JCM 900 100 Watt Power Trans Specs..

DaveMcLain

Member
2007-10-03 1:07 am
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.
 

DaveMcLain

Member
2007-10-03 1:07 am
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.
 
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
 
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
 

DaveMcLain

Member
2007-10-03 1:07 am
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?