Time Bandit Audio 91A

built for Harald van Haaren
Eindhoven, Netherlands

April 2008



Here's my latest project, another 300B amp based on the WE91A theater amplifier from the 1930s.
This one uses James 6123HS output transformers, the same as I used in my silver amp and others.

  The power transformer is a Hammond unit with multiple primary windings so that I could build the amp for the 230VAC 50Hz 
power used in Europe.  The secondary is rated at
400-0-400 at 465mA.  It weighs in at about 18 pounds, pretty hefty.  I have used
these power transformers for years and they always perform well. The power supply is filtered by 50uF 440VAC motor run capacitors
paralleled with a 100/100uF at 500V JJ electrolytic. I polish the aluminum cases of the motor run caps for
appearance purposes.  The output tubes are Emission Labs 300Bs, very nice, well built tubes.  Get yours from
TubesUSA.com
. The rectifier is a NOS RCA 5Z3.  In the years I've been using them, I have had only one failure, an open filament.
The driver tubes are NOS 5693s, a special version of the 6SJ7 pentode.

The B+ on the first cap is about 400VDC and there's 380VDC on the plates of the

300Bs.   They bias at 65VDC on the cathodes, about 23 watts plate dissipation, very conservative for this tube.

Here's a few more shots for your viewing pleasure.

 

I wanted to try something different with this amp, well actually two things.  I wanted to minimize the number of exposed fasteners for appearance
purposes and I wanted to try DC heating for the 300B filaments because filament hum is very difficult to minimize with AC heating and hum pots.  For the DC
heating I used a pair of Tent Labs heater modules, which are current and voltage regulated for a 5V 1.2A load, perfect for the 300B.

 

This is the chassis as received from TubesUSA.com.  It's 1/8" thick aluminum folded on the front and back with welded end
panels and a recessed bottom panel with countersunk screws fitting into aluminum blocks welded to the chassis and tapped
for 10-32 flat head machine screws.  It takes a lot of sanding after the holes are cut to get the chassis ready for painting.

This is a shot of almost all of the components to be used in the construction of this amp.  I think the only thing missing from this shot is the
chassis and bottom cover and the miscellaneous hardware.

Because I wanted to minimize the number of exposed fasteners on this amp, I devised a way to mount the oval motor run
caps by welding an aluminum bar alonside the cutouts and tapping it for screws to clamp an angle section over the rims of the caps.
I also welded eight spacers, also tapped for screws, to mount aluminum sub-chassis panels with the amplifier and
power supply sections of the amp.

This chassis had a small defect on the left side which I filled with auto body filler and sanded smooth.  It's invisible under the paint.
All of the outer aluminum surfaces are sanded with 400 grit wet paper to remove all of the tool marks before priming.

I cut all of the holes on the chassis except for the oval ones which I have cut at a machine shop on a mill with a fly cutter.  Accuracy pays here.
 I use hole saws, high-speed drill bits and UniBits to cut the round holes.

Here's the chassis after priming with an etching primer to insure that the successive coats of undercoat, color coat and clear coat
adhere to the aluminum.

On the last few amps I have built, I have done the painting myself. I have discovered that I'm not a very good painter.
I can get the job done, but only with a *lot* of hand sanding, so I decided to have a car painter do the work for me.  Best decision
I ever made!  On the Hal Hudgins amp I spent $175 for primer, paint, sandpaper, etc. and the fellow that did this job suppliled
everything (except the etching primer) and did the job for $150.  That's a no-brainer if I ever saw one.



Here are the parts back from the painter.  He did a very nice job and it was done overnight.  I carefully removed the badges from the
James transformers so that I wouldn't have to mask them, and I glued them back into place after attaching the transformers to the chassis.


Here you can see how I clamped the motor run caps into place.  I was able to use an extension of the welded bars to mount the
heater transformer for the 5693s.


Here's a shot with the transformers mounted.  I mount the power transformer and all of the filament/heater transformers using
rubber grommets to not transmit any physical vibrations to the chassis.

Here's a shot of the amplifier sub-chassis with all of the parts mounted and wired.  This turned out to be a very good way to
build this amp, because I was able to access all of the components easily.  I attached long leads to the various points which
required a ground or a positive voltage so that I could attach them after mounting the sub-chassis.

I mounted the sockets on spacers to provide a small clearance between the pan-head scres and the chassis above.  The rectangular
cutout you see below the 300B sockets are for ventilation of the filament supply modules.  I also provided ventilation holes around the
tube socket openings in the chassis and air inlets on the bottom cover.  There's a pretty good convective flow with the heat from the 
300Bs keeping things moving.  I also wanted to provide some air to cool the sub-chassis with cathode resistors attached since they get
fairly hot when the amp is running.  I experienced no excessive heat after the amp had been operating for over four hours.

Here's a shot of the underside of the power supply sub-chassis.  It made a nice mounting spot for the 300B filament
transformers (6.3V 2A units with universal primary windings for 230V operation).

Here's the top side.

Here's a shot of the two 10Hy 125mA chokes mounted with countersunk screws to the rear panel of the amp.

Here are the input RCA jacks, the speaker terminals, the IEC power inlet and one of the fuse holders.

Now you can see how sloppy my soldering is :-)  This is the six position two pole selector switch with ten shielded cables from the RCAs.
The first position was configured as a mute position, since Harald requested five inputs and I couldn't locate a five pole switch.

Here's a look toward the power supply side of the amp with everything mounted.

Here's a view of the amp section.

And the whole thing.  

Here's the bottom cover showing the ventilation holes below the heat sinks on the filament supply modules.  The round
screens are nifty little things that fit in a 1" hole with tabs that I bent over to secure them.

This is the most quiet amp I have ever built.  When I first powered this amp up with a CD player for a source, I was worried
because I didn't hear any music from the speakers regardless of what position I set the selector switch.  I stuck my ear directly
into the speaker cone and could hear nothing.  I happened to look at the display on the CD player and realized that it had a fault.
When I reset it, instant music!  The DC filament supplies will be in all of my amps from now on.  No more fiddling with hum pots!


Here's a link to the article from Sound Practices magazine that inspired me to try this circuit.
WE91A.PDF

Here's a link to the schematic (finally!).

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.