Wednesday, December 01, 2010

It's been an obstacle course lately. Since the workshop proper is still a living mess I was unable to locate my box of octal sockets, so I had to scrounge around until I located one in the piles. This naturally equates to my only addressing the really bad socket and hoping for the best.

On the upside, the clearly weathered nature of this socket matches the rest of the amplifier completely.

While desoldering this socket the wax flowed freely, there was enough to plug one of the holes. I take back what I said previously about the candle color, it must have been turquoise; I don't think corrosion would flow a solid color. I guess my mind refused to visualize turquoise candles in my house.

Anyway, while perusing the guts I took note of this unused tap at the power transformer.

This amp has always run a bit hot, and in the past I've elected to just place a variac in line with the amplifier and knock off a few volts from the wall. Wiring up an unused filament winding to buck the primary down would free up the variac and simplify installation.

To determine the polarity of the secondary windings, I desoldered one of the primary wires and left it to hang free. Then it was a matter of using a pair of jumpers: one from the hanging primary wire to one of the secondary winding taps, and the second jumper from the other secondary tap to the point at which the primary winding tied into the electrical circuit (in this case it was an auxiliary electrical socket).

With the amplifier sans tubes and on a variac (at roughly 10%), I threw the power switch and read the voltage on the secondary B+ leg. I then cut power and swapped connections at the unknown secondary winding and repeated the test. The lower reading of available voltage at B+ is the bucking arrangement while the higher is the boost (to be avoided at all costs in this circumstance).

So, determining arrangement of wires for bucking the line voltage down closer to design intent was a snap, if it happened to be a 6.3 volt filament winding, which it is not. I was expecting to read a difference of a couple volts at most, whereas the swing here was too much given the highly reduced voltage from the variac.

This lead me to clarify the coloring through scuffing, clearly not a green 6.3 volt leg. Looks like a mid voltage secondary for bias (an old RMA color code chart allocates blue to plate). Since the HF87 is a cathode bias amplifier I can live without this tap, as I have done for years.

Won't happen again. So it looks like I've still got to build a fixed step down/bucking transformer to temper the wall voltage so I can have my variac back.

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