Graaf GM 200 OTL power amplifier
Of course, nothing's free; eliminating the output transformer (and its distortions, claim rabid OTLers) engenders other problems. For instance, the primary purpose of an output transformer (the bad puns never stop) is to reduce the intrinsically high source impedance of power tubes to one more appropriate for the amp/speaker interface. This accounts for the thicket of PL504s in the GM 200: each doubling of the number of tubes halves the total source impedance. And, as you might guess, choosing the right speaker is of great concern.
OTLs are also widely cited for blast-furnace levels of heat. The GM 200 is certainly no exception. The blessed thing ran incredibly, furiously HOT. Powerful thermals shimmered in the air above it. The lights in our loft (in our neighborhood?) dimmed every time I fired it up. The huge toroidal power transformer remained too hot to handle for several hours after shutting down! A real Cat on a Hot Tin Roof.
Importer/distributor Gary Kaye counters that while the amp sure does run hot, no arguing about that, an entire set of 32 PL504 output tubes costs an almost-reasonable $500. And, he added enthusiastically, "They're easy-to-source TV tubes with an expected lifetime of around 10,000 hours!" Unsurprisingly, Gary also reminded me that nothing's free; output transformers have their own well-documented problems. Too true.
Arte e industria...
The GM 200 is completely Italian in origin, function, and spirit. You might call it the Ferrari of tube amps—Graaf is located near Ferrari Central in Modena, Italy. In fact, Graaf uses the same finish as their neighbors. (I looked carefully but found no flies in the paint.)
I even found an item on the GM 200 while flipping through the magazine Graphis at Barnes & Noble ("Industrial Design Highlights," Jan./Feb. '97, No.307 Vol.53). And no wonder—the amp is, as they say, drop-dead gorgeous, a single-chassis stereo unit somewhat smaller than you'd imagine from the photos. On paper the eye is drawn to the bristling carpet of power tubes. But PL504s are actually rather petite, and the whole affair, when seen in the metal, is beautifully proportioned and takes up surprisingly little room.
Nevertheless, at just over 66 lbs it proves a hefty two-person lift. The polished roll-bars fore and aft of each set of 16 push-pull pairs of output tubes provided a useful way of hauling the amp around our listening room. (The hoops are meant to locate and support the tube cages, of course.)
When seen head-on, the uncaged power tubes resemble the exposed top-end of a racing engine. But let me give you a serious warning about running the amp without its cages: If you touch the anode at the top of an exposed power tube and the chassis at the same moment, you'll frolic'n'cavort to the tune of 150 Italian volts. For those less inclined to risk, the amp, with its polished tube cages in place, remains an icon of elegant industrial design.
Why the PL504 output tube? Manual: "This tube is able to supply a high value of current not obtainable with the most blazoned [sic] of tubes." I suppose Graaf is in a position to know about such things; they've been making amplifiers of one kind and another for almost 15 years.
On the rear apron I found two sets of high-quality WBT gold-plated 5-way binding posts, and a choice of balanced or single-ended inputs. The IEC mains-in is protected by a whopping 20 amp Slo-Blo. Set into the rear panel are a pair of test-point receptacles for adjusting bias and DC offset.
The left and right channels of the GM 200 are physically separated on four circuit boards in a dual-mono configuration. The power supply comprises five separate stages, of which two float in respect to ground and supply the output stages. The amp is biased into class-A for 20 of the 200Wpc available, and makes use of a relatively modest 17.5dB of overall negative feedback.
If you go in balanced with XLRs, the input signal traverses an additional "class-A Buffer Separator" module using a single 12BZ7 per side, then to a single 5965 per side to split the phase for push-pull operation. This stage also serves as the input when running single-ended. I got the best sound by avoiding the additional tube and circuitry of the "Buffer Separator" module and going in single-ended.
The voltage gain to drive the power stage is derived from a pair of tall, skinny EFL200 double-pentodes per side, mounted inboard of the squat 5965s. There are no solid-state devices in the signal path, or in the offset and bias controls. Power output is signaled by a pair of Telefunken EM81 Magic Eyes, one mounted each side of the industrial-strength power switch. In the dark, the fiercely glowing power tubes and jumping green Tellies are quite a sight.
The amplifier is a fully DC-coupled OTL (output-transformer–less) and OCL (output-capacitor–less) design. Scanning the complete and very technical manual (including parts list and schematics), I found the following: "The use of only one high-quality capacitor in the input stage avoids any unbalancing of the offset caused by undesirable DC voltages unintentionally produced by the preamplifier."
Since there's no blocking capacitor on the output, protection circuits are vital to avoid exploding tweeters and other speaker-related mishaps. As a result, firing up the amp is accompanied by a cascade of chattering relays (plug-in modules for easy field servicing). The circuits are protected by two current-limiting systems, and there's another protection circuit on the output. Importantly, the first limiter works on the power-tube filaments, thus improving longevity. All circuits are deactivated by a timed relay during normal operation.
Protection goes down to board level on the output tubes. Here's Graaf's somewhat idiosyncratic explanation: "In the soldering part of the output circuit board there are 1 ohm fuse resistors, connected in series to the cathodes of the power tubes. In case of tube trouble these resistors will break themselves and light an LED diode on the upper part of the circuit board near each power tube."
If a couple of tubes do go down, you can still operate the amp and enjoy music. Total power levels are somewhat reduced this way, but it's actually hard to tell the difference. How do I know? A cranky early-production GM 200 blew its way through three power tubes and shut down one channel seemingly at random. This beat and scruffy demo unit was sent for a brief compatibility check with the Joseph Audio RM-50s used as evaluation tools in the review. (This speaker's relatively high impedance curve—it never drops below 7 ohms—makes it eminently suitable for use with OTLs.) The second unit, an up-to-date example from current production, proved altogether better-sounding and behaved like a gentleman.
Being the very embodiment of Italian engineering and style, the GM 200 must be tuned up for best performance. The tiny bias and offset trimmers are set into the high-gloss chassis on the front of the amp. Measurements are taken across a pair of speaker terminals for the DC offset (ideally, less than ±30mV) and between the Test Point and a negative speaker terminal for the bias (ideally, in the 75–80mV range). A good strategy would be to check these values during setup—about five minutes after power on—and again an hour or so later.
The amp held its bias and offset well. However, one evening I was taken aback by a suddenly grainy and rather bright treble. I quickly found the culprit: a touch of DC drift in one channel. A few moments later, tutto va bene.