Morrison Micro build begins

Vaughn

AK Subscriber
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I've started building an amplifier in a very backwards fashion. I designed the chassis first (assuming it would be a small amp) and have waited for a suitable schematic to present itself to me:)

Redboy reminded me of a design I had seen years ago as a subscriber to Sound Practices, the J.C. Morrison "Micro". The Micro is a SE 2A3 amp that uses no coupling capacitor between the driver stage and output tube. Just two tubes and a transformer in the signal path! I fell in love with this years ago but had forgotten about it. I knew when I saw it that it was the amplifier to go in this chassis.

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I'm building this amp a little differently than any other I have in the past. Typically builds have consisted of everything being mounted to a metal plate. This works well because it is easy. I'm not sure that, mechanically, it is a good idea to have everything on the same plane. For instance, power transformers have a tendency to vibrate and tubes are microphonic:scratch2:
I decided to mount the transformers on a very heavy, dead plate made from 1.25" Paperstone. This material is similar to Corian and other solid surface countertop materials in that it is extremely dense and relatively inert. As you might have guessed, Paperstone is made from highly compressed paper with an epoxy binder. It is extremely close to Phenolic, in my experience.

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The top is a a roasting pan of .125" thick aluminum. In this instance it is acting as more of a cowl to cover the electronics and protect curious fingers from sudden death. It attaches at two points (the center hole at each end where the original handles were riveted) to the plinth via threaded fasteners in paperstone blocks.

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Machined brass banana jacks rescued from some old stacking plugs I had make up the speakers connections. Other connections will be flush mounted as well, I don't like anything to stick out and get broken off.

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The sockets for the 6SL7's and 2A3's will be mounted to an aluminum plate mounted to standoffs machined from the same material. I made a dress plate as well that will go on top of the aluminum and cover the fasteners. This shows the approximate position on the chassis, although it will be beneath the pan.

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So there you go. I will be looking for recommendations on iron or any other part of this you have an opinion on! In an amplifier this simple I believe, to quote Depeche Mode, "Everything counts in large amounts":D
 
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It will be partially visible. In the cowl a 2.5" hole will be centered over each of the sockets, which will leave the vent holes revealed.
 
I made a pair of these amps when the article came out but being the cheap guy that I am, I used Hammond iron. Even with the downgrade they're really good sounding amps. Mine were in daily use for at least 5 years with the original tubes and no issues. A very reliable design.
 
Very cool. I love those flush mount banana jacks, they look killer.

Thanks, I was just happy I didn't have to buy anything:D

I made a pair of these amps when the article came out but being the cheap guy that I am, I used Hammond iron. Even with the downgrade they're really good sounding amps. Mine were in daily use for at least 5 years with the original tubes and no issues. A very reliable design.

Thanks for the word, I haven't been able to find a lot of reviews of this amp. So you built a set of monoblocks or two stereo amps?
 
I built a pair of monos. Originally I used them with Lowthers in Mauhorn IV cabs but switched to Altec 604-8g's. The amps drove both speakers nicely and were quiet enough as well.
 
I built a pair of monos. Originally I used them with Lowthers in Mauhorn IV cabs but switched to Altec 604-8g's. The amps drove both speakers nicely and were quiet enough as well.

Very good to know, I will be using mine with Mark Audio Alpair 12P's in a smallish room. I would love to hear the 2A3/604 combination some day!
 
Ordered sockets, caps and resistors today. Built standoffs and an aluminum sub plate that the sockets mount to. Still need to get in and clean up the inside edge of the holes in the cowl.

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I've got most parts on the way and am about to place an order with Edcor for the power transformer/choke/filament transformers. I think I'm going to go with Transcendar outputs.

Power transformer - https://www.edcorusa.com/p/736/xpwr134_120

XPWR134-120

Power transformer for a 120V, 60Hz. line to 750V (375-0-375) at 200mA center tapped, two 6.3V at 3A and 5V (2.5-0-2.5) at 3A center tapped.

Choke -

https://www.edcorusa.com/p/782/cxc125-10h-200ma

CXC125-10H-200mA

Filament transformers -

https://www.edcorusa.com/p/947/lvp2_5-3-120
 
Any reason I shouldn't use a 47uf poly cap on the filament transformers?:scratch2:
 
This is coming along nicely! I'm looking forward to your review of this circuit, Vaugn, because it's pretty obvious that you'll finish yours long before I finish mine... :)
 
Thanks Nate, I've definitely got a bee in my bonnet about it now:). Although I think it is going to be a good month until all parts are trickled in.

I started to realize that I was cutting it very close for being able to pack everything into the space available. I decided to knock together some quick models from card stock which describe the volume of each of the major components. This way I can "play house" a little bit and move the pieces where I think they might work best. I think I can get everything in but switching out the 47UF 250v cap for a poly model will definitely make it more difficult.

Maybe someone can chime in on whether or not it is acceptable to have the filament transformers for the 2A3's so close to the driver stage. Filament transformers are front and center laid on their side with the 47uf 250V cap (here shown the size of a Dayton poly of this value) on top. I would also need to work the 2.5K 20w resistor into that assembly. I'm also unsure about having the laminations oriented in this direction.

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This might be a better layout, with the filament transformers at the back near the PT and main capacitor bank near the tubes. Any opinions?

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The power transformer and choke are huge. I'm going to have to countersink both into the bottom plate in order for them to clear the cowl. I think I might be able to find a smaller PT, this one is 200ma and I only need 150ma. It also has a second 6.3V tap I don't need.
 
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forgot to mention the small cylinder tucked between the choke and power transformer is the 10uf 630v cap. This will be probably be mounted underneath the rectifier somewhere in this area:scratch2:
 
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