Power Transformer Take-A-Part

we need some lams for a W.E 6L6 OPT, so why not take them out of an unused transformer that used to be inside an HP 400D AC voltmeter?

what happened to the voltmeter? well, analog meters are nice, but they can be a pain,

or a boat anchor,  you know what i'm sayin?

so we get free lams and gain insight into the world of power transformers,

another black tar affair to take another 10 years off my life, so we detox with siberian ginsing and L-Tyrosine, plus some benzocaine for this tooth ache which is unrelated to the task at hand,

this is just like opening up a hot pocket only different,

we will pull the top and bottm lids as this is a fat stack o lams which means a tight squeeze and a pain to get the guts out as this tar takes a million degree heat gun just to get it loose, jus sayin...

took a while, but we finally extracted the carcass,

copper shield is wrapped around the whole transformer and does not represent a shorted turn as it is outside the core,

used to keep radiated stuff from the sensitive voltmeter

another two hours to take out the lams without bending them and messing up the grain structure if there is any, could be barn roof lams (non grain),

notice the over square bobbin as HP like a pwr trans that will run cool and last a long time, like 55 years,

these are EI 125,  or 1.25 inch tongues,  perfect for a 6L6 OPT, 1/8" bigger tongue than the stock W.E we are trying to duplicate, since this is for a Portaflex bass amp, we can always use a lil more metal, wtf, over?

outer layer is a heavy duty 6.3 volt wind, 20 Turns of monster wire, 

low ohms, about 0.2

next layer in is a 6.3 v-ac on the left and a 7.3 on the right,

these get combined in series and rectified to keep AC out of the voltmeter amplifiers,

the 6.3 powers a pilot light, notice heavier wire on the left,

every transformer you take apart will have something different than all the others,

in this case, the difference is a reverse wind on the left chamber which means that the center tap is at the finish of both winds of the high voltage B+

why did they do this?

my guess is that they wanted the fields to cancel in order to keep the 770 volts of AC out of the volt meter amps,

never seen this on a push pull OPT, might be worth trying to see what it does to frequency response,

split chambers also keep the DCR at 140 ohms for both HV winds,
if you wind plate-CT-plate on a single chamber, you always get a mis match due the outer winds having a longer  turn length,

two copper foil faraday shields between pri and sec>

inner layer, as with most power transformers, its the primary,

344 turns each,

layers are wound right on top of the pvc lead wires, as opposed to bringing out the enameled wire and splicing on top of the coil,

dual chamber bobbin has smaller inner flange as the last wind is done as 1 layer,

sec turns are 1,180 ea, so 2,360 total,

so pri to sec ratio is 344/2.360 = 1:6.8

so if 120 on pri, sec total volts is 823 minus losses, so 411 AC into the dual wave rect minus losses (DCR and leakage inductance) HP says 395 V-ac from each sec, so 96 percent efficiency,

DCR on sec is 280 ohms, so figure 16 volts/280 ohms = 57 ma total plate current in the voltmeter,

> reverse wind on the left chamber which means that the center tap is at the finish of both winds of the high voltage B+.... never seen this on a push pull OPT, might be worth trying to see what it does to frequency response

Puts the high-side of both windings adjacent. Increases end-to-end capacitance. At higher Z I would expect this to worsen treble response.

here is a drawing for the HP pwr trans>