JCM 900 100 Watt Power Trans Specs.. - diyAudio
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Old 9th April 2008, 01:31 PM   #1
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Default JCM 900 100 Watt Power Trans Specs..

I'm repairing an amp for a customer right now and it has a bad power transformer. His amp is the Marshall 9005 power amp and I can't find a realistically priced power transformer anywhere. I'm thinking of trying a power transformer from a JCM 900 series 100 watt top because it looks very similar from what I've seen listed by Hammond online but they don't list the dimensions. Does anybody have one that they can measure so I can see if the bolt pattern happens to be the same. Other than that I can see no other reason why it wouldn't work.
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Old 9th April 2008, 02:11 PM   #2
nhuwar is offline nhuwar  United States
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Are the tube complements the same in both amps? If they are, at least the output tube and it's fits then go for it. But I haven't had any luck with the dimensions sorry.


Nick
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Old 9th April 2008, 04:12 PM   #3
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The amp I'm repairing has four EL34 output tubes and four 12AX7's as drivers and a line stage so yes, it's very close to the 100 watt top. I just need to know the dimensions of the transformer for mounting purposes. It's the laydown style with four bolts in a square pattern. If worse comes to worse I could always make an adapter plate and fit a different style tranny but I'd rather use something that's closer.
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Old 9th April 2008, 05:30 PM   #4
cerrem is offline cerrem  United States
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The JCM900 power transformer I believe is a EI-125 lamination...
With these power requirements, it's either going to be EI-125 or EI-150.... The OEM Marshall replacement transformer I get for $110..
I don't care for recent Hammond power transformers...every single one I have ever bought had noise and buzzing issues...the coils were simply not well vacuum varnished...

cerrem
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Old 9th April 2008, 05:46 PM   #5
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This tranny is a much larger dimension with a bolt pattern of 3 15/16 X 3 1/8.

Thanks for the info now that I know it won't fit I'll just use a more universal transformer and make a few parts so it'll work correctly.
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Old 9th April 2008, 06:46 PM   #6
cerrem is offline cerrem  United States
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Sounds like you have a EI-150 lamination.....
Here is the quick and dirty method to get this amp up and running as fast/cheap as possible....
Get the 100W Marshall power transformer replacement for JCM800...aftermarket they are like $65.....
Pull the vertical bell covers off and then mount it as a lay-down then re-use old top cover...if need be you may need to drill a few holes, not a big deal...
Then squeeze the wires through the lamination window so they all come out on one side, the bottom.... Now you will need to re-wire the rectifier and first filter cap that comes after the bridge rectifier.. add another filter cap in series with the 50uF x2 thats in there now...get a 100uF blue axial ATOM sprague and put in series with the cap there now..you take the YELLOW wire, Center-Tap from the H.V. tranny winding and stick it betwen these filter caps that are in series to make a voltage doubler.... The rest is just cake...
$65 + cap+ an hour = DONE ......

Chris
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Old 9th April 2008, 07:09 PM   #7
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I believe that you've read exactly what I had in mind. I can't see any reason why that idea won't work fine.
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Old 19th April 2008, 03:41 AM   #8
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This evening I installed my JCM 800 replacement transformer and I have the amp up and running. I have a question about using the voltage doubler circuit vs not using it in this application. Right now I have the transformer hooked up with the center tap unused. The B+ is about 440 volts at the output tube with them biased. The amp produces a nice waveform all the way up to clipping. Question is... According to my book there are some JCM800 100 watt heads that don't use the voltage doubler yet the older Super leads do. They seem to use the same transformer, is there something I'm missing here?
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