V72a Power Transformer NT/V74

here goes the pwr xfmr for the weird V72a preamp,

this one happily features a split primary so we can electrocute ourselves with either 120 or 240,

there is just 1 secondary winding and one non center tapped heater winding, so it is only 4 sections,

core is the snap in type again, every transformer has something new and weird about it, for this model it was the screen made out of some type of foil besides copper, maybe brass,

and the purple wrapper is uber cool,

heater is wound last,

poly wrapper over winding,

different screen foil,

start of the first layer which is one of the primaries,

brown interlocking  insulator parts are taped together,

here is the print,


the label say that you use 110 x 2 , = 220,

or 110 in parallel, but nobody uses 110 any more so you will get a higher sec voltage than the 205 printed on the label if you wire it for U.S 120.,

the turns ratio and voltage readings seem to bear this out,

heater turns ratio looks like it will be too high on the sec side, there is about a volt lost in the winding, maybe more due to leakage as the heater winding is furthest from the center leg of the core,

but 2670 / 102 = 26.18,

if we wire the primaries in series, and use 220 VAC power, then we will have

220 / 26.18 = 8.4 volts, minus a volt for the 2.1 ohms of dcr at 500 ma heater draw for the 6AK5 and the E80f = 7.4 VAC times .95 for efficiency and another  3 percent for the winding being last and we have 6.8 volts,