Faceplate designs sent to BNP Lasers

I finally got the amp plate designs off to BNP lasers. I have to give a big thanks to Jeanne Kangas for walking me through a lot of steps and providing examples needed to get to these finished. I designed them in Adobe Illustrator CS5 which was a big learning curve for me. Another big thanks to Tylor over at The Rock File for his Illustrator instructional help and doing the number dials for me. These will be laser engraved on satin gold AlumaMark with black lettering. Here is a pic of the final proof.


Merren Transformers have landed…

I wanted to use the best transformers I could locate. I was very happy with my Marstran transformers that I used in my JTM45, but I wanted something different and unique for this build. I have heard of the legendary hand-wound transformers created by Chris Merren. After many emails and and a lot of time passing, I have finally got my greedy hands on his 1202-118 power transformer and 784-139 output transformer. I can’t wait to get these babies wired up! Chris is an awesome guy. He talked to me on the phone and made sure I was using bakelite sockets and carbon comp resistors in the right spots. He even went out of his way to send me links to where I could find the sockets on eBay. Awesome customer service.

Beautiful Iron

Beautiful Iron

Neutrik powerCON Connector

I haven’t had much tome to work on the amp but I am getting back to it! Instead of using a I.E.C. power cable that can be stepped on and yanked out, I am employing a Neutrik powerCON series connector. This is a locking connector that can take heavy gauge cable. I used a step bit to drill out the hole to accept the fatter body of the Neutrik.


Bias points and 10-turn potentiometer

I decided when it comes time to bias this amp I don’t want to have to pull the chassis out. I remember seeing a Komet amp designed by the late great amp legend Ken Fischer that had a crazy looking potentiometer knob that looked like a combination lock. With a little research I found out knob is connected to a 10-turn potentiometer. A regular pot turns less than 360 degrees meaning that minute adjustments to bias are hard to make. With the 10-turn, the pot takes 10 rotations to travel through it’s resistance, making it a great pot for adjusting bias. The knob has a concentric dial, and with each turn, the outer ring clicks up or down by a number. It also has a lock on it, so once adjusted it won’t move.

I also drilled holes for banana jack bias points as a lot of companies are doing now. This set up will make biasing the power tubes a cinch.

***Note – I ended up not using this fancy expensive pot because it just stuck too far out the back when I mounted the amp in a standard head cabinet. I ended up using a PEC locking pot that I got from DigiKey***

Mounting the board

Since I am using my own board design I will have to drill custom holes in the chassis for mounting. I am using stainless steel stand offs as opposed to the standard steel so that nothing rusts. I use Loctite thread locker to keep any screws or bolts from backing out. I also have the power tube sockets bolted in using Nyloc nuts, again so nothing comes loose. It’s got to be heavy duty.

Yet more design changes

I decided to move the two 68k grid resistors I had mounted on the board right to the pins of the tube socket. This will help reduce unwanted noise by keeping a longer portion of the cable shielded according to a thread on the Metroamp site. I chopped off two rows of the perf board and now it will fit more easily into the chassis.

New circuit board drawing after removing the grid resistors.

New circuit board drawing after removing the grid resistors.

Chopped off two rows

Chopped off two rows


Really Cool Parts

Part of the fun for me when building an amp is hunting down cool parts. I try to find as many vintage parts as I can, but then I combine these with some modern items for dependability and functionality. I have scored some holy grail parts already but I still need to fill in some gaps. There is a certain mystique associated with vintage parts. Amp builders call it “mojo.” While the older parts often pale in comparison spec-wise and noise-wise to modern parts, builders use them to come as close as they can to the original tone and dare I say soul of the original Plexi and JTM45 amplifiers. The parts are expensive but my idea is to build this amp once and it will be the last amp I ever own. Here are some of the parts I have acquired:

1970s Mullard “Mustard” Capacitors – These are a must when seeking the tone, feel and look of a vintage Marshall amp. I found these N.O.S. (New Old Stock) on eBay from a seller in Greece. I still need to find a .68uF 160 volt which is pretty tough to do. They look like mustard from a squeeze bottle, hence the name.

Radiospares 560pF Capacitors – I had one of these already that I bought a long time ago, but I need 2 for the build. One is for the bright channel mix capacitor and the other for treble tone stack. I found one from a seller in France on the Metroamp Forum. paid a bunch. Ouch.

Carling Switches with Plexi-style Chrome Toggle Rings – These were both readily available on the Valvestorm.com site, a very cool place to find Marshall parts.

Piher Resistors – These are the same brand of resistors used in Marshall JMP and JCM800 amps of the late 70’s and early 80’s. I sourced them from guys in France and Spain. The 1/2 watt ones I found are from the 60s and 70s while the 2 watters are 90s vintage. I am still missing 2K7 and 4K7 values in 1/2 watt. The hunt continues…

Lemco Dogbone 47pF – This will be the phase inverter capacitor. Valvestorm.com

BC 10uF 160 volt – I found some 400 volt versions on eBay.uk that I was going to use but then I stumbled on the proper 160 volt versions at Valvestorm. These will be in the bias capacitor positions.

Belling Lee Fuse Holders – Very cool N.O.S. fuse holders I found on eBay from a guy in Pulborough, England.

ARS Dual Can Capacitors – These beauties will serve as the filter capacitors. When filter caps sit for a while they need to be reformed. These are sold reformed from Valvestorm.com.

1K 7watt Welwyn W22 Wirewound Resistors – These will live in the screen resistor position on each power tube socket. Very cool classic resistors from Valvestorm.

Metroamp 50 Watt Plexi chassis – This build is being done on a standard 50 watt Plexi chassis. The original JCM800 amplifiers had a much longer chassis. I want to keep the build compact so I am going with a the smaller chassis. On my previous JCM800 amps I used this same sized chassis as well. Once again – Valvestorm.com.

Staking Turrets

I took my perfect 9 hole high board and sliced a thin strip off to modify it to an 8 hole high which is what I had originally. On the first board I attempted, the holes were a little off, so modified this one. I carefully sanded the edge and the board was ready for turrets. I dug up my turret tools that I bought a few years ago. I bought the staking tool from Watts Tube Audio and the anvil from Hoffman Amps. I stake turrets my own way. I found the drill press method to not seat the turrets firmly enough so I use drilling vise to clamp the anvil. I set the height so that when doing turrets close to each other, the adjacent turret will not get bent.

To stake, I insert a turret into the hole I drilled in the circuit board. Then I flip the board over and place the turret into the anvil. It has a hole in it for the turret to sit in. I use a cardboard box to support the circuit board. I give the turret 4 strong taps on the back and the turret is seated.

One problem that came up was that I had some double turrets left from an old build. They use a #33 drill bit which is .113.” I knew I was going to run out of these turrets so I ordered more from Watts Tube Audio. Well, it seems the size has changed! The new double turrets they sell require a #32 bit which is .116.” You wouldn’t think 3 thousands of an inch would be that much different, but sure enough the new turrets would not press into the holes. I used a #32 bit and drilled the holes out for the new turrets. The good news is that they look almost exactly the same (the hole in the top is slightly larger – won’t matter when soldered) and I don’t have to re-stake the work I have already done.

So the lesson of the day is buy all of your turrets at the same time!