Studio Monitor Amp


Time Bandit Audio
September 2010

This amp was built for Shawn Phillips, a very talented musician, for use in his recording studio.  The circuit
for this amp was designed by Pete Millett, who also furnished the big red circuit board and a few other components.

Shawn has been using a vintage H. H. Scott integrated tube amp as his monitor amp and he knew I was a tube amp guy, so that was a
common interest for us.  I met Shawn through a mutual friend over 20 years ago.  Shawn heard my Carmen 71A amp and liked the
sound, and I offered to build an amp for him.  He specified that it have a bit more output than my 71A amp :-), and he wanted
no tone controls and dual volume controls for balance.  I looked around the web and couldn't find what I was looking for and then I
heard Pete's version of this amp.  It's a really accurate-sounding amp, being all-pentode, and
that was exactly what Shawn was looking for.  Listen to some of his music and you'll understand what I mean.
You can Pete's amp at his website, look for the "Engineer's Amp" link.  Pete has a cool design philosphy.  He likes
to go against the mainstream and use "orphan" tubes that work well for unintended applications.  He designed this amp
to use inexpensive television tubes which were made by the zillions and are basically unwanted today.  His novel design
uses 6JN6 output pentodes with duodecar bases, making the only really odd part in the amp the tube sockets.  I bought mine
from Pacific TV in Canada.  The input and phase-splitting duties are handled by a pair of 6CB6 seven-pin pentodes.  The
remainder of the components were sourced from Mouser (except the transformers from Edcor).

Here's a shot of the stuffed circuit board.  (See if you can spot the components I inadvertently omitted.)
I have always used point-to-point wiring in my amp, apart from an occasional module for heater power or such and
I found stuffing a big board like this one a little like paint-by-numbers.  It's really easy to do and the nice thing is that
it's a tried and true design and pretty much guaranteed to work (assuming you don't dumb out and omit a part or two).

Here's the top side of the circuit board with the tube sockets installed.  They go on one side of the board and all the other components
hang down on the other side.  The two rectangular holes are for mounting a couple of regulators to the chassis to act as a heat sink.

This is a shot of the chassis after I did the machining.  I had the chassis bent from 1/8" aluminum at a local sheet metal fabricator
and did the chassis layout and milling myself using hand tools and a 1/2" drill with Unibits.  The aluminum is easy to machine.

Here's a shot of the bottom with the ventilation holes cut.

I had the sheet metal shop weld some tabs to the inside of the chassis so that I could drill and tap them for screws.  The end tabs
are drilled through, while the tabs supporting the bottom are drilled and tapped for machine screws.

Here's a close-up of some of the tabs.  This approach works very well.

Here's a shot of the mahagony side panels I used.  They're cut from 3/4 solid wood and finished with hand-rubbed
lacquer from spray cans (available at Home Depot, Lowes, etc.).  The technique for getting what I refer to as a
"piano" finish is fairly easy.  I sand the wood with 320 grit paper and shoot on a couple of light coats of lacquer.  I then
sand that finish completely flat with 400 grit paper on a wood block.  Next come several more coats, each one sanded flat.
By "flat" I mean that I sand until there is no shine left and all of the "orange peel" is removed from the finish, completely matte.
The final coat of lacquer is then sanded with successively finer grits of paper, going from 600 grit to 1000, 1500, 2000, 2500
and finally to 3000 grit paper.  I get these fine grits from an auto paint supplier near my home.  After sanding, I then
polish the finish with auto polishing compound
(the white stuff) and I finish with a good coat of carnuba wax.
Be sure to let the lacquer completely dry between sandings.

Here's a shot of the chassis after I got it back from the powder coat shop.  This is a really durable finish that you can get in
a huge variety of colors.  Seems like everybody likes the red amps I build more than any others.

The transformers are made in New Mexico by Edcor and are good quality iron.  A really neat thing they do, is when you order
a custom transformer, as Pete did for the power transformer, it becomes a stock item that you can order from their website.
They even have a special price for the "Engineer's Amp" package which includes two output transformers as well.

Here you can see how I attached the end panels using 1/2" sheet-metal screws.

Here's a shot with the circuit board mounted.  It sits on 1/2" high nylon spacers, which is perfect for the output
tube sockets to fit snug against the bottom of the chassis.  I used stainless steel countersunk screws for hardware mounting.

I built the little terminal strip to be mounted on transfomer mounting screws.  I needed a place to make the AC power connections and
to tie the unused taps on the output transformers.  A friend sent me a big bag of the porcelain insulated terminals when he found them at a hamfest.
  They were threaded into tapped holes in the aluminum channel.

Here's a shot with the major components mounted.  You have to be really accurate marking the holes for the
tube sockets and chassis mounting screws.  I drew everything out on graph paper and punched the centers
of the holes through the paper.  I then started with a 1/16" drill bit at that point and graduated up to what you see.

Here's the inside with the transformers mounted and the wires going everywhere.

Here's a shot looking down.  The heat sink you see in the center of the chassis is mounted above the two regulators which
are bolted to the chassis with conductive grease. Probably not necessary with the weight of the chassis, but it
can't hurt and I think it looks nice.

Here's some feedback from Shawn to a mutual friend:
The new amp is awesome, and please thank Raymond for me. Kicks the shit out of my JBL 4331's.

I liked this amp so much that I'm in the process of building two more similar amps, one for me and one for a friend.

If you have any questions or comments about this amp I welcome email from other hobbyists.

© 2010 Raymond Koonce, Time Bandit Audio, all rights reserved.