March 13th 2011: Getting Started

This blog is going to chart my attempt at building a replica 1960's Marshall JTM45 amplifier. I’ve always wanted one, and was considering buying the Marshall Reissue JTM45 until I stumbled upon George Metropoulos’s MetroAmp website. George builds replica Marshall Plexi amps to order, from JTM45s to 100 watt Superleads. He also, until recently, offered kit versions. After hours of reading the Metro Forum, I became hooked, and planned to buy a kit. Unfortunately the kits were discontinued just as I was about to order one. There’s a few other kit manufacturers around, but none seemed to be as good, or as accurate as the Metros'. Luckily, the instructions and bill of parts for the Metro kits are available to download from the site, and after reading them, I decided to see if I could gather all the parts to build one from scratch.

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So, after about 2 months searching the net, I’ve managed to get hold of almost all I need. The amp is going to be built on a chassis I bought from steves025ukon eBay. He offers the chassis either pre-drilled or blank, I opted for the blank version. There’s still a few more bits to get, but I have enough to get started. The first step will be drilling the chassis...

April 14th 2011: Chassis Drilling

The first job was to mark out and drill the rear of the chassis.  I wanted to incorporate a voltage selector switch into the build, but this left a problem of finding a suitable rear panel. I'd toyed with the idea of getting a JTM50 panel from Ceriatone, but I wanted to keep the rear white. Another option was to drill the existing JTM45 panel I'd bought from Ceriatone, but there wasn't quite enough room to fit the switch in.

  

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In the end I decided to design my own panel using Photoshop to create a template. An  engraving company made it for me. It's been reverse engraved in London Tube font, to keep it looking authentic, and I'm petty pleased with the way it's turned out.

After drilling, I had a test fit, and everything lined up perfectly. The fiddly part was cutting the rectangle hole for the power socket.

Later, I'll remove the voltage markings from the switch as these don't follow what the transformer tap values are. I had the correct values engraved on the panel instead.

The next step will be drilling the front panel...

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April 17th 2011: Chassis Drilling Part 2

The next job was to mark and drill the front of the chassis. This was more straight forward than the rear as there's no square holes to cut. I bought Alpha pots with metric mounting threads by mistake, so the six holes were drilled at 8mm diameter instead of the usual 3/8". I also drilled holes for the pot retaining pins.

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Again, as with the rear, everything lined up when test fitted. The front panel in the picture is from  CeriaTone, although I've since ordered another one from an eBayer in the States that is closer to the original.



April 23rd 2011: Chassis Drilling Part 3

I've finished the final drilling and cutting on the top of the chassis and it's now ready for building. One more job to do first will be making a bracket for the power transformer. I'm also going to have a stab at making some sort of cradle for the chassis to hold it during the build.

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April 25th 2011: Transformer Bracket

I've tried unsuccessfully to get hold of a transformer bracket. In the end I had a go at making one. I mitred 4 pieces of mild steel angle and welded them together to form the frame.

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Next, I cut out a piece of aluminium sheet to form the base of the bracket. After shaping and drilling, I fixed it to the steel frame using epoxy resin.

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Finally, after a couple of coats of paint it's ready to go into the chassis.

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August 20th 2011: Initial Assembly

Not had much time to get on with the build during the summer, but I've finally made a start on bolting things in place. Before that, I made a cradle  to hold the amp during the build, just some scrap bits of chip board and some L brackets, not pretty but it does the job.

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I've opted for Magnetic Components Classic Tone transformers and choke. The power transformer's main secondaries are rated at 690 volts. The valve bases are Beltons, and everything is held in place with brass round head machine screws and nuts in either 2, 4 or 6BA... These took some hunting for, eventually finding them on eBay.

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I've decided to incorporate parts of the star grounding scheme as described on the Metro Forum to avoid having to solder a bus wire down the back of the pots, the additional 4 solder lugs can be seen in the picture below. The circuit board standoffs were made from 4mm aluminium tube from a local model shop.

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I had a problem with the paint on the P.T. bracket. It never set off properly, and was still soft after 2 weeks. In the end, I stripped it and resprayed with Plastikote silver paint.

The fuse holder (and circuit board) was made by Brian Haberman in the States.

Now comes the fun part... wiring...