Many months have passed and the unit is finally complete. But getting there was quite the journey.
First the chassis was in pretty rough shape, servicable, but beat up looking. The lettering was pretty weak as well, so I decided to try to clean it up. I tried polishing with Flitz polish, but the finish was still beat up looking so I stripped off the paint. In so doing, I saw that there were underlying corrosions that explained why the polishing didn'dt work. The pic on the right shows the result of sanding the chassis down to remove the corrosions.
My first plan for chassis refinishing was to try my own screen printing, but alas, I failed ;-). After that defeat, I consulted with my 10 year old grandson Eli who has a real eye for art and color, and he helped me select the final color scheme of a flat black chassis with gloss dark metallic gray transformers.
Stu Hegeman interviews revealed in his philosophy for fatigue-free listening that one factor is wide bandwidth, well above the standard frequency range of 20Hz to 20kHz. The photos below show a remarkalbe square wave response for a transformer coupled tube amp! From left to right, 1kHz, 10kHz and 20 kHz at 1 Watt. In all three scope photos, the input signal is the top trace and the output is the bottom.
Overall gain is very well matched between channels, at 23.25 L and 23.22 R. The distortion spectrum above, right, shows a THD of 0.005% at 1W. Power output with both channels driven at 1kHz met the 60W spec at at a line voltage of 118 in both channels, 62W Left , 61W Right.
My friend Blake Hocevar (of Audio Video Choices, audiovideochoices.com, here in Phoenix) was kind enough to break-in the amp in their shop for a couple of weeks on a variety of musical and home theater programs. He offered some helpful hints on where to go from here, not the least of which is to upgrade the power cord from the old original stock item to something with a little more heft and less oxidation! I will do that!
Soundwise I think this amp works well in my system. I really like the detail it conveys through the Altec 19's and when you turn up the volume, the sound character doesn't change, it just gets louder! Audible noise level is quite low even with my ear up to the horns, and I really like the image it throws up. And I think Hegeman was onto something: both my wife and I can listen through this amp for hours watching movies or listening to music. We like to watch "The Voice" at elevated volume and it effortlessly delivers dynamic sound as smoothly as a hot knife through butter.
After painting the chassis and the transformers and before any wiring, I set the transformers on the chassis sitting on top of the bottom cover just to check the color scheme. It's a go! I definitely like these colors better than the original brown. So assembly begins first with the power supply rebuild following the McShane (mcshanedesign.net) recipe. First the bracket needed to be punched to accept the larger diameter caps and their mounting bracket holes. Then wiring and parts installation began, including a terminal strip board on which to mount the fast recovery diodes. Note that this mod also includes the separate bias transformer that McShane recommends.
After final assembly, I installed my best 12BY7 tubes and a fairly well matched set of Svetlana 6550C tubes. It's currently installed in my main system driving the Altec Model 19's, which are currently augmented with subwoofers and super tweeters, connected to the Citation II with DanaCables (danacables.com).
Next the board assembly (note the resistors, more on that later), then harness construction, chassis mounted parts and final wiring.
I've had this broken down chassis sitting around for several years waiting for time and inspiration. Well, now is the time and reading what others have done, including my friend Steve Geiger with the help of the master, Jim McShane, I'm now inspired. The goal is to make this unit look as healthy and beautiful looking as it will sound with McShane's mods. Right now the unit is completely torn down. Right on the heels of the Heathkits, the chassis will be completely torn down every nut and bolt, thoroughly cleaned, paint removed, chassis bends straightened, sanded, primed, painted with Sherwin Williams enamel matched to the original, and finally re-screened to make the lettering look new. Of course the transformers will be painted as well. The current plan is clear coat the paint to help protect it from those ugly scrapes that the originals were vulnerable to. I plan to make the clear coat glossy to enhance the colors a little and make it smooth to the touch.
Electrically, it will get the full McShane Level II Plus mods with PIO capacitor upgrades. It will get things like new taller feet to aid cooling and lifting and a new 3 prong power cord update for safety.