Heathkit Model WA-P2 Preamplifier (1957)

           

This Heathkit WA-P2 preamplifier is a companion to my Heathkit W-6M Amplifier. Both of them were made in 1957, during the golden days of monophonic "hi-fi."

I bought this little preamp in 2014 after I had restored my W-6M amplifier. The first photo shows my WA-P2 alone and the second shows it next to the W-6M.

  

Here is a 1957 ad from High Fidelity magazine:

Description

Heathkit called the WA-P2 the "master control" for any of its Williamson-type amplifiers, of which (in 1957) my W-6M was the most recent, and most powerful, example.

The WA-P2 offers these features:

  • Volume control
  • Treble and bass controls
  • Rolloff and turnover controls for equalization of different LP recording types
  • Five inputs (microphone, phono, tuner, 1, 2) with individual level controls
  • Frequency response within +-1½ db from 15-30,000 kHz

When used together, the W-6M amp provides the power for both units, feeding the WA-P2 through a thick cable, and the preamp's power switch turns both units on and off.

The WA-P2's rear connectors allow an elaborate setup, if you're so inclined. In the illustration, the WA-P2 controls a phono turntable, microphone, AM/FM radio receiver, television, and tape recorder.

  

Click the next icon to read the condensed WA-P2 factory manual:

The manual contains operating instructions and many technical details, including the schematic diagram. I omitted most of the assembly instructions.

First Look

Let's take a look under the hood. The top view shows three tubes (one 12AU7 and two 12AX7) and a big aluminum can containing four electrolytic capacitors:

The next photo shows the underside of the chassis, with several plastic-coated paper capacitors (colored black and aqua) and a small electrolytic cap:

At the left in the previous photo you can see the WA-P2's thick grey power cable. Its eight-pin plug goes into the back of the W-6M amplifier.

You could use the WA-P2 with some other amp, but in that case you'd need to provide the power that it expects (6.3 volts AC for its tube filaments and 300 volts DC for its tube plates). If you want to switch the other amp on and off using the preamp, you would also need to route its 120-volt AC line through the WA-P2. A note to this effect appears on the last page of the manual referenced above.

Restoration

Like all 1950s tube devices, a WA-P2 will need restoration before you can expect it to perform reliably. As explained in my recapping article, paper and electrolytic capacitors degrade with age whether or not the device is used, so replacing them is mandatory. But first, I'll test the tubes and perform other preliminaries (see First Steps in Restoration).

All three of the tubes checked out nicely on my tester—as strong as if they're brand-new.

The tubes' printed labels are crisp and I didn't find a speck of dust anywhere inside the chassis. Perhaps this little preamp never got much use.

These are the original Mullard tubes, not replacements. The backs of the tubes have black etched code letters that are hard to read and nearly impossible to photograph, but I was able to decipher them with the assistance of the Antique Radios tube forum (see discussion). In my codes, the letters B7C and B7D mean that the tubes were made at the Mullard factory in Blackburn, England (B) during 1957 (7), in the months of March and April (C and D).

This preamp is not very complex, compared to many of the radios and TVs I have restored, but some of its circuitry is rather crowded. Furthermore, every high-fidelity device is carefully designed to minimize hum, distortion, or cross-talk from one circuit to another, so I'll be careful not to disturb any shielding or change the routing ("dress") of any lead wires.

The next photo shows a crowded corner. Notice how the aqua and black capacitors are wired tightly into place. Some are located underneath other components and others are half-blocked by parts of the chassis.

Speaking of corners, here's another that demands attention. We're looking at the base of the can that contains four electrolytic capacitors:

In that photo, I had already removed the two screws that hold the base to the chassis; I'll also need to unsolder all of those wires, of course. Then I can slide out the can and install new caps on this base. (See my recapping article for various methods of dealing with electrolytics in cans.)

The next photo shows the base after I had removed the can and drilled holes to insert the new capacitor leads next to the original terminals.

Notice how I left three resistors attached to the can's terminals. When I checked those resistors, their values still matched what's specified in the manual, so there's no point in disturbing things unnecessarily.

In the next photo, I have installed new capacitors on the old base, screwed the base back onto the chassis, and restored the original wiring. You have to look pretty closely to spot any of the new leads sneaking up along the original terminals.

The Turnover control has three capacitors mounted across the switch. To replace these, I loosened the switch's mounting nut and brought the control out where I could reach its terminals. There's enough slack in the connecting wires to do this without unsoldering all of them, but you should be careful not to break the thin hookup wires if you try this method.

Before long, the job was done. Here are top and bottom views of the restored chassis.

  

Fighting That Old Demon Hum

After restoration, the preamp behaved normally and all of its tube voltages matched those in the manual's voltage chart. The audio quality was impressive when used with my W-6M amp and a large speaker, but I could hear a distinct background hum, even with the volume turned all the way down.

When troubles arise, human error is the first thing that you want to eliminate. I carefully double-checked all of the work I had done on both the amp and preamp. Then I double-checked all of the work that the original owners had done when assembling both units, comparing the component values and lead dress against the many illustrations in both assembly manuals. I also tried adjusting the WA-P2's onboard "hum control" and substuting tubes in the preamp, as directed in the manual. Still no joy.

In one place, the manual suggested disconnecting the shield from the RCA-type signal cable that runs from the preamp's output to the amplifier's input. I temporarily disconnected the shield, but the hum persisted.

When I reviewed the manuals again, I came across two notes addressing hum, involving slight modifications to both the amp and the preamp. (Somewhat confusingly, the change to the amp is described in the preamp manual, and vice versa.)

The first modification, to the W-6M amplifier, involves disconnecting a green-yellow power-transformer wire from the preamp power socket. I included this note on the final page of the WA-P2 Manual.

The second mod involves moving a little wire in the preamp from a terminal strip to the ground lug of the phono input jack. This note from the W-6M manual uses identifiers (GG and the like) from the WA-P2 manual:

After I made these changes, the hum was noticeably reduced, but I could still hear it faintly with the volume turned down. There must be a second noise source!

In a forum discussion at Antique Radios, a fellow named Tube Radio suggested disconnecting the 120-volt AC power line from the preamp power socket on the amp. Since I already had a switch on the amp, I could route the AC through that switch and eliminate the AC as a source of hum in the preamp. (Remember, in the stock configuration, AC power for the amp is fed all the way out to the preamp's power switch, and back again to the amp, through the big power cable. The preamp doesn't need the AC to operate—it merely provides a switch for the amp's power supply.)

Bingo! That little change got rid of the noise for good. Perhaps 60-Hz AC hum was leaking somewhere inside the preamp, either through the wires themselves or across that switch, which carries the potentiometer for the treble control as well as AC power.

Final Thoughts

My little WA-P2 preamp performs beautifully now, and I'm glad I took the trouble to restore it. As I mentioned in my W-6M article, in the long term I'd like to use that amp to provide audio for my RCA TM-10 color monitor, and it will be nice to have tone controls as well as a volume control for that setup.

To be honest, the construction of the WA-P2 seems a little cheap, compared to the robust build quality of the W-6M amp. Of course, the preamp was a twenty-dollar item, while the amp cost more than a hundred! Within the limits of its design, the preamp works well, and that's all we can expect of any device, in the end.

©1995-2017 Philip I. Nelson, all rights reserved