  ;D Also, it doesn't matter to what side you connect the negative or positive... either way will work and give you the total volts present. Remember, however, if your tranny is center tapped, and you connect between the CT and one end, then you'll only measure half the voltage of the total secondary winding, which is good for estimating what you'll get in terms of + and - DC rails after rectification (multiply the AC voltage by 1.4 to get approximate DC voltage)...
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MonkeyBoi

OK, with no load across the secondary legs it is 29.36VAC and to the center tap 14.66VAC on each one of the legs. Having a hard time locating a power transformer with those specs.

Enzo

Well, if you are looking for 29.36v transformers, you won't find them, but 30v center tapped transformers should be easier to find.

Remember that if a transformer is rated at some output voltage - 29.36 or 30 or whatever - it will ONLY produce that voltage when the primary voltage is ALSO at the specified level.  In other words if the part is specified as 30v out with 120 on the primary, and your local mains are sitting at 124VAC, then your output will be 31v.  SO don't sweat those partial volts.

If you are confused by VA ratings, you can think of VAs as Watts, it is close enough for this discussion.

yustech

good info about the VA thing,now I know what it is.

Thanks a lot.
:) :)
http://yustech.blogspot.com/
That Tech & Tutor from greater KD

MonkeyBoi

I wanted to thank everyone for all of their help and the information was also very valuable. I decided to just replace the filter caps and clean up all the places where they leaked and just reused the original transformer. I also decided to socket the op amp to try a few different ones as well. The amp has never sounded better, no more crackling pots or input jacks and all is well with the world.

I am contemplating not selling it now or maybe modding it a bit.

Thanks again all!!