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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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6th Jul 2020, 10:52 am | #1 |
Diode
Join Date: Sep 2018
Location: Cardiff, South Gamorgan, Wales, UK.
Posts: 6
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Marshall Amp Transformer
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. |
6th Jul 2020, 11:16 am | #2 |
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 1,465
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Re: Marshall Amp Transformer
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. |
6th Jul 2020, 11:50 am | #3 |
Pentode
Join Date: Oct 2018
Location: Coventry, West Midlands, UK.
Posts: 220
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Re: Marshall Amp Transformer
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. |
6th Jul 2020, 12:05 pm | #4 |
Banned
Join Date: Nov 2014
Location: Derry, Northern Ireland, UK.
Posts: 167
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Re: Marshall Amp Transformer
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. Last edited by tritone; 6th Jul 2020 at 12:10 pm. |
6th Jul 2020, 12:36 pm | #5 | |
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 1,465
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Re: Marshall Amp Transformer
Quote:
Colin. |
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6th Jul 2020, 1:02 pm | #6 |
Diode
Join Date: Sep 2018
Location: Cardiff, South Gamorgan, Wales, UK.
Posts: 6
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Re: Marshall Amp Transformer
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 |
6th Jul 2020, 1:51 pm | #7 |
Pentode
Join Date: Oct 2018
Location: Coventry, West Midlands, UK.
Posts: 220
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Re: Marshall Amp Transformer
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!
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6th Jul 2020, 2:00 pm | #8 |
Pentode
Join Date: Oct 2018
Location: Coventry, West Midlands, UK.
Posts: 220
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Re: Marshall Amp Transformer
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6th Jul 2020, 3:15 pm | #9 |
Banned
Join Date: Nov 2014
Location: Derry, Northern Ireland, UK.
Posts: 167
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Re: Marshall Amp Transformer
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.
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6th Jul 2020, 5:38 pm | #10 |
Diode
Join Date: Sep 2018
Location: Cardiff, South Gamorgan, Wales, UK.
Posts: 6
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Re: Marshall Amp Transformer
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 |
6th Jul 2020, 10:08 pm | #11 |
Dekatron
Join Date: Jun 2009
Location: Staffordshire Moorlands, UK.
Posts: 5,274
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Re: Marshall Amp Transformer
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
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6th Jul 2020, 10:36 pm | #12 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,400
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Re: Marshall Amp Transformer
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! |
7th Jul 2020, 6:32 am | #13 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,669
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Re: Marshall Amp Transformer
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.
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7th Jul 2020, 5:41 pm | #14 |
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 1,465
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Re: Marshall Amp Transformer
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. |
10th Jul 2020, 5:14 pm | #15 |
Dekatron
Join Date: Jan 2003
Location: Bradford on Avon, Wiltshire, UK.
Posts: 3,310
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Re: Marshall Amp Transformer
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.
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