Western Electric CW-926A Restoration

(Please note: several pictures on this page have clickable links to higher resolution versions.)

The dirt inside is so pervasive and so inaccessible that it quickly became clear that only a complete disassembly would make cleaning possible.  This was obviously not going to be an evening project.

The wood cabinet had suffered considerably.  The bottom panel had warped and split, and the rodents had further damaged the wood by gnawing at it.  Several pieces of the brass cabinet hardware were missing and the original shellac finish was in bad condition.  The schematic diagram that was glued to the inside back of the cabinet was almost completely destroyed.

This unit was probably sold as surplus by the Navy in the early 1920's.  Once in civilian hands, two binding posts were added to allow the insertion of headphones into the plate circuit of the output tube as an alternative to the low impedance CW-929 horn.  Fortunately, the new owner used the existing tube inspection holes for this, rather than drilling new holes.  One resistor was replaced, which is visible just behind the tubes in the previous picture.  One of the seven binding posts had been broken off and lost.

The schematic diagram in this cabinet was about 95% intact and mostly readable, which was a big help in the restoration process.

Most of the wiring is no. 14 solid copper wire (identical to house wire of the same size).  All connections to the shock-mounted tube socket assembly were made with flexible stranded wire.  All of the wire was insulated by sleeving that had a cloth base with a rubber coating.  Almost without exception, the old sleeving was badly deteriorated, and would crack or crumble under any stress.  At least the mice did not find this material to be edible.

I located a modern product consisting of woven fiberglass with a PVC coating that looks astonishingly like the original, so I decided to replace the old sleeving with this material.

The key components in this amplifier are the three transformers.  A continuity test revealed them to be functional.  At this point, I decided that it made sense (?) to restore the unit to full operating condition since it would be apart anyway.

The socket on the left was apparently hacked up to allow a failed tube to be removed.  The remaining tubes could only be removed once the socket shells were removed from the phenolic base, and that was only possible once the entire assembly was removed from the amplifier.

A report that was issued within the Western Electric company after the end of WW-I mentioned this melting wax problem as it was experienced in service on the sub chasers.

The original wax that they used had a melting point around 160° F.  They found that the temperatures experienced in operation went as high as 225° F!  That is almost incomprehensible, and it speaks volumes for the fortitude of the sailors who served on those ships.  A higher temperature wax filling compound was developed which was an improvement, but a recommended change to a phenolic insert as used in the VT-2 tubes was never implemented as the war ground to its conclusion.

The two tubes that were still in the unit were apparently there since the amplifier was installed on a sub chaser.  Once removed, they tested well, so I remelted the wax and pressed it back into the shells.  The original tubes are now back in the sockets which held them for the last 90 years.

The socket shell on the left (for the output tube) has two bayonet slots in it, allowing either the VT-1 or VT-2 type tubes to be used in that position.  The VT-2 tubes had bayonet pins offset by 45 degrees from the standard position.  This prevented inserting one of these tubes into the wrong socket.  When I acquired this amplifier, a VT-2 was installed in this socket, which was still usable for that tube type.  A replacement was fabricated, however.

A standard size of aluminum tubing was almost the right size for the replacement shell, but it had to be ground out on the inside to provide sufficient clearance.  Note the two original shells have vertical slots which allow them to expand a bit, though this slot was probably only a result of their having been fabricated from sheet stock that was rolled into the necessary round shape.

After several hours of hand work, the new shell looked so nice that I didn't want to risk ruining it by cutting the slot, so I left it that way.

On either side of the transformer are groups of resistors.  The large resistors running front-to-back are plate resistors for the three stages, or voltage dropping resistors in the series-connected filament circuit.  Each of these resistors has an asbestos washer at each end which will be removed.  Underneath are two additional groups of resistors running left-to-right which are in the grid circuits of the first and third stages.

The one above consists of a 2K ohm plate supply resistor for the output stage, and a 48K plate supply resistor for the second stage.  The one below consists of a 48K ohm plate supply resistor for the first stage and two resistors that are used to generate grid bias voltages for the three stages (8 and 15 ohms) as part of the series-connected filament circuit.  The two 48K resistors actually consist of three 12K units in series due to limitations in the technology of the time. 

At each end is a piece of 1/8 inch thick Bakelite (at the panel end) and a piece of a much thinner phenolic material at the free end.  Screws run through the hollow cores of each resistor and hold the assembly together.

These resistors did not have solder terminals.  Instead, connections were made through uninsulated stranded wire leads.  These were insulated with ceramic beads which were appropriate for high temperature operation.  I have seen the same beads used in early electric heaters.

Fortunately, the two bias resistors are still usable, but most of the plate resistors are open.  They are almost identical in size to modern 50-watt resistors, but because of their unique appearance, I sought a different option.  Actual measurements on a breadboarded version of the circuit indicated that a 10-watt unit would suffice for the third stage plate resistor, and 2 watt units would suffice for the others.  I bridged them with modern resistors with black ceramic coatings that would be difficult to see once installed.

Since most of these resistors will never get hot again, I replaced the asbestos washers with ones that I made from white felt.  They are totally safe and look very much like the originals.  For the two bias resistors, I used fiberglass washers that were also light in color.

They are a conventional wax paper and aluminum foil design and were sealed with tar.

They were gutted, cleaned, painted and a modern capacitor (tiny, by comparison) installed inside.  They were then resealed with the original tar and reproduction labels applied.

Note the number "87" stamped into the rear of the panel in the upper left corner.  I believe this is the serial number for the unit.  There are no external numerical markings on this unit (other Western Electric equipment of the same vintage have serial numbers stamped into metal plates on the outside of the cabinet).

As has been mentioned, the CW-926A (and some other Western Electric equipment of the same vintage) had the tube filaments connected in series.  In this case, the supply voltage was nominally 30 volts DC, as derived from the ship's batteries.  The 350 volt plate supply was provided by dynamoters (motor-generators) driven from the same 30 volt supply.

Dropping resistors in the filament circuit were used to generate negative grid bias for each of the tubes.  The first two stages had bias voltages of approximately -15 volts, and the output stage had a bias of approximately -20 volts.

The VT-1 and VT-2 tubes were designed to use filament voltages of 2.3 and 7.0 volts, respectively.  In this amplifier, at the rated supply voltage with a VT-2 installed, the VT-1's will operate at very close to their rated voltage, and the VT-2 will have about 6.5 volts on its filament.  If three VT-1's are installed, the first two VT-1's will have about 2.8 volts on their filaments, and the final stage will have about 4 volts on its filament.

Some VT-2's that I have seen have "4.8" ink stamped on their bases.  Presumably, Western Electric had at some point backed off from the 7.0 volt rating in the interest of longevity.  The lower voltage was in line with the later versions of this tube, most notably the 205-D, which had a 4.3 volt filament rating.

In either case, it is clear that the designers were driving the output tubes very hard, and they probably didn't last very long. 

The front panel of the later unit has two additional holes that the earlier unit doesn't have, as shown in the above picture.  Other pictures of CW-926A units that I have seen also have these holes.

The explanation is found in the way the resistor subpanel is mounted.  The two holes shown in the picture below never had screws in them (this side of the panel was closest to the front of the unit).  If the resistors were installed on the panel first (the logical approach from a production standpoint), there is no way these screws could be installed.

What they apparently did was to add additional brackets which strengthened the mounting system for this panel, hence the additional holes.

There was no clearance issue with this one.  This was strictly a quality control problem.  Only five of the eight screws were present, which might have led to breakage in severe conditions, especially considering that this panel was the sole support for the three transformers which collectively weigh about 15 lbs.

There were other quality control problems as well.

One of the volume control switch points was improperly installed, though it didn't affect the operation.

Each of the small brass tags identifying the binding posts along the bottom of the panel is held in place by two brass escutcheon pins (tacks).  Eight pilot holes were drilled for these pins.  Then, for whatever reason, the workman decided they were in the wrong place, and he proceeded to drill another series of holes about 1/4 inch above the first set.  The first set of holes were then filled in.  Again, this was a cosmetic problem only.

One of the phenolic pieces which hold the ends of the large resistors in position had not been made correctly, and was an interference fit once installed (the "U" shaped support bracket forced it out of the correct position so all of the resistors leaned a bit from perpendicular).  I trimmed it to match the base piece, which is obviously what the designers intended.

Finally, the workmanship exhibited by whoever wired this unit left much to be desired.  It is generally not acceptable, particularly in military equipment, to lay the end of a wire against a lug and rely on the solder alone to make the connection secure and permanent.  But that's exactly what was done in this unit in at least half of the connections.

From a maintainability standpoint, the design was a nightmare.  The difficulties caused by the punched or soldered hardware must have been considerable.  Replacing any component, particularly a failed resistor would have required a major disassembly.

So, this is an early unit, built as production was just starting up and, as always seems to be the case, in a rushed atmosphere of impending contract deadlines.

Military electronics, in the 1917-18 period, was in its infancy.  There was much yet to be learned.

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