Carmen 71A

Carmen 71A
by
Time Bandit Audio

April 2009

And now for something completely different...

This is a push-pull amplifier using 71A triodes. It produces a whopping 2 watts per channel. Believe it or not, this is plenty of power
for my Klipsch Cornwalls. The driver tubes are 56s which are transformer coupled to the 71A grids. The input transformers, which are the
green cylinders on the chassis, were salvaged from a Newcomb PA amplifier. In that service they were used as microphone transformers.

The chassis is made of 2 pound copper which I sanded and polished by hand, then coated with several coats of lacquer to prevent
oxidation. It's a lot of work! I created a silk screen to label the amp and the rear panel which has several connections.

The power supply capacitors are the big aluminum cans you see above. They are 440VAC rated motor run capacitors, so they should
last indefinitely in this application. From left to right above you have the right speaker terminals, input jacks (I only needed one
set for this amp, since it's to be used to stream computer audio), 0.25A fast blow fuse, 1.5A slow blow fuse, standby switch,
IEC power inlet and the left speaker terminals.

The amp has a total of sixteen transformers! The output transformers were salvaged from a Stromberg-Carlson 6BQ5 amp. I had
a NOS Stancor power transformer which I modified to use horizontal mounting to match the output iron. The phase-splitting is done
using a Hammond 124B with a 10K:90K impedance and a 49% nickel core. Surprisingly good for an inexpensive part. The remaining
transformers are all Hammond parts which I have found to be quite good at their price point. Each output tube has its own filament
transformer, a Hammond 166F5. The center tap of each of these transformers is connected to a separate bias network, which consists
of two 600 ohm power resistors in series bypassed with 100uF. I did this so that I could try something new to me, but really old:

The Blumlein Garter Circuit

This image courtesy of John Broskie, www.tubecad.com

This circuit was created by Alan Blumlein in the 1930s and was brought to my attention by Al "Happy Ears" Marcy of Joe List fame.
Thanks Al!
I built this amp on a breadboard to test my circuit design and when I first built it using a conventional bias network, I was sorely
disappointed with the bass response, which was quite thin-sounding, not good at all. I happened to read a post on the Joe List from
Al touting this circuit and when I tried it, I was astounded at the change it made. The bass became much stronger and tighter.

The tube line-up is, back row: 71A, 71A, 82, 71A, 71A; front row: 56, 0C3, 0C3, 56.

Since this amp was built for my office, I wanted to use remote control for the volume control. Several years ago I bought a
kit from a fellow named Mikkel Simonson of Denmark and I had not used it. Get yours here: http://electronics.dantimax.dk/
This was the perfect opportunity, since it was designed to use a pair of relays to provide a time delay for a mercury
vapor rectifier (the 82) and to run a motorized pot for the volume control. I provided it with a separate
6.3V 1A transformer and it works perfectly.

The 0C3 tubes are gas regulator tubes which regulate at 105V and I used two of them in series to provide 210VDC for the
output tubes. The output tubes are biased to run at 20mA per each with about 180V across the tube.

Here are some additional photos:

This is the completed chassis after the silk screen was applied. The wood end panels are made of mahogany which I coated with
clear lacquer and polished to a high gloss.

This is a shot of the inside of the chassis. I had a sheet metal shop bend the copper sheet for me and I had them make some
3/8" x 3/8" copper angles which I soldered to the chassis for stiffness and to provide a way to fasten the end panels.
I used plumbing solder and flux with a MAPP gas torch after sanding the contact areas to provide good contact.
Soldering these angles seemed simple, but it proved to be a little harder than I thought. I clamped the angle to the
chassis at the desired spot, using shims to avoid marring the copper. When I heated the copper sheet, it expanded in
length and actually buckled between the clamps. It fell back into place when it was cool. I ended up soldering close
to the clamps and moving them along to maintain tight contact between the angles and the chassis.
I also used the angles to provide a mounting system for the bottom cover. I soldered brass nuts to the top
side of the angles so that I can use brass machine screws to fasten the bottom cover. I soldered six #10-32 brass
flat-head machine screws to the chassis and used a pair of nuts on each one to attach the aluminum
subchassis. This avoids surface mounted hardware and gives a more finished appearance.

This is the subchassis on which I built the main circuitry of the amp. The large rectangular openings are for access to the transformer
windings and the oval holes in the rear are for the filter capacitors. The black sockets on the lower corners are nine-pin "octal" type
sockets for the input transformers. I could have removed the ones installed in the PA amplifier, but I found some on-line. They are
used in electronic organ connecting cables. The remainder of the sockets are porcelain types. You can see how I mounted the
remote control circuit board and relays using perf-board and nylon standoffs. I make the standoffs using CPVC tubing and a
tubing cutter to make them any length I need.

Here's the subchassis installed in the chassis with the transformer leads sticking out everywhere. They were a mess to tie off.

Here's a shot with the amp assembled and ready to wire. The wiring is by far the easiest part of building this amp. I find the chassis
work to be quite a challenge. The copper looks very nice and is soft and easy to drill and punch, but its softness makes it subject to
scratch with the slightest touch of a steel tool. A scratch requires much sanding with successively finer grits of sandpaper starting with
400 grit and progressing through 3000 grit with a final polish of automotive polishing compound. It must be coated with lacquer
immediately to prevent tarnish.

Here's a shot of the inside ready to go.

Here's a night shot, about a 5 second exposure. Looks good :-)

The amp is very quiet running with no noticable hum in my Cornwalls, which are quite sensitive speakers. I suppose it's the
push-pull circuit that causes hum cancellation, even with AC on the 71A filaments. All told, I'm really pleased with this amp
and I look forward to getting it installed in my office.

Here's a link to the schematic (PDF).

If anyone would like to see more detailed photos or if you have any questions or comments, please
email me at rkoonce@dsityler.com. I always enjoy email from other hobbyists.