Ampex 120W Monoblock Transformer Sets

. . . .Presented here are five sets of Ampex 120 iron, PTs, OTs and chokes, as used in the Ampex 6446 amplifier; details on which can be downloaded here. The iron was manufactured by Oregon Electric Co., OECO, in Milwaukie, OR. (The Left Coast spelling is correct). Noteworthy are the facts the company is still around, largely as a defense contractor, and that 50 years ago it was one of Boeing’s preferred suppliers.

. . . .After much experimentation Bruce Moore and Scott Frankland, then of Moore-Frankland Assoc., producers of the renown Luminescence preamp, settled on the iron used in the Ampex 60 and 120W amplifiers as the best of the “Golden Era” and scoured the Bay area pretty much clean of them.

. . . .Because the ‘120 was in production for about 20 years there are slight differences in the sizes of the potting cans, slight variations in the terminal styles and slight differences in the lead-outs on the chokes. The bolt patterns for all remained the same, making all the parts interchangeable.

. . . .I have grouped the iron into the five sets you can see below and made note of the variations, not that any of them is substantial. With the exception the chokes in sets three and four, all devices are potted into sealed cans and all have been very carefully refinished using two-part Endura, a bulletproof paint used on fuselages. The other four chokes are similarly potted but the enclosure ends are screwed in place and they have wire leads.

. . . .The power transformers are in 4.5" x 6.0" x 6.25" cans and weigh 25 lbs. ea., the outputs are in 4.5" x 6.0" x 6.75" cans and also weigh 25lbs. ea. while the chokes are in 3.6" x 4.25" x 5.5" cans and all weigh 12 lbs. ea. All are individually packed 2" hard-Styrofoam lined cardboard boxes and taped shut. These parts weigh too much to be packed into 55 or 65 lb. boxes, UPS would trash them to bits.

. . . .All the PTs have been hi-pot tested at 3KV AC, all windings to each other; all the chokes have been measured at 400Vrms @ 120 Hz plus 200mA DC conditions and all measure 8H at 35-ish ohms DCR. The GenRad 1630AV was used.

. . . .All the OTs have been very carefully measured into matched pairs using the test rig shown below along with some of my great HP and B & K gear and the average frequency and phase responses are seenhere. Note that the source impedance of the driving amps used was "padded up" with 390 ohm resistors to be effectively that of 3 push-pull pairs of 6550Cs run at 575B+ volts and 400 screen volts; 1.5K© near enough. Seeing two resistors of only 390 ohms in series with the 50 ohm output Z of the Tek amps might seem like an error but it correct . . .

. . . .A PDF showing the actual frequency responses of the individually matched pairs of OTs can be downloaded here. On average the -1dB point is 50KHz and -3dB is 80KHz. While is there is some, expected, unit-to-unit variation, it is slight; OECO held the spec. very well. The 1 KHz square wave performances of the 8 ohm loaded secondary and the RC terminated tertiary windings can be seen here.

. . . .Measuring the open circuit (primary) inductance, OCL, with the 1630AV at 100VAC @ 100Hz with no DC in the winding it averaged some 90H and that is right in the "rule of thumb" ballpark of 80-to-100H per 1K ohm of plate-to-plate source impedance. And, that OCL does not significantly collapse with DC unbalance. At 10mA DC it drops to 80H and given the bandwidth these devices evince, that is very good.

. . . .In stark contrast, the OT in the modern day Sonic Frontiers Power 2 measures some 250H OCL under no-DC conditions but with only a 10mA DC unbalance the OCL collapses to 50H, an 80% drop ! ! And some DC unbalance is always present . . .





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General Radio 1630AV
200W Incremental Inductance Bridge

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Here are just a few pix of the lab.