Monday, February 25, 2013

Rebuilding a pair of Heathkit W-2 tube amplifiers

 The Heathkit W-2M amplifier is a fairly rare beast compared to the more later versions of the Williamson line of amplifiers.  It is also perhaps one of the earliest incarnations of a commercial ultralinear circuit.  The earlier predecessor, the W-1, used 807s connected as triodes, with pretty much the same power supply and driver circuitry.  Of note, it's would be an easy project to convert the W-2 to match the W-1, since the layout and even the output transformers are so similar.

Anyway, for my W-2 rebuild I decided to stick with a retro-modern ethos: keep the same circuit but improve the parts and biasing method.  Since I consider myself a bit of an audio historian, I prefer to hear the amplifier as the designer intended.  If someone wants to tackle an improved Williamson design, I would strongly suggest the Chimera Labs modification which will also reduce distortion, provide cleaner power, and improve square wave performance. 

(right click and open to see more detail)

Power Supply Chassis: The stock power supply is rather anemic with barely enough capacitance to reproduce transients and extended output levels.  In my case, I replaced all of the electrolytics, also increasing the values except for the first bucket since it is attached to the 5V4 rectifier.  I also added bypass resistors to normalize the voltage between the two series capacitors.  Parts required per unit: (2) 22uF/350V capacitors, (2) 150uF/450V capacitors, (4) 470k/2W resistors, capacitor mounting hardware, and nuts & bolts.

(right click and open to see more detail)

Signal Chassis: Once again, new electrolytics, also increasing the values to improve power supply stability.  Potentially leaky paper coupling capacitors were replaced by new film units.  The input capacitor was removed and replaced by a small value (1K) carbon composition grid stopper resistor.  Aged resistors should also be checked to see if they are within specification.  This is especially important for the ones requiring matched pairs.  Parts required per unit: (2) .047uF/400V capacitors, (2) .22uF/600V capacitors, (2) 1K carbon composition resistors, (1) 47uF/450V capacitor with clamp and nuts & bolts, and (1) 22uF / 450V capacitor with two lug terminal strip for mounting.

 (right click and open to see more detail)

Bias Supply: The 1/4" phono test jacks were replaced by easier to use banana jacks and 10 ohm resistors which now allow the output tubes to be balanced by using a multimeter: set the meter to the lowest voltage range and adjust the 100-ohm potentiometer until a reading of zero is found.  This biasing method is pretty much straight from the Heathkit W-5 schematic and is much easier than trying to juggle current meters with the clunky phono jacks.  Parts required per unit: (2) 22uF / 150V capacitor, (2) 10 ohm, .5W resistors, (2) banana female jacks with washers and nut, and one terminal strip.

Miscellaneous: The stock RCA jack can easily be replaced with a modern gold-plated unit.  On the other hand, the single speaker screw terminal strip is a slightly more difficult proposition.  At this time I don't have an easy solution that doesn't require a drill press and new hardware.  For now I suggest using speaker wire with small "vintage" spades, or bare solid-core wire.

Listening results: The stock pair of Heathkit W-2s, even with ancient electrolytic capacitors, were still quite pleasant.  They sounded a little muddled and lacking in power, but still possessing a musical charm that many modern units have a hard time matching.  The excellent Peerless output iron certainly helps.  However, this simple upgrade really helped, making the bass firmer and the dynamics better; all while keeping the golden midrange and easy going treble detail.  With new parts, the W-2s really excel at musicality and now rate among my favorite vintage amplifiers.  At least in my listening situation, I would still give the nod to the Eico HF-60s, but that's due to the higher power and the overall sound of the EL34, which is a tube I prefer over the 6L6GC family.  But for a different listener and a different setup with more efficient speakers, it really would be a matter of personal preference.

Tubes: With the rated 4A filament supply, it is possible for the power transformer to handle an output tube with a little more heft than the 6L6GC.  Since I'm not one to trust vintage iron too far, I probably wouldn't go beyond a KT66 type.  I would be curious to see how a set of Gold Lion re-issue KT66s would stack up against the vintage Tungsol 5881s.  Another option for the cheapskate would be to rewire the output sockets to use the 6AR6 which is still cheap and plentiful.

With two 6SN7s handling gain and phase-splitting duties, there is a vast world of tube-rolling opportunities here.  For now, I'm sticking with GE 6SN7GTAs since they are easy to get and sound great for the dollar. 

The 5V4 rectifier can be replaced with the 5AR4 which will slightly bump up the voltage and offer some of that classic Mullard sound.  However, with the high price of good vintage versions, one may want to stick with the more common 5V4 with all the various manufacturers and versions.

Conclusion: Bringing an old tube amplifier back to life and ready for another twenty (or more) years of service is always a feeling of accomplishment.  The Heathkits aren't the easiest to rebuild, not with some of the strange capacitor can sizes and small interior chassis depth, but the time and work was worth the effort.  These are wonderful amplifiers with that vintage smoothness but still modern enough for electronic music and pop.  Highly recommended.