Marshall Amp Transformer

[izoomer]

I have a large power transformer from an old Marshall amplifier I think (model unknown), it has no markings. I want to test it. It has the normal 2 wire primary, but has 3 wires on the secondary. 2 wires are the same size and are both orange, the other is thicker and is blue. I think this is a 40-0-40, but how do I test this to see if it is a 40-0-40?

Cheers, Izoomer.

[ColinTheAmpMan1]

A photo would be informative. Going from your description, though, it sounds as though it is an output transformer. I can't see a 40-0-40 mains transformer being used in a Marshall amplifier.
Also, the mains primary should have more that two wires to cater for different mains voltages.
Colin.

[Paul JD]

Marshall JMP solid state amps used a dual rail +40v/-40v supply so could easily be a power transformer from one of those.

First step would be to apply a low voltage AC to the primary and measure the secondary, gradually increase the voltage until you get a sensible reading that you can calculate the voltage ratio from.

[tritone]

An old Marshall valve amp power transformer has several taps on the primary side to select for the different world wide mains voltages so it would have about 5 taps on the primary side.

Also, the power transformer output taps are very high, like up to around about 300vac - 0 – 300vac or there about, plus a bias voltage tap at about 55vac or there about-ish, and also the two 3.15vac taps on the secondary side as well.

Drake were the early manufacture Marshall used then they changed over to Dagnell, There should be an ink I.D. stamp code or else an I.D. sticker that may have fallen off over the years.

Yes, some pics please.

[ColinTheAmpMan1]

Originally Posted by Paul JD

Marshall JMP solid state amps used a dual rail +40v/-40v supply so could easily be a power transformer from one of those.

That's a fair comment that I didn't think of, but I was perhaps persuaded that this was going to be from the valve era of Marshall by the description of "an old Marshall amplifier".
Colin.

[izoomer]

A number ink stamped on the top looks like 1708, there are a pair of brown and black primaries, ignore the white wires. With mains applied to the primary side I get 63.2 volts across the two orange and 31.4 volts across the blue and either orange, 3 ohms across the either orange and blue,

Cheers
Izoomer

[Paul JD]

Originally Posted by ColinTheAmpMan1

I was perhaps persuaded that this was going to be from the valve era of Marshall by the description of "an old Marshall amplifier".
Colin.

Marshall have been producing valve amplifiers since the early 60's and solid state amplifiers since the early 70's (and are still producing both today). Quite a few of them could be described as old!

[Paul JD]

Originally Posted by tritone

plus a bias voltage tap at about 55vac or there about-ish

I can't think of any vintage era Marshall that has a separate bias tap on the transformer. They all take the bias supply from the HT.

[tritone]

Originally Posted by Paul JD

I can't think of any vintage era Marshall that has a separate bias tap on the transformer. They all take the bias supply from the HT.

I do not wish to get into a debate about this, But yes, after about the early mid 60's they did, and do have a seperate dedicated bias voltage tap on the sec side.

[izoomer]

so, would this be a centre tap/0v type transformer? Can I test this to see if it is a 40-0-40, or is the test I did show that it is a centre tap/0v type transformer?
cheers

[McMurdo]

I think its fair to say if you have secondary leads of a single of one colour and a pair of another, the pair are going to be the main winding and the single the centre tap (0V if you like).
That's because on the production line they're not going to care which way round the main windings go but the centre tap is crucial.

No point looking for a bias winding if its solid state! (or a heater winding for that matter!). I doubt 40V into a pair of EL34's would do much in the way of rocking

[turretslug]

Plus the single wire of a different colour being thicker is a good indicator of two ends of windings slid into an overall sleeve making a centre-tap. I think the OP pretty much had it sussed in the first post, with rectified and smoothed 31.4VAC likely to make around 40VDC nominal with a bit of class AB-type circuit quiescent drain, plus a bit for typical usage (maybe not quite to 11).

It's been a long time since I delved into any solid-state Marshall amps but they definitely did make them!

[Diabolical Artificer]

From the pic there looks to be more wires than a two wire pri and three wire sec, also a 4 digit # might indicate a Partridge or Parmeko tfmr, there looks to be 06 (or 8) 75 stamped on it. It doesn't look like a Partridge tfmr though.

A.

[ColinTheAmpMan1]

I just took a look at my collection of Marshall circuit diagrams and it would seem that a fair number of mains transformers for the solid-state amps had two primary windings which were wired in series for UK 240VAC mains and in parallel for US 120VAC mains. Using just one of these primaries for 240VAC would probably not be a very good idea. Come to think of it, wiring them in antiphase wouldn't be too great, either.

Colin.

PS: I tend to be rather dismissive of solid-state guitar amplifiers, except perhaps the Roland Jazz Chorus.

[Tim]

Not sure what facilities you have, but you could apply 24volts AC to what you think is the primary and see what you get at your secondaries. Output would be a factor of ten less obviously.
Alternatively apply a 50Hz signal from a signal generator, and have a look at the output on a ‘ scope.
If there is a number on the transformer?If you google it there are probably lots of sites that will help you identify your windings.